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Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media {LLC}  

Abstract: A quantitative evaluation method for a reduced-order model of the flow field around an NACA0015 airfoil based on particle image velocimetry (PIV) data is proposed in the present paper. The velocity field data obtained by the time-resolved PIV measurement were decomposed into significant modes by a proper orthogonal decomposition (POD) technique, and a linear reduced-order model was then constructed by the linear regression of the time advancement of the POD modes or the sparsity promoting dynamic mode decomposition (DMD). The present evaluation method can be used for the evaluation of the estimation error and the model predictability. The model was constructed using different numbers of POD or DMD modes for order reduction in the fluid data and different methods of estimating the linear coefficients, and the effects of these conditions on the model performance were quantitatively evaluated. The results illustrates that forward (standard) model works the best with two to ten significant DMD modes selected by sparsity promoting DMD. Graphical abstract: [Figure not available: see fulltext.]

DOI： 10.1007/s00348-021-03205-8

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Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers ({IEEE})  

The randomized subspace Newton convex methods for the sensor selection problem are proposed. The randomized subspace Newton algorithm is straightforwardly applied to the convex formulation, and the customized method in which the part of the update variables are selected to be the present best sensor candidates is also considered. In the converged solution, almost the same results are obtained by original and randomized-subspace-Newton convex methods. As expected, the randomized-subspace-Newton methods require more computational steps while they reduce the total amount of the computational time because the computational time for one step is significantly reduced by the cubic of the ratio of numbers of randomly updating variables to all the variables. The customized method shows superior performance to the straightforward implementation in terms of the quality of sensors and the computational time.

DOI： 10.1109/LSP.2021.3050708

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Other Link： https://dblp.uni-trier.de/db/journals/corr/corr2009.html#abs-2009-09315

Publisher：Mechanical Systems and Signal Processing  

The present study proposes a novel dynamic mode decomposition (DMD) that can simultaneously estimate the reduced-order model, the original signal, and the system/observation noise model only from the noisy data. An expectation–maximization (EM)-algorithm DMD (EMDMD) combines DMD and the parameter adjustment of the linear dynamical system (LDS) based on the EM algorithm. The initial parameters based on the linearity of the reduced-order data are set by using DMD. Subsequently, the log-likelihood of the complete data is maximized by adjusting the LDS parameters while separating the noise. The proposed algorithm is applied to the benchmark data of the short-fat and tall-skinny data matrices with different noise and the time-series velocity fields of the flow around a circular cylinder and the separated flow around an airfoil. The performance of EMDMD in terms of system identification and noise separation from the noisy data is evaluated, and the EMDMD shows the highest system identification and noise separation performance in all data.

DOI： 10.1016/j.ymssp.2024.111864

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Language：English   Publishing type：Research paper (scientific journal)  

The pressure fluctuation distribution on the floor surface behind a supported square cylinder in a turbulent boundary layer was measured by using pressure-sensitive paint (PSP). Specifically, the accuracy and frequency response of PSP at a Mach number around M = 0.3 were examined. Four square cylinders with different dimensions were investigated in the same turbulent boundary layer, and the detailed relationship between these conditions and Kármán vortex shedding structures was discussed. The values measured with PSP had a similar trend to those measured with a pressure transducer at up to 5 kHz. The peak power spectral density (PSD) of the pressure fluctuations due to Kármán vortex shedding was observed within an error of approximately 30 % at up to frequencies greater than 3 kHz. Moreover, the peak frequency of Kármán vortex shedding was found to decrease with the cylinder height in a manner similar to a previous empirical equation for the Strouhal number. The peak PSD value for the shortest square cylinder (h/w = 3.5, h/δ = 1.1) was twice as high as that for the other cylinders; also, the high-pressure fluctuation area in this case did not spread downstream, which suggests that the flow from the top of the cylinder affected the Kármán vortex shedding generated from the cylinder's sides. One contour of the two main modes extracted for the highest PSD via the right singular vector vi at the Kármán vortex frequency had an asymmetric distribution behind the supported taller square cylinders (h/w ≥ 7.0, h/δ ≥ 2.3). This result is considered to be derived from the Kármán vortex shedding being mainly generated from the mainstream rather than the boundary-layer flow.

DOI： 10.1016/j.expthermflusci.2024.111226

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Authorship：Last author   Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1016/j.expthermflusci.2024.111210

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DOI： 10.1088/1361-6501/ad2b41

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Authorship：Last author   Language：English   Publisher：Review of Scientific Instruments  

In this paper, we propose a photodegradation correction method for the dual-luminophore pressure-sensitive paint (PSP) measurement using lifetime-based imaging, which was proposed for correction of the temperature-induced error but has suffered from photodegradation in the previous studies. We introduced a parameter that characterizes the photodegradation of a dual-luminophore PSP as the intensity ratio between the two luminophores. The changes in the calibration coefficients for the pressure and the temperature due to photodegradation were corrected based on this parameter. In this study, a coupon-based calibration test was performed, and the luminescence characteristics of the dual-luminophore PSP including photodegradation were investigated. Then, the proposed method was applied to a coupon-based validation test and a jet impingement test, and the effectiveness of the method was evaluated by comparing results with and without correction. The pressure measurement accuracy was significantly improved by photodegradation correction.

DOI： 10.1063/5.0194749

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AIP Publishing  

This study proposes a framework that reduces the calculation cost of sound source localization with the Amiet model, using a data-driven sparse sampling method. This method accelerates the calculation of the steering vector used in conventional beamforming. An aeroacoustic wind tunnel test was conducted in a 2 × 2 m2 low-speed wind tunnel, and the proposed method was verified. During the test, a monopole laser sound source, which does not interfere with the flow, was used, and its acoustic signals were measured using a microphone array. Next, steering vectors were reconstructed by discovering dominant modes and optimized sampling points from the training data based on the Amiet model and the modified data-driven sparse sampling method. Finally, the sound-source positions when the steering vector of the proposed model was used were compared with the positions observed when the steering vector of which all the grid points were calculated was used. The error was less than 2 mm when 16 modes were used, and the calculation time was reduced to ∼1/33 of that of the previous Amiet model.

DOI： 10.1063/5.0205567

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Applied Sciences (Switzerland)  

Dielectric barrier discharge plasma actuators (DBD-PAs) have the potential to improve the performance of fluid machineries such as aircrafts and wind turbines by preventing flow separation. In this study, to identify the multiple flow control mechanisms in high Reynolds number flow, parametric experiments for an actuation parameter (Formula presented.) with a wide range of Re values (105–106) for NACA0015 airfoil was conducted. We conducted wind tunnel tests by applying a DBD-PA to the flow field around a wing model at the leading edge. Lift characteristics, turbulent kinetic energy in the flow field, shear layer height, and the separation point of the boundary layer were evaluated based on pressure distributions on the wing surface and velocity of the flow field, with the effect of DBD-PA on the post-stall flow around the wing and the mechanism behind the increase in the lift coefficient (Formula presented.) analyzed based on these evaluation results. The following phenomena contributed to the increase in (Formula presented.) : (1) increase in turbulent kinetic energy; (2) increase in circulation; and (3) acceleration of the flow near the leading edge. Thus, this study effectively investigated the dependence of the increase in lift on (Formula presented.) and the lift-increasing mechanism for a wide range of Re values.

DOI： 10.3390/app14114652

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Publisher：Journal of Visualization  

The surface pressure field generated by a supersonic impinging jet on a vertical flat plate was measured using a pressure-sensitive paint (PSP) and a double-pulsed laser. The Mach number of the jet was Mj = 1.23 and the position of the flat plate was h/D = 4.5. A significant peak at St = 0.41 (15.2 kHz) was observed in the spectra measured by a microphone, and unsteady pressure transducers. The feedback loop involved in the acoustic loading occurred at this frequency. Coherent structures of the flow were extracted by applying the azimuthal Fourier decomposition and the dynamic mode decomposition (DMD) to PSP images on the impingement plate. Axisymmetric modes expanding outward from the impingement point of the jet were observed for the azimuthal mode m = 0. Two helical modes related to the feedback loop were identified for |m| = 1. It was confirmed that the RMS values of the amplitudes of these DMD modes were larger than the other modes. The frequencies of these DMD modes were St = 0.39 (14.3 kHz) and St = 0.37 (13.7 kHz), respectively. Coherent structures associated with phenomena faster than 10 kHz were successfully extracted from PSP measurement data. Graphical abstract: (Figure presented.)

DOI： 10.1007/s12650-024-01000-1

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DOI： 10.1007/s00348-024-03778-0

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DOI： 10.3390/aerospace11040291

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DOI： 10.2514/1.J063448

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DOI： 10.1063/5.0179847

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Publisher：Journal of Visualization  

In this study, robustness of sparse processing particle image velocimetry (SPPIV) of high spatial resolution was improved, and the flow velocity field was measured in real time by improved SPPIV, whereas SPPIV estimates the entire flow field from limited results of sparsely located PIV analysis interrogation windows in real time but suffers from estimating high spatial resolution field because of outliers appearing in the cross correlation analysis. The high-resolution velocity field estimation was conducted by reducing the interrogation window size from 32×32pixel2 to 16×16 and 8×8pixel2, and the robustness of the improved SPPIV was investigated. We developed two methods of high-resolution SPPIV which is capable of real-time flow field measurement. One is robust SPPIV which incorporates with robust Kalman filter and eliminates the outliers, while the other is multiresolution SPPIV which adopts the large interrogation area for real-time measurements and projects it into the high-resolution velocity fields. Robust and multiresolution SPPIV can estimate the velocity fields more accurately than high-resolution standard SPPIV with 16×16 or 8×8pixel2 interrogation windows. The detailed discussion and comparison of those two methods are conducted. In addition, the sensor optimization is compared in the present framework and it shows that the sensors optimized by the Kalman filter index are better than those by the snapshot-to-snapshot index for SPPIV application. Graphical abstract: (Figure presented.).

DOI： 10.1007/s12650-024-01016-7

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DOI： 10.1109/TGRS.2024.3352817

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DOI： 10.1109/JSEN.2024.3388849

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Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers (IEEE)  

In this study, a new greedy sensor selection algorithm with a cost constraint is proposed based on a nondominated-solution-based multi-objective-greedy (NMG) method, and its performance is investigated by comparing it with a previously proposed method. The cost function is simultaneously considered with the D-optimality criterion, and the sensor set is selected based on the idea of the nondominated solution. Although a multi-objective optimization method for sensor selection with a cost constraint was previously considered based on the linear combination of the objectives, it requires a hyperparameter that determines the balance between the actual objective and the cost of the optimization. On the other hand, the proposed algorithm can obtain the Pareto solution without tuning the balance between the actual objective and the cost. We demonstrate the effectiveness of the proposed algorithm on the three different real datasets that are related to the sea surface temperature field, the flowfield around an airfoil, and the combustion field in a rocket chamber. A binary cost function is virtually imposed for each potential sensor location, and a sensor selection problem with a cost constraint is simulated. The results of the numerical experiments demonstrated that the NMG method could field a Pareto solution, and the objective values of almost all the sensor sets at a certain cost selected by the proposed method are superior to those selected by the previous method.

DOI： 10.1109/JSEN.2023.3328005

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Publishing type：Research paper (scientific journal)   Publisher：Oxford University Press (OUP)  

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Reconstruction of the distribution of ground motion due to an earthquake is one of the key technologies for the prediction of seismic damage to infrastructure. Particularly, the immediate reconstruction of the spatially continuous wavefield is valuable for decision-making of disaster response decisions in the initial phase. For a fast and accurate reconstruction, utilization of prior information is essential. In fluid mechanics, full-state recovery, which recovers the full state from sparse observation using a data-driven model reduced-order model, is actively used. In this study, the framework developed in the field of fluid mechanics is applied to seismic wavefield reconstruction. A seismic wavefield reconstruction framework based on compressed sensing using the data-driven reduced-order model (ROM) is proposed and its characteristics are investigated through numerical experiments. The data-driven ROM is generated from the data set of the wavefield using the singular value decomposition. The spatially continuous seismic wavefield is reconstructed from the sparse and discrete observation and the data-driven ROM. The observation sites used for reconstruction are effectively selected by the sensor optimization method for linear inverse problems based on a greedy algorithm. The proposed framework was applied to simulation data of theoretical waveform with the subsurface structure of the horizontally stratified three layers. The validity of the proposed method was confirmed by the reconstruction based on the noise-free observation. Since the ROM of the wavefield is used as prior information, the reconstruction error is reduced to an approximately lower error bound of the present framework, even though the number of sensors used for reconstruction is limited and randomly selected. In addition, the reconstruction error obtained by the proposed framework is much smaller than that obtained by the Gaussian process regression. For the numerical experiment with noise-contaminated observation, the reconstructed wavefield is degraded due to the observation noise, but the reconstruction error obtained by the present framework with all available observation sites is close to a lower error bound, even though the reconstructed wavefield using the Gaussian process regression is fully collapsed. Although the reconstruction error is larger than that obtained using all observation sites, the number of observation sites used for reconstruction can be reduced while minimizing the deterioration and scatter of the reconstructed data by combining it with the sensor optimization method. Hence, a better and more stable reconstruction of the wavefield than randomly selected observation sites can be realized, even if the reconstruction is carried out with a smaller number of observations with observation noise, by combining it with the sensor optimization method.

DOI： 10.1093/gji/ggac443

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Other Link： https://academic.oup.com/gji/article-pdf/233/1/33/47386322/ggac443.pdf

Publishing type：Research paper (scientific journal)  

In this study, strut support effects on a slanted cylinder afterbody were investigated by a magnetic suspension and balance system installed in a wind tunnel. The results showed that the aerodynamic characteristics with respect to the critical Reynolds number where the flow field and aerodynamic forces change significantly. In the cases with the dummy strut, the range of the critical Reynolds number decreases, and the variation depends on the location of the strut. The size of the recirculation region and the separation bubble on the wake center plane also changes depending on the location of the strut. Moreover, additional weak vortices are observed to be formed behind the strut, which changes the wake structure. That change affects the variation of the vortex core wandering, and is a factor in the power spectral density peaks observed in previous studies. It is suggested that the strut support strongly interferes with the flow around the test model and should be carefully considered.

DOI： 10.1016/j.expthermflusci.2023.110952

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This study numerically investigated the mechanisms of separation control using a synthetic jet (SJ) and plasma actuator (PA) around an NACA0015 airfoil at the chord Reynolds number of 63,000. Both SJ and PA were installed on the leading edge with the same order of input momentum ((Formula presented.) – (Formula presented.)) and the same actuation frequencies in (Formula presented.) –30. The momentum coefficient (Formula presented.) is defined as the normalized momentum introduced from the SJ or the PA, and (Formula presented.) stands for the actuation frequency normalized by the chord length and uniform velocity. A number of large-eddy simulations (LES) were conducted for the SJ and the PA, and the mechanisms were clarified in terms of the exchange of chordwise momentum with Reynolds shear stress and coherent vortex structures. First, four main differences in the induced flows of the SJ and the PA were clarified as follows: (A) wall-tangential velocity; (B) three-dimensional flow structures; (C) spatial locality; and (D) temporal fluctuation. Then, a common feature of flow control by the SJ and the PA was revealed: a lift-to-drag ratio was found to be better recovered in (Formula presented.) –20 than in other frequencies. Although there were differences in the induced flows, the phase decomposition of the flow fields identified common mechanisms that the turbulent component of the Reynolds shear stress mainly contributes to the exchange of the chordwise (streamwise) momentum; and the turbulent vortices are convected over the airfoil surface by the coherent spanwise vortices in the frequency of (Formula presented.).

DOI： 10.3390/act12080322

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In this study, fast-response polymer-ceramic pressure-sensitive paints (PC-PSP) were developed and evaluated for pressure measurement under low-pressure conditions. The PC-PSP using poly[1-trimethylsilyl)-1-propyne] (poly(TMSP)), which has high oxygen permeability under low-pressure conditions, was developed in this study. The static and dynamic characteristics of the developed poly(TMSP)-based PC-PSP were evaluated in comparison with those of conventional poly(isobutyl methacrylate) (poly(IBM)) binder and ruthenium-complex-based PC-PSPs, which have been used for pressure measurements under atmospheric pressure conditions. The particle mass content of titanium dioxide of PC-PSPs with poly(TMSP) was changed from 90 wt% to 98 wt% to increase the frequency response. The critical pigment volume concentration, so called CPVC, of the PC-PSP with poly(TMSP) and hydrophobic particles and hydrophilic particles were 95-98 wt% and 90-95 wt%, respectively. The PC-PSP using hydrophilic particles with poly(TMSP) and a particle mass content of 98 wt% could provide a cut-off frequency of approximately 4.5 kHz and a high local Stern-Volmer coefficient of 0.5 at low pressure of 2 kPa.

DOI： 10.1088/1361-6501/acc5a0

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Sound-pressure level of acoustic waves from the flow around a 1/8-reduced-scale simplified train model and pressure-fluctuation distribution of the bottom surface of the bogie were measured simultaneously by a microphone and a pressure-sensitive paint (PSP). A high-pressure-fluctuation area was observed on the upstream side on the bogie bottom surface at the peak sound frequency. The phase distribution of the peak frequency of the PSP data was observed to be uniform in the spanwise direction and delayed in the downstream direction. This result indicates that propagation speed of peak surface pressure fluctuation was 66 % of the freestream wind velocity. Thus, the measured peak sound frequency was found to be the same as the theoretical cavity peak frequency given by the Rossiter equation with that propagation speed as a vortex convection velocity. Therefore, the peak sound is concluded to be generated from acoustic feedback in the cavity, which is the gap between the upstream cavity edge and the bogie. Moreover, the difference between the measured and correlated peak sound levels of the bottom surface of the bogie, was no more than 3 dB, where the correlated sound level was calculated by using the Lighthill-Curle equation with coherent output power data.

DOI： 10.1016/j.expthermflusci.2023.110885

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The extended phase-consistent dynamic mode decomposition (DMD) method, which reconstructs density fields from density gradient fields in multiple directions, was developed and applied to schlieren images in the low-density wind tunnel tests. Schlieren images were acquired in the Re = 3000, 10,000, and M = 0.15, 0.50 flows around a triangular airfoil, and the density gradient fields were calculated from the calibration of the optical system. The proposed density field reconstruction method adopts the extended phase-consistent DMD principle for the estimation of the DMD modes of the density field. The density field was reconstructed with good accuracy in a numerical simulation for comparison, and the density fluctuation region caused by vortex shedding around a triangular airfoil was visualized by the experimental data.

DOI： 10.1007/s00348-023-03668-x

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：MDPI  

Optimization approaches that determine sensitive sensor nodes in a large-scale, linear time-invariant, and discrete-time dynamical system are examined under the assumption of independent and identically distributed measurement noise. This study offers two novel selection algorithms, namely an approximate convex relaxation method with the Newton method and a gradient greedy method, and confirms the performance of the selection methods, including a convex relaxation method with semidefinite programming (SDP) and a pure greedy optimization method proposed in the previous studies. The matrix determinant of the observability Gramian was employed for the evaluations of the sensor subsets, while its gradient and Hessian were derived for the proposed methods. In the demonstration using numerical and real-world examples, the proposed approximate greedy method showed superiority in the run time when the sensor numbers were roughly the same as the dimensions of the latent system. The relaxation method with SDP is confirmed to be the most reasonable approach for a system with randomly generated matrices of higher dimensions. However, the degradation of the optimization results was also confirmed in the case of real-world datasets, while the pure greedy selection obtained the most stable optimization results.

DOI： 10.3390/s23135961

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Flapping flight of animals has captured the interest of researchers due to their impressive flight capabilities across diverse environments including mountains, oceans, forests, and urban areas. Despite the significant progress made in understanding flapping flight, high-altitude flight as showcased by many migrating animals remains underexplored. At high-altitudes, air density is low, and it is challenging to produce lift. Here we demonstrate a first lift-off of a flapping wing robot in a low-density environment through wing size and motion scaling. Force measurements showed that the lift remained high at 0.14 N despite a 66% reduction of air density from the sea-level condition. The flapping amplitude increased from 148 to 233 degrees, while the pitch amplitude remained nearly constant at 38.2 degrees. The combined effect is that the flapping-wing robot benefited from the angle of attack that is characteristic of flying animals. Our results suggest that it is not a simple increase in the flapping frequency, but a coordinated increase in the wing size and reduction in flapping frequency enables the flight in lower density condition. The key mechanism is to preserve the passive rotations due to wing deformation, confirmed by a bioinspired scaling relationship. Our results highlight the feasibility of flight under a low-density, high-altitude environment due to leveraging unsteady aerodynamic mechanisms unique to flapping wings. We anticipate our experimental demonstration to be a starting point for more sophisticated flapping wing models and robots for autonomous multi-altitude sensing. Furthermore, it is a preliminary step towards flapping wing flight in the ultra-low density Martian atmosphere.

DOI： 10.1038/s41598-023-36174-5

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The present study proposes a reservoir computing reduced-order model (RCROM) of the post-stall flow around the National Advisory Committee for Aeronautics 0015 airfoil based on the time series velocity field, and the estimation accuracy of the RCROM is evaluated compared to that of a linear reduced-order model (LROM). The data were experimentally obtained by particle image velocimetry at a chord Reynolds number of 6.4 × 104 and an angle of attack of 18°. The low-dimensional description of the velocity field can be obtained by decomposing the velocity field with a proper orthogonal decomposition (POD) technique and by employing the leading POD mode coefficients as temporal variables of the data instead of the velocity field. Reservoir computing (RC) is adopted as a nonlinear function that predicts several steps ahead of the leading POD mode coefficients. The hyperparameters of RC are tuned by Bayesian optimization, and the optimized RCROM outperforms the LROM in terms of estimation accuracy. The estimation accuracy of the RCROM can be investigated under different numbers of the predicted dominant POD modes and prediction step conditions. As a result, the RCROM shows higher estimation accuracy than the LROM.

DOI： 10.1063/5.0150947

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This paper presents the experimental results of separation and reattachment transient flow processes over a NACA0015 airfoil wing when using a plasma actuator for flow control. In addition, it addresses the flow behavior in the transient processes when the flow control device is activated or deactivated, providing insights for future feedback-based active flow control. This approach offers the benefit of enhanced aerodynamic capabilities. The experiments were conducted at a Reynolds number of 66,000 and an angle of attack of 13 degrees for leading-edge separation without control. The plasma actuator was installed on the leading edge of the wing, with a voltage of 8 kV, base frequency of 30 kHz, and burst frequencies ranging from 100 Hz to 600 Hz. Particle image velocimetry was employed for the flow field velocity measurements, and surface pressure data were obtained using eight piezoelectric pressure sensors. The first proper orthogonal decomposition mode of the transient flow velocity field is the focus of this paper and the flow behavior is quantitatively discussed. The results reveal details about the flow separation and reattachment transient processes such as their flow structures and their evolution over time. It is concluded that the time asymmetry between the separation and reattachment transient processes could be leveraged for further improvements to the efficiency of actuators.

DOI： 10.3390/act12060218

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This study investigates a mechanism for controlling separated flows around an airfoil using a synthetic jet (SJ). A large-eddy simulation (LES) was performed for a leading-edge separation flow around an airfoil at the chord Reynolds number of 63 000 and the angle of attack of 12°. The present LES resolves a turbulent structure inside a deforming SJ cavity with a deforming grid. An optimal actuation-frequency band is identified between the normalized frequencies of F + = 6.0 and 20, which suppresses the separation and drastically improves the lift-to-drag ratio. In the controlled flows, the laminar separation bubble near the leading edge periodically releases multiple spanwise-uniform vortex structures, which diffuse and merge to generate a single coherent vortex in the period of F+. Such a coherent vortex plays a significant role in exchanging a chordwise momentum between a near-wall surface and the freestream away from the wall. It also entrains smaller turbulent vortices and eventually enhances the turbulent component of the Reynolds stress throughout the suction surface. Linear stability theory (LST) was subsequently compared with the LES result, which clarifies the applicability of the LST to the controlled flows. In the optimal F+ regime, both linear and nonlinear modes are excited in a well-balanced manner, where the first mode is associated with the Kelvin-Helmholtz instability and contributes to a quick and smooth turbulent transition, while the second mode shows a frequency lower than that of the linear mode and encourages a formation of the coherent vortex structure that eventually entrains smaller turbulent vortices.

DOI： 10.1063/5.0148943

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The randomized group-greedy (RGG) method and its customized method for large-scale sensor selection problems are proposed. The randomized greedy sensor selection algorithm is applied straightforwardly to the group-greedy (GG) method, and a customized method is also considered. In the customized method, a part of the compressed sensor candidates is selected using the common greedy method or other low-cost methods. This strategy compensates for the deterioration of the solution due to compressed sensor candidates. The proposed methods are implemented based on the D-and E-optimal design of experiments, and numerical experiments are conducted using randomly generated sensor candidate matrices with potential sensor locations of 10000-1000000. The proposed method can provide better optimization results than those obtained by the original GG method when a similar computational cost is spent as for the original GG method. This is because the group size for the GG method can be increased as a result of the compressed sensor candidates by the randomized algorithm. Similar results were also obtained in the real dataset. The proposed method is effective for the E-optimality criterion, in which the objective function that the optimization by the common greedy method is difficult due to the absence of submodularity of the objective function. The idea of the present method can improve the performance of all optimizations using a greedy algorithm.

DOI： 10.1109/JSEN.2023.3258223

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A pressure distribution inside a two-dimensional rectangular cavity with a ratio L/ D= 5.0 of the cavity length L to the depth D over a supersonic flow was obtained using an improved fast anodized-aluminum pressure-sensitive paint (AA-PSP). The freestream Mach number was M∞= 1.9 and the frequencies of the dominant cavity modal phenomena, called “Rossiter” modes, were on the order of 1–10 kHz. The fast AA-PSP using a free-based porphyrin luminophore and a high-frequency-repetition double-pulsed laser were applied, and instantaneous pressure fields were captured. This measurement system has a 60 kHz sampling rate and is suitable for the purpose of a direct measurement of the Rossiter mode phenomena. The result of the power spectral density (PSD) of the PSP measurement showed that this measurement system is capable of observing phenomena of approximately 18 kHz. Time-resolved pressure transducer measurement was also employed, and PSD obtained by the PSP measurement was validated. The frequency spectra and the amplitudes of PSD obtained by the PSP measurement were in good agreement with those obtained by the pressure transducer data. A spectral proper orthogonal decomposition (SPOD) analysis was also conducted, and the symmetric and asymmetric spatial modes were extracted at each peak frequency. In this analysis, the seventh Rossiter mode phenomena of approximately 21 kHz were visualized. The peak frequencies of symmetric modes were in good agreement with those of the Rossiter mode, and lower than that of asymmetric modes. The energy of the symmetric modes at each peak frequency was several times higher than that of the asymmetric mode. The phase of the SPOD mode revealed the propagation direction of the pressure waves. It showed the existence of the upstream propagation waves and standing waves inside the cavity.

DOI： 10.1007/s00348-023-03648-1

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The ‘big’ seismic data not only acquired by seismometers but also acquired by vibrometers installed in buildings and infrastructure and accelerometers installed in smartphones will be certainly utilized for seismic research in the near future. Since it is impractical to utilize all the seismic big data in terms of the computational cost, methods which can select observation sites depending on the purpose are indispensable. We propose an observation site selection method for the accurate reconstruction of the seismic wavefield by process-driven approaches. The proposed method selects observation sites suitable for accurately estimating physical model parameters such as subsurface structures and source information to be input into a numerical simulation of the seismic wavefield. The seismic wavefield is reconstructed by the numerical simulation using the parameters estimated based on the observed signals at only observation sites selected by the proposed method. The observation site selection in the proposed method is based on the sensitivity of each observation site candidate to the physical model parameters; the matrix corresponding to the sensitivity is constructed by approximately calculating the derivatives based on the simulations, and then, observation sites are selected by evaluating the quantity of the sensitivity matrix based on the D-optimality criterion proposed in the optimal design of experiments. In this study, physical knowledge on the sensitivity to the parameters such as seismic velocity, layer thickness, and hypocentre location was obtained by investigating the characteristics of the sensitivity matrix. Furthermore, the effectiveness of the proposed method was shown by verifying the accuracy of seismic wavefield reconstruction using the observation sites selected by the proposed method.

DOI： 10.1093/gji/ggad165

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An effect of an angle of attack on the aerodynamic characteristics of a freestream-aligned circular cylinder is investigated and discussed. The experiment without support interference was conducted using a magnetic suspension and balance system (MSBS) which can levitate and support a model. A cylindrical model with a fineness ratio of 1.0 was employed in wind tunnel tests. The Reynolds numbers based on the diameter of the model were 3.3×104 and 6.7×104. The range of the angle of attack is from 0 to 15°. Aerodynamic forces and velocity fields were obtained by the MSBS and particle image velocimetry. The results of the time-averaged aerodynamic force and moment coefficients and the time-averaged velocity field illustrate that the flow reattachment occurs at an angle of 9° or more. The flow reattachment changes the aerodynamic characteristics, especially the lift and pitching moment coefficients and the lift force fluctuations.

DOI： 10.1103/PhysRevFluids.8.024701

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DOI： 10.1016/j.sna.2022.114140

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DOI： 10.1016/j.sna.2022.114140

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A new model-position-sensing method for the levitation of models with a low fineness ratio (ratio of the longitudinal length to the diameter) in a magnetic suspension and balance system (MSBS) is proposed. MSBS is an ideal model-support device for wind-tunnel testing, which enables the study of flow fields around blunt bodies without flow disturbances introduced by mechanical support devices, with the aerodynamic forces determined from the magnetic forces using a pre-calibrated relationship. The new method allows wind tunnel experiments without mechanical supports with a low fineness ratio model. This method adopts two line sensors placed parallel to the central axis of the model image and measures the position with a resolution finer than 0.06 mm or deg even for thin model geometries. In addition, measurement errors were reduced by correcting a second-order term in the depth direction of the camera. A low fineness ratio circular cylinder model was levitated following sensor calibration. The model was supported in conditions with and without freestream flow. This position measurement method was also applied to a reentry capsule model. The model was levitated while keeping its position and attitude stabilized near the origin.

DOI： 10.1063/5.0113806

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In this study, an experimental investigation of separation control using a dielectric barrier discharge plasma actuator was performed on an NACA0015 airfoil over a wide range of Reynolds numbers, angles of attack, and nondimensional burst frequencies. The range of the Reynolds number was based on a chord length ranging from 2.52 × 10 (Formula presented.) to 1.008 × 10 (Formula presented.). A plasma actuator was installed at the leading edge and driven by AC voltage. Burst mode (duty-cycle) actuation was applied, with the nondimensional burst frequency ranging between 0.1–30. The control authority was evaluated using the time-averaged distribution of the pressure coefficient (Formula presented.) and the calculated value of the lift coefficient (Formula presented.). The baseline flow fields were classified into three types: (1) leading-edge separation; (2) trailing-edge separation; and (3) the hysteresis between (1) and (2). The results of the actuated cases show that the control trends clearly depend on the differences in the separation conditions. In leading-edge separation, actuation with a burst frequency of approximately (Formula presented.) 0.5 creates a wide negative pressure region on the suction-side surface, leading to an increase in the lift coefficient. In trailing-edge separation, several actuations alter the position of turbulent separation.

DOI： 10.3390/act12010043

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Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers (IEEE)  

Objective functions for sensor selection are investigated in linear time-invariant systems with a large number of sensor candidates. This study compared the performance of sensor sets obtained using three types of D-optimality-based indices as objective functions for sensor selection based on the greedy method. The compared indices are computed based on the snapshot-to-snapshot Fisher information matrix, the observability Gramian and the Kalman filter-based matrix. Both random systems and systems with eigenmodes are considered, indices for selecting the best-performing sensor set for each are identified, as well as computational complexity and corresponding wall clock times. The sensor optimized for each index works best for that index, as expected. We also clarified the trend of the sensor sets selected by the greedy method based on each objective function in terms of the other objective function.

DOI： 10.1109/ACCESS.2023.3291415

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Antioxidants were applied to a dual-luminophore pressure-sensitive paint (PSP), and the effects on photodegradation caused by exposure to excitation light were studied. Three types of antioxidants that are commonly used for the photostability enhancement of polymers were added to a dual-luminophore PSP, and degradation rates and pressure/temperature sensitivities were investigated by coupon-based tests. One-hour-long aging tests were performed in a pressure chamber with a continuous excitation light source under dry air and argon atmospheres at 100 kPa and 20 °C. As a result of the aging tests, a singlet oxygen quencher type antioxidant was found to reduce the degradation rate by 91% when compared with the dual-luminophore PSP without antioxidants. This implies that singlet oxygen has a dominant role in the photodegradation mechanism of the dual-luminophore PSP.

DOI： 10.3390/s22239470

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The present study proposed the framework of the spatiotemporal superresolution measurement based on the sparse regression with dimensionality reduction using the proper orthogonal decomposition (POD). The non-time-resolved particle image velocimetry (PIV) and the time-resolved near-field acoustic measurements using microphones were simultaneously performed for a Mach 1.35 supersonic jet. POD is applied to PIV and microphone data matrices, and the sparse linear regression model of the reduced-order data is calculated using the least absolute shrinkage and selection operator regression. The effects of the hyperparameters of the superresolution measurement were quantitatively evaluated through randomized cross-validation. The superresolved velocity field indicated the smooth convection of the velocity fluctuations associated with the screech tone, while the convection of the large-scale structures at the downstream side was not observed. The proposed framework can reconstruct the unsteady fluctuation with multiple frequency phenomena, although the reconstruction is limited to the phenomena that are associated with the microphone output. Graphical Abstract: [Figure not available: see fulltext.]

DOI： 10.1007/s12650-022-00855-6

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DOI： 10.2514/1.J062054

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Publishing type：Research paper (scientific journal)   Publisher：AIP Publishing  

Unsteady drag, unsteady lift, and movement of one or two moving particles caused by the passage of a planar shock wave are investigated using particle-resolved simulations of viscous flows. The particle motion analysis is carried out based on particle-resolved simulations for one or two particles under a shock Mach number of 1.22 and a particle Reynolds number of 49, and the particle migration and fluid forces are investigated. The unsteady drag, unsteady lift, and particle behavior are investigated for different densities and particle configurations. The time evolution of the unsteady drag and lift is changed by interference by the planar shock wave, Mach stem convergence, and the shock wave reflected from the other particle. These two particles become closer after the shock wave passes than in the initial state under most conditions. Two particles placed in an in-line arrangement approach each other very closely due to the passage of a shock wave. On the other hand, two particles placed in a side-by-side arrangement are only slightly closer to each other after the shock wave passes between them. The pressure waves resulting from Mach stem convergence of the upstream particle and the reflected shock waves from the downstream particle are the main factors responsible for the force in the direction that pushes the particles apart. The wide distance between the two particles attenuates these pressure waves, and the particles reduce their motion away from each other.

DOI： 10.1063/5.0101365

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Authorship：Last author   Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media LLC  

A novel measurement method is developed for a simultaneous measurement of pressure and temperature on an airfoil by sensitive paints. The proposed method requires two sets of measurements: in the first set, the temperature distribution is measured on the entire surface of the airfoil by temperature-sensitive paint (TSP). This temperature field is further utilized to evaluate sparse sensor locations based on the sensor selection methods. For the second set of measurements, TSP was sprayed on sparse points and pressure-sensitive paint (PSP) on the remaining airfoil surface. A full temperature field can be reconstructed using temperature data measured at those sparse locations. The temperature-induced error due to temperature sensitivity of PSP is corrected, and a time-averaged pressure field is compared with the pressure tap data. The proposed method is demonstrated on a flow over a NACA 0015 airfoil. Time-averaged and spanwise averaged pressure agrees very well with pressure sensor data measured simultaneously with PSP giving further confidence in our measurement. The present results also show that the Bayesian estimation and a corresponding sensor selection method overperform the linear least squares estimation and a corresponding sensor selection method, and the Bayesian estimation framework is recommended for the practical sparse sensor for temperature reconstruction.

DOI： 10.1007/s00348-022-03501-x

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Other Link： https://link.springer.com/article/10.1007/s00348-022-03501-x/fulltext.html

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The exact dynamic mode decomposition (DMD) was applied to the nonsequential image dataset obtained by the double-pulsed schlieren measurement of a supersonic impinging jet, and the effect of the dataset length on the obtained spatial modes and estimated frequencies of the aeroacoustic fields was investigated. The Mach number of the jet was 2.0, the Reynolds number based on the diameter of the nozzle exit was 1.0 × 10 6 and the distance between the nozzle exit and the flat plate was four times the nozzle diameter long. The DMD modes extract the characteristic pattern and its frequency that relate to the aeroacoustic fields. The estimated frequencies of DMD modes were compared with the acoustic spectra measured using microphones. The estimated frequency of the DMD mode that has the largest amplitude approximately coincides with that of the highest peak in the acoustic spectra regardless of the dataset length. However, the variation in the estimated frequencies of the high-order DMD modes increases when the dataset length is short. Although the estimated frequencies of the second and third DMD modes did not match the peak frequencies of the acoustic spectra, the estimation accuracy of the frequency of the modes can be improved by recalculating the frequency based on the wavelength of the corresponding spatial mode. The order of the amplitude of DMD modes did not agree with the order of the peak magnitude in the acoustic spectra, except for the first mode. This is because the schlieren method visualizes the density gradient resulting in emphasizing the high-frequency fluctuations. This mismatch was mitigated by correcting the acoustic spectrum considering the first derivative of the acoustic spectrum. Therefore, the verification of the estimation accuracy considering the data characteristics is important when the exact DMD analysis is applied to the noisy experimental data. Graphical abstract: [Figure not available: see fulltext.]

DOI： 10.1007/s12650-022-00836-9

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Publishing type：Research paper (scientific journal)   Publisher：{IOP} Publishing  

In this study, a new method that optimizes a measurement condition in a lifetime-based simultaneous measurement of a pressure-sensitive paint (PSP) and a temperature-sensitive paint (TSP) is proposed for the improvement of the accuracy of the pressure measurement. An optimal gate is selected based on a pressure measurement error when calculating the pressure and the temperature simultaneously from measurement values of a PSP and a TSP. A shot noise of a PSP, a temperature error, and a fluctuation in an emission intensity ratio due to blurring were considered error factors of the PSP measurement. The pressure measurement error propagated from each error source was considered as an evaluation index in an optimization of a measurement condition. We evaluated 17 types of TSP characteristics and selected an optimal TSP and a measurement condition for the PSP measurement. Further, the optimized measurement condition was evaluated in a PSP/TSP simultaneous measurement using a coupon-based test. The optimal measurement condition obtained based on the proposed method and an empirical selection method were compared by a PSP/TSP simultaneous measurement using a coupon-based test. A small-pressure measurement error, i.e. high pressure-measurement accuracy, was realized by the proposed method in the simultaneous lifetime-based method of a PSP and a TSP. In addition to the analyses above, the blurring effects were found to be minor and briefly summarized in appendices.

DOI： 10.1088/1361-6501/ac769b

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We developed and evaluated an anodized-aluminum pressure-sensitive paint (AA-PSP) with new formulations of free-base porphyrin, H2TCPP, as an optical unsteady pressure sensor. The luminophore H2TCPP has quite a short fluorescent lifetime (2.4 ns on the condition of the AA-PSP). The fluorescence spectroscopy result shows that the excitation wavelength of H2TCPP corresponds to violet-colored (425 nm) and green-colored (longer than 520 nm) lights. The pressure sensitivity is sufficiently high for the pressure sensor (0.33-0.51%/kPa) and the temperature sensitivity is very low (0.07-1.46%/K). The photodegradation of the AA-PSPs is not severe in both excitation light sources of the green LED and the Nd:YAG laser. The resonance tube experiment result shows the cut-off frequency of the AA-PSPs is over 9.0 kHz, and the results of the shock tube experiment show the 10 µs order time constant of the normal shock wave.

DOI： 10.3390/s22176401

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Publishing type：Research paper (scientific journal)   Publisher：American Institute of Aeronautics and Astronautics ({AIAA})  

Distributions of the pressure and mass transfer coefficient on rotating blades under low-pressure (low-Reynoldsnumber) conditions were visualized, whereas the latter is closely related to the skin-friction distribution. Two types of optical measurement techniques, lifetime-based pressure-sensitive paint (PSP) measurements and sublimation visualization, were implemented for the experiment inside a low-pressure chamber. For the lifetime-based PSP measurement, different types of PSP were compared, and the one most suitable in low-pressure applications was selected. In addition, the gate time setting for the low-pressure condition was determined. For the sublimation method, naphthalene was selected as the sublimation surface based on previous studies. The rotating blade test model was a 0.3-m-diam rotor system with two rectangular blades with an aspect ratio of two. The experiments were carried out at a rotational speed of 2400 rpm and at an ambient pressure of 10 kPa. The three-fourth-span Reynolds number was 9000. The pitch angle of the blades was set to 0–20 deg. Both methods successfully illustrated clear images of the distribution of pressure and mass transfer coefficients on the upper surface of the blade, and the measurement in the low-pressure environment was successful.

DOI： 10.2514/1.J061638

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Publishing type：Research paper (scientific journal)   Publisher：American Physical Society ({APS})  

DOI： 10.1103/PhysRevFluids.7.084605

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Other Link： http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevFluids.7.084605/fulltext

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Abstract: Signal processing methods that remove noise due to atmospheric fluctuation and image sensors and extract fluid phenomena from schlieren images obtained in the low-density wind tunnel test were developed together with the highly sensitive schlieren measurement setup. Time-series schlieren images of the flow around a triangular airfoil were analyzed, and the effectiveness of noise reduction methods using the randomized singular value decomposition and band-pass filtering using the fast Fourier transform (FFT) and the inverse FFT were investigated. The proposed method succeeded in removing noise by taking advantage of the frequency difference between the noise and fluid phenomena, and the fluid phenomena around the airfoil were clearly visualized at a Reynolds number of 3000 and a Mach number of 0.15. Graphical abstract: [Figure not available: see fulltext.]

DOI： 10.1007/s12650-022-00829-8

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In this study, a cylinder with a slanted afterbody was magnetically levitated without support structures by means of a magnetic suspension and balance system (MSBS) with six-degrees-of-freedom (6-DOF) control. The levitation stability during the 6-DOF levitation was first confirmed and accurate to less than 8.9 μm and 18.6 mdeg at 15 m/s freestream. The results show that the model can be rigidly levitated with the 6-DOF control. Then, aerodynamic force on the levitated model was measured and its wake was visualized. Hystereses in aerodynamics and wake structure were observed around the critical Reynolds number between 3.3 × 10 4 and 3.6 × 10 4. Graphical abstract: [Figure not available: see fulltext.]

DOI： 10.1007/s00348-022-03470-1

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Publishing type：Research paper (scientific journal)   Publisher：American Institute of Aeronautics and Astronautics ({AIAA})  

Martian conditions present various challenges when designing rotorcraft. Specifically, the thin atmosphere and low sound speed require Martian rotor blades to operate in a low-Reynolds-number (1000–10,000) compressible regime, for which conventional airfoils are not designed. Here, we use PyFR to undertake high-order direct numerical simulations (DNS) of flow over a triangular airfoil at a Mach number of 0.15 and Reynolds number of 3000. Initially, spanwise periodic DNS are undertaken. Extending the domain-span-to-chord ratio from 0.3 to 0.6 leads to better agreement with wind-tunnel data at higher angles of attack, when the flow is separated. This is because smaller domain spans artificially suppress three-dimensional breakdown of coherent structures above the suction surface of the airfoil. Subsequently, full-span DNS in a virtual wind tunnel are undertaken, including all wind-tunnel walls. These capture blockage and wall boundary-layer effects, leading to better agreement with wind-tunnel data for all angles of attack compared to spanwise periodic DNS. The results are important in terms of understanding discrepancies between previous spanwise periodic DNS and wind-tunnel data. They also demonstrate the utility of high-order DNS as a tool for accurately resolving flow over triangular airfoils under Martian conditions.

DOI： 10.2514/1.J061454

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In the present study, characteristics of a shear layer around a freestream-aligned circular cylinder and the relationship between the shear layer motion and the aerodynamic force were investigated under supportless condition. A 0.3-m magnetic suspension and balance system was employed, and experiments were conducted without a mechanical supporting device. Velocity fields were measured using particle image velocimetry with a sufficient temporal and spatial resolution, and high-frequency velocity fluctuations caused by small Kelvin-Helmholtz (KH) vortices were captured. The power spectral densities of velocity fluctuations represent phenomena such as KH vortex convection, vortex pairing, and convection of multiple vortices. Furthermore, fluctuations of the shear layer position were investigated. The results illustrate that the dominant frequency of the shear layer position is lower than the frequency of the velocity, and it shows good agreement with the characteristic frequency of lift force fluctuations. The present results together with the report in the previous study illustrate that the pressure fluctuations are considered to drive both fluctuations of the shear layer position and lateral aerodynamic force.

DOI： 10.1063/5.0091044

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Abstract: In this study, the optical flow method for the skin-friction-stress estimation from the unsteady oil film distribution was reformulated based on the variational method, and the validity of the proposed method was verified in comparison with the conventional method. The regularization is proposed to be added directly to the skin-friction-stress field in the proposed method while the regularization is added to the amount of oil movement in the conventional method. As a result, the smoothness of the skin-friction-stress field can be controlled by adjusting the regularization parameter in the proposed method whereas it was difficult in the conventional method. The performance of the proposed method was evaluated to be superior to the previous method through the numerical and experimental data. Graphic Abstract: [Figure not available: see fulltext.]

DOI： 10.1007/s12650-021-00794-8

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The objective of this study is to obtain accurate flow field analysis results in a short computational time by using data assimilation, which increases the accuracy of Reynolds averaged Navier-Stokes (RANS) simulations with low grid resolution. The large-eddy simulation (LES) results are assimilated into RANS simulations. In those simulations, the turbulence-model parameters are optimised by an ensemble Kalman filter with a proposed method for adaptive hyperparameter optimisation. The target of calculations is the flow field around a square cylinder of the Reynolds number of approximately (Formula presented.). Only the surface pressure of the square cylinder is used as an observation variable. For this shape, the assimilated RANS flow field is similar to that given by the LES analysis, and the drag coefficient reproducibility is improved by (Formula presented.). The turbulence-model parameters are also used in the analyses of different cross-sectional shape and are found to improve the reproducibility of the flow field.

DOI： 10.1080/10618562.2022.2085257

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In this study, the dynamical behavior of the vortex pair produced in the wake of a cylinder with a slanted afterbody, relevant to cargo aircraft applications, is examined experimentally through time-resolved particle image velocimetry (TR-PIV) and fast-response pressure sensitive paint (PSP). The vortex wandering phenomenon, characterstic of this wake, is measured with state of the art, time-resolved diagnostics at Reynolds numbers ranging from ReD=25000 to 750000 and a slant angle of φ=45°. Spectral proper orthogonal decomposition reveals a consistent and narrow normalized frequency band (0.56<StD<0.64) with significant energetic peaks in both surface pressure and near-field vortex velocity fields across all Reynolds numbers examined. Pressure sensitive paint measurements at free stream velocities ranging from 15 m/s (ReD=150000) to 75 m/s (ReD=750000) reveal a strong unsteady signature of the flow structures that are related to the vortex wandering at these normalized frequencies. These structures consist of traveling pressure waves at the surface that excite the helical mode of the vortex core. These pressure patterns could be detected at free-stream velocities as low as 15 m/s, where the pressure fluctuations are of the order of 20 Pa.

DOI： 10.1103/PhysRevFluids.7.024701

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We propose a markerless image alignment method for pressure-sensitive paint measurement data replacing the time-consuming conventional alignment method in which the black markers are placed on the model and are detected manually. In the proposed method, feature points are detected by a boundary detection method, in which the PSP boundary is detected using the Moore-Neighbor tracing algorithm. The performance of the proposed method is compared with the conventional method based on black markers, the difference of Gaussian (DoG) detector, and the Hessian corner detector. The results by the proposed method and the DoG detector are equivalent to each other. On the other hand, the performances of the image alignment using the black marker and the Hessian corner detector are slightly worse compared with the DoG and the proposed method. The computational cost of the proposed method is half of that of the DoG method. The proposed method is a promising for the image alignment in the PSP application in the viewpoint of the alignment precision and computational cost.

DOI： 10.3390/s22020453

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Publishing type：Research paper (scientific journal)  

Abstract: A high-spatial resolution measurement system by the background-oriented schlieren (BOS) method is proposed in this note. The single-pixel correlation method is applied for the image pairs obtained by the BOS method, and the displacement is calculated. The backgrounds are projected by a high-speed projector and changed at 900 Hz for calculation of the pixel-to-pixel correlation. This method is compared with the conventional method which utilizes the constant background and calculates the displacement by the conventional spatial correlation. The results show that the proposed system can obtain the displacement distribution in the single-pixel resolution which is eight times as high resolution as the conventional one. Graphical abstract: [Figure not available: see fulltext.].

DOI： 10.1007/s00348-021-03373-7

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Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers (IEEE)  

DOI： 10.1109/TSP.2022.3212150

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Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers (IEEE)  

Optimization of sensor selection has been studied to monitor complex and large-scale systems with data-driven linear reduced-order modeling. An algorithm for greedy sensor selection is presented under the assumption of correlated noise in the sensor signals. A noise model is given using truncated modes in reduced-order modeling, and sensor positions that are optimal for generalized least squares estimation are selected. The determinant of the covariance matrix of the estimation error is minimized by efficient one-rank computations in both underdetermined and overdetermined problems. The present study also reveals that the objective function with correlated noise is neither submodular nor supermodular. Several numerical experiments are conducted using randomly generated data and real-world data. The results show the effectiveness of the selection algorithm in terms of accuracy in the estimation of the states of large-dimensional measurement data.

DOI： 10.1109/ACCESS.2022.3194250

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Publishing type：Research paper (scientific journal)   Publisher：Japan Society for Aeronautical and Space Sciences  

DOI： 10.2322/tjsass.65.221

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Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers (IEEE)  

In the present study, we organize the existing sparsity-promoting dynamic mode decomposition (DMDsp) in terms of noise robustness, propose faster optimization algorithm for DMDsp, and evaluate its characteristics. Two kinds of DMDsp, namely system-based DMDsp (sDMDsp) and observation-based DMDsp (oDMDsp), combined with three kinds of optimization algorithm, namely the fast iterative shrinkage thresholding algorithm (FISTA), the alternating direction method of multipliers (ADMM), and a greedy algorithm, are investigated. For both sDMDsp and oDMDsp, FISTA yields the shortest processing time. The processing time for sDMDsp with FISTA is shorter than that for oDMDsp with FISTA. The original data reconstruction errors for sDMDsp and oDMDsp are similar among the three optimization algorithms. The noise robustness for sDMDsp and oDMDsp is evaluated. sDMDsp and oDMDsp have similar robustness to observation noise, except for a system with large system and observation noise, for which oDMDsp outperforms sDMDsp.

DOI： 10.1109/ACCESS.2022.3193157

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Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers (IEEE)  

DOI： 10.1109/TSP.2022.3224643

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Publishing type：Research paper (scientific journal)   Publisher：Japan Society for Aeronautical and Space Sciences  

In the present study, the visualization of compressible flow around a particle/particles including wake vortices and drag estimation were conducted through shock-particle interaction experiments. An experimental method that can investigate flow over isolated and clustered particle(s) (with a minimum diameter of 0.3 mm) interacting with a planar shock was established. For flow visualization, the Mach number (M) and Reynolds number (Re) based on the relative velocity between the particle and the quantities behind the planar shock wave were 0.46 ¯ M ¯ 1.24 and 3500 ¯ Re ¯ 9800, respectively. The present measurement system succeeded in visualizing flow structures not only for shock waves, but also wake structures formed behind the particle(s) under subsonic and transonic conditions, and the Mach number effect was provided. The mean drag coefficient was estimated from the time-position data of the particle at 3100 ¯ Re ¯ 9800 and M = 0.46. The estimated drag coefficient was close to that of the value estimated by the drag model and previous experiments. The flowfield around clustered particles was visualized and its breakdown process was observed. The particle cluster dispersed due to aerodynamic interference. Particularly, the particles located on the upper side of the particle cluster moved upward against the gravitational force.

DOI： 10.2322/tjsass.65.185

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Abstract: Three-dimensional density fields of the twin jets were numerically and experimentally investigated. The present study focused on the comparison of the density distribution for the twin jets. The results obtained by the computational fluid dynamics (CFD) and three-dimensional background-oriented schlieren (3D-BOS) indicate that the periodic density fluctuation appears in the potential core each nozzle, and the flow structure of the twin jets is quite similar. The distribution of the normalized density value at the nozzle centerline agrees well with CFD and 3D-BOS. The density value of the shear layer between the nozzles increases as the interaction of the twin jets occurs. The trend of increasing and decreasing the interference between the nozzles was almost the same as each other. On the other hand, the position where the interaction of the twin jets starts and the growth rate of interaction were different. This is probably due to the effect of the laminar-to-turbulent transition occurred in the results of CFD. This result indicates that the laminar-to-turbulent transition can be estimated from the velocity fields obtained by CFD and particle image velocimetry (PIV). Graphic Abstract: [Figure not available: see fulltext.]

DOI： 10.1007/s12650-021-00765-z

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Publishing type：Research paper (scientific journal)   Publisher：American Institute of Aeronautics and Astronautics ({AIAA})  

The wind tunnel experiments of separation flow control by a nanosecond-pulse-driven dielectric-barrier-discharge plasma actuator (ns-DBDPA) for three airfoil shapes of NASA-Common-Research-Model (NASA-CRM), Gottingen387, and NACA0015 airfoils were conducted and the results were compared. Aerodynamic forces, surface pressures, and particle image velocimetry images were obtained in the experiments. The results of the aerodynamic force and surface pressure measurements showed that the characteristics of the flow separation control by ns-DBDPA are clearly different depending on the airfoil shape, and particularly the type of stall. NACA0015 exhibited leading-edge stall under the conditions investigated, and ns-DBDPA achieved flow control by producing vortices. NASA-CRM exhibited thin-airfoil stall, and ns-DBDPA increased the maximum lift of the lift curve even after the stall. On the other hand, Gottingen387, which exhibited trailing-edge stall, achieved almost no flow control at any actuator position, actuation frequency, or voltage amplitude. The flow control mechanisms for different types of airfoils are discussed based on the particle image velocimetry measurement results.

DOI： 10.2514/1.J060486

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Authorship：Lead author   Publishing type：Research paper (scientific journal)   Publisher：{IOP} Publishing  

The methods that measure the turbulence statistics distribution of high-speed flow with high spatial resolution using particle-image-velocimetry images are proposed, and their performances are verified. Two methods are proposed, and the problems caused by blurring the particle shape due to the high-speed movements are resolved. While the conventional method approximates the unique particle shapes with a circular distribution for image pairs, the first proposed method approximates the different particle shapes with an ellipse for first and second images of pairs and reduces the effects of the blur in the flow direction of the particles. Meanwhile, the second proposed method treats the general particle shapes such as the blur of temporally changing laser intensity and adopts the deconvolution analysis using a Fourier transform. Synthetic particle images were created and a supersonic jet test was performed, and the proposed methods were evaluated to be superior to the previous method for the estimation of turbulent fluctuation using those data.

DOI： 10.1088/1361-6501/ac27e9

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ACADEMIC PRESS LTD- ELSEVIER SCIENCE LTD  

A greedy algorithm is proposed for sparse-sensor selection in reduced-order sensing that contains correlated noise in measurement. The sensor selection is carried out by maximizing the determinant of the Fisher information matrix in a Bayesian estimation operator. The Bayesian estimation with a covariance matrix of the measurement noise and a prior probability distribution of estimating parameters, which are given by the modal decomposition of high dimensional data, robustly works even in the presence of the correlated noise. After computational efficiency of the algorithm is improved by a low-rank approximation of the noise covariance matrix, the proposed algorithms are applied to various problems. The proposed method yields more accurate reconstruction than the previously presented method with the determinant-based greedy algorithm, with reasonable increase in computational time.

DOI： 10.1016/j.ymssp.2021.107619

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

In this study, the effect of the burst ratio (BR) on flow separation control using an alternating-current dielectric barrier discharge plasma actuator (DBDPA) was investigated. The effects of BR on the lift force on the NACA0015 airfoil were quantitatively evaluated by force measurements, and those on the flow field were measured by particle image velocimetry (PIV) and Schlieren visualisation. The force measurements showed that the recovery of the lift force decreased as BR increased, indicating that the flow separation control effect of the DBDPA decreased as BR increased. The PIV measurements showed that the temporal variation in vorticity was smaller at BR = 80% than at BR = 20% and that the vorticity did not change periodically at BR = 100%. Schlieren visualisation showed that DBDPA-induced flow produced a density gradient near the DBDPA electrode; the gradient was periodically observed and was found to be proportional to BR. In contrast, the density gradient in the separated shear layer varied at the end of each burst waveform, and the degree of variation decreased as BR increased. In addition, the root mean square value of the Schlieren signal intensity in the separated shear layer decreased with increasing BR. This result suggests that not only the strength and frequency of the disturbance input of the DBDPA but also the switching timing of the disturbance input are important for producing vorticity and thereby ensuring flow control.

DOI： 10.1088/1361-6463/ac00ee

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Institute of Aeronautics and Astronautics ({AIAA})  

A comprehensive review of all relevant experimental data was completed, including recent data for the drag coefficient for a sphere in supersonic and hypersonic flows. The primary characterization parameter included the relative Mach, Knudsen, and Reynolds numbers based on the relative velocity, the sphere diameter, and other parameters. This review of data showed that the previously proposed nexus at a Reynolds number below 45 was not strictly met, and it instead included a weak transonic bump, which was identified numerically for the first time with the present simulations. New continuum-gas and rarefied-gas simulations were conducted and were combined with the expanded experimental dataset to improve the quantitative description of the drag coefficient in this region. The results indicated that a quasi nexus bridges the rarefaction regime and the compressible flow regimes. The comprehensive dataset was then used to develop new empirical models for the drag coefficient that showed improved robustness and accuracy as compared to previous models. These models are limited by the critical Reynolds number associated with boundary-layer transition on the sphere, which was found to increase substantially with the sphere Mach number.

DOI： 10.2514/1.J060153

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Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers ({IEEE})  

The present study proposes a sensor selection method based on the proximal splitting algorithm and the A-optimal design of experiment using the alternating direction method of multipliers (ADMM) algorithm. The performance of the proposed method was evaluated with a random sensor problem and compared with previously proposed methods, such as the greedy and convex relaxation methods. The performance of the proposed method is better than the existing greedy and convex relaxation methods in terms of the A-optimality criterion. Although, the proposed method requires a longer computational time than the greedy method, it is quite shorter than that of convex relaxation method in large-scale problems. Then the proposed method was applied to the data-driven sparse-sensor-selection problem. The dataset adopted was the National Oceanic and Atmospheric Administration optimum interpolation sea surface temperature dataset. At a number of sensors larger than that of the latent variables, the proposed method showed similar and better performance compared with previously proposed methods in terms of.

DOI： 10.1109/JSEN.2021.3073978

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We propose a noise reduction method for unsteady pressure-sensitive paint (PSP) data based on modal expansion, the coefficients of which are determined from time-series data at optimally placed points. In this study, the proper orthogonal decomposition (POD) mode calculated from the time-series PSP data is used as a modal basis. Based on the POD modes, the points that effectively represent the features of the pressure distribution are optimally placed by the sensor optimization technique. Then, the time-dependent coefficient vector of the POD modes is determined by minimizing the difference between the time-series pressure data and the reconstructed pressure at the optimal points. Here, the coefficient vector is assumed to be a sparse vector. The advantage of the proposed method is a self-contained method, while existing methods use other data, such as pressure tap data for the reduction of the noise. As a demonstration, we applied the proposed method to the PSP data measuring the Kármán vortex street behind a square cylinder. The reconstructed pressure data agreed very well with the pressures independently measured by pressure transducers. This modal-based approach will be applicable not only to PSP data but other types of experimental data.

DOI： 10.1063/5.0049071

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER  

Abstract: The transonic buffet on an ONERA-M4 model was experimentally investigated using an unsteady pressure-sensitive paint (PSP) in the present study. Wind tunnel tests were conducted in a blowdown-type transonic wind tunnel at a Mach number of 0.84 and a chord Reynolds number of 2.0 × 10 6. The angle of attack was varied in between - 3. 0 ∘ and 4. 0 ∘. The left wing was painted with a polymer/ceramic PSP with low surface roughness, and the right wing was painted with a temperature-sensitive paint. The measured PSP data were processed to calculate time-series pressure coefficients, root-mean-squares pressure-coefficient fluctuations, power spectral density, coherence, and phase shift. The behavior of the unsteady pressure field was different from that observed for the NASA Common Research Model (CRM) in a previous study. The dominant frequency of the shock oscillation shifted from the low-Strouhal-number component (St< 0.05) to the bump Strouhal number (St= 0.11 for the center frequency) with increasing angle of attack. The separation processes with an increasing angle of attack were also found to be different for the two models. The separation starts from the mid-span region in the CRM, while the separation starts from the wingtip in the ONERA-M4 model. The characteristic pressure fluctuations known as “buffet cells” were not observed for the ONERA-M4 model. These differences are considered to be caused by the difference in model geometries, such as the wing twist and the airfoil cross-sectional profile. Graphical abstract: [Figure not available: see fulltext.]

DOI： 10.1007/s00348-021-03228-1

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The accurate measurement of flows and aerodynamic characteristics around a bluff body has been a challenging task due to the existence of interference between the wake and mechanical model supports in wind-tunnel experiments. The present study focuses on a freestream-aligned circular cylinder with an extremely low fineness ratio (the ratio of the axial length to diameter) ranging from 0.30 to 0.50, which has never been investigated without interference from a mechanical model support. We employed a magnetic suspension and balance system to eliminate interference from the model support and measured the drag and velocity fields in the diameter-based Reynolds number between 2.0×104 and 7.7×104. As the fineness ratio decreases below 1.50, the size of the recirculation bubble increases and the velocity distribution on the central axis inside the bubble gradually converges to that of the circular disk. Furthermore, large-eddy simulations were performed in the Reynolds number of 4.0×104, whose drag coefficient agrees well with experiments. Based on those results, it was found that the drag coefficient monotonically converges to that of the circular disk without local maximum. This study revealed that in the low-fineness-ratio regime (0.10-0.50), a critical geometry, at which the drag coefficient shows a local maximum, does not exist in the circular cylinder. Subsequently, unsteady flow analyses were performed, where two characteristic frequencies, i.e., St=0.05 and 0.155, were identified from power spectral densities of the drag coefficient and the pitching moment coefficient. The associated flow structures are then extracted by a phase-averaging procedure, where the phase-averaged flows with St=0.05 represent the recirculation bubble pumping while the phase-averaged flows with St=0.155 show nonaxisymmetric structures inside the recirculation bubble.

DOI： 10.1103/PhysRevFluids.6.054704

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Publishing type：Research paper (scientific journal)   Publisher：MDPI AG  

In this study, the propagation time and attenuation rate distributions of each sound source grid point (200 × 200) to a microphone of an arbitrary position across the shear layer, which are required for beamforming, were reconstructed by the reduced-order model with sparse sampling for acceleration of the computation. First, the propagation time and attenuation rate distributions, including the refraction of sound by the shear layer were calculated over 100 patterns of combinations of the wind speed and the microphone position, as training data. The dominant modes and optimum sampling points were discovered from the training data. Subsequently, data-driven sparse sampling for reconstruction was applied and the propagation time and the attenuation rate from each grid point (200 × 200) to a microphone were quickly calculated for the given microphone position and wind speed. The error of the obtained calculation result is 1% or less, and the approximation by data-driven sparse sampling is concluded to be effective.

DOI： 10.3390/app11094216

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Publishing type：Research paper (scientific journal)   Publisher：{MDPI} {AG}  

The characteristics of fast-response pressure-sensitive paints (PSPs) in low-pressure conditions were evaluated. Three representative porous binders were investigated: polymer-ceramic PSP (PC-PSP), anodized-aluminum PSP (AA-PSP), and thin-layer chromatography PSP (TLC-PSP). For each PSP, two types of luminophores, Pt(II) meso-tetra (pentafluorophenyl) porphine (PtTFPP) and tris(bathophenanthroline) ruthenium dichloride (Ru(dpp)3 ), were used as sensor molecules. Pressure sensitivities, temperature sensitivities, and photodegradation rates were measured and evaluated using a pressure chamber. The effect of ambient pressure on the frequency response was investigated using an acoustic resonance tube. The diffusivity coefficients of PSPs were estimated from the measured frequency response and luminescent lifetime, and the governing factor of the frequency response under low-pressure conditions was identified. The results of static calibration show that PC-PSP/PtTFPP, AA-PSP/Ru(dpp)3, and TLC-PSP/PtTFPP have high pressure sensitivities that exceed 4%/kPa under low-pressure conditions and that temperature sensitivity and photodegradation rates become lower as the ambient pressure decreases. Dynamic calibration results show that the dynamic characteristics of PSPs with PtTFPP are dependent on the ambient pressure, whereas those of PSPs with Ru(dpp)3 are not influenced by the ambient pressure. This observation indicates that the governing factor in the frequency response under low-pressure conditions is the lifetime for PC-PSP and TLC-PSP, whereas the governing factor for AA-PSP is diffusion.

DOI： 10.3390/s21093187

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER  

Abstract: This study aims to isolate and evaluate the influence of a corrugation on flow structures and aerodynamics under compressible low Reynolds number conditions, and to compare it to simpler but well-known model: the flat plate. The simplified corrugated model was made by a flat surface with only two corrugations on the leading edge. The models only differ for the corrugations on the leading edge. Force values were measured at a Reynolds number ranging from 10,000 to 25,000 and at a Mach number from 0.2 to 0.6. Pressure sensitive paint was used at the same flow conditions and the pressure distribution over the models was obtained. Schlieren visualization was also conducted and flow characteristics were observed. Detailed analysis showed that the corrugated model experiences strong depression on the leading edge caused by the separation of the boundary layer. Because of the presence of the corrugation, the shear layer transitions to turbulent rapidly and reattaches to the surface before reaching the summit of the first corrugation, separating again at its peak. Instabilities in the shear layer were dissipated thanks to the shape of the corrugation allowing pressure recovery and discouraging flow separation. The flow reattaches before reaching the trailing edge. The results showed that the transition of the boundary layer was accelerated as the Reynolds number increases on corrugated model, leading to a stronger negative pressure zone in the leading edge. Due to pressure recovery being less effective, lead to similar performances for the range of studied Reynolds numbers. The compressibility effects resulted in a delay on the transition of the instability of the shear layer, negatively affecting the intensity of the pressure gradients as well as pressure recovery. This contributed to the variation in the performance of the wing. As a result, the corrugated model has a better aerodynamic performance compared to the flat plate at low Reynolds numbers, but not for higher Mach numbers. Graphic abstract: [Figure not available: see fulltext.]

DOI： 10.1007/s00348-021-03164-0

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：MDPI  

The effects of the oxygen mole fraction on the static properties of pressure-sensitive paint (PSP) were investigated. Sample coupon tests using a calibration chamber were conducted for poly(hexafluoroisopropyl methacrylate)-based PSP (PHFIPM-PSP), polymer/ceramic PSP (PC-PSP), and anodized aluminum PSP (AA-PSP). The oxygen mole fraction was set to 0.1–100%, and the ambient pressure (P ) was set to 0.5–140 kPa. Localized Stern–Volmer coefficient B increased and then decreased with increasing oxygen mole fraction. Although B depends on both ambient pressure and the oxygen mole fraction, its effect can be characterized as a function of the partial pressure of oxygen. For AA-PSP and PHFIPM-PSP, which are low-pressure-and relatively low-pressure-type PSPs, respectively, B peaks at P ref < 12 kPa. In contrast, for PC-PSP, which is an atmospheric-pressure-type PSP in the investigated range, B does not have a peak. B has a peak at a relatively high partial pressure of oxygen due to the oxygen permeability of the polymer used in the binder. The peak of S , which is the emission intensity change with respect to normalized pressure fluctuation, appears at a lower partial pressure of oxygen than that of B . This is because the intensity of PSP becomes quite low at a high partial pressure of oxygen even if B is high. Hence, the optimal oxygen mole fraction depends on the type of PSP and the ambient pressure range of the experiment. This optimal value can be found on the basis of the partial pressure of oxygen. ref local local local O2 local local PR local local

DOI： 10.3390/s21041062

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Publishing type：Research paper (scientific journal)   Publisher：{IOP} Publishing  

Flow over a flat plate with a 5% thickness ratio is investigated by schlieren visualization in compressible low-Reynolds-number conditions. The results show that flow separates at the leading-edge and laminar separation-bubble forms. The position of the maximum root mean square of the schlieren image which is related to the position of the vortex shedding moves downstream as a Mach number increases. Furthermore, the two-dimensional structure of generated vortices is maintained up to the trailing edge at the Mach number of 0.66. The frequency analysis of the time-series intensity value of the schlieren images also shows that the flow is stabilized with increasing the Mach number. The position of the end of the pressure plateau region matches the position where the root-mean-square value of the intensity image becomes a maximum due to vortex shedding.

DOI： 10.1088/1873-7005/abe04c

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media {LLC}  

Abstract: The wake structure of a freestream-aligned circular cylinder and its aerodynamic characteristics were investigated using a magnetic suspension and balance system (MSBS), which can levitate a model to eliminate support interference. The objectives of the present study were to investigate the flow field around a freestream-aligned circular cylinder and to clarify its effect on the aerodynamic force and base pressure. Experiments were conducted using a 0.3-m MSBS for support-free wind tunnel tests. Six models with fineness ratios (length to diameter, L/D) of 1.0, 1.25, 1.5, 1.75, 2.0, and 2.25 were used. The freestream velocity was set to 10 and 20 m/s, which correspond to Reynolds numbers based on model diameters of 3.3 × 10 and 6.7 × 10 , respectively. The velocity field, aerodynamic force, and base pressure were measured for each freestream-aligned circular cylinder. Two characteristic fluctuations with frequencies of St < 0.05 and St ≈ 0.13 were observed in a nonreattaching flow field. A low-frequency axisymmetric fluctuation related to the drag force and base pressure was observed in the axial direction. A high-frequency antisymmetric fluctuation related to the lift force was observed in the radial direction. These features are in good agreement with those of both recirculation-bubble pumping and vortex shedding observed in the wake of a disk reported in previous studies (Berger et al., J Fluids Struct 4(3):231–257, 1990; Yang et al., Phys Fluids 27(6):064101, 2015). Graphic abstract: [Figure not available: see fulltext.] 4 4

DOI： 10.1007/s00348-020-03101-7

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER  

Unsteady pressure fields on a civil aircraft wing near the onset of transonic buffet have been investigated experimentally to identify flow unsteadiness that relates to the buffet onset determined by global criteria. An 80%-scale NASA Common Research Model was tested in the JAXA 2 m x 2 m Transonic Wind Tunnel at a Mach number of 0.85 and a chord Reynolds number of 2.27 x 10(6). The angle of attack was varied in small increments around the buffet onset angle determined by global criteria based on the lift curve and wing-root strain-gauge data. Unsteady pressure fields over the wing were measured using unsteady pressure-sensitive paint (PSP) with temperature-effect correction by temperature-sensitive paint (TSP). Characteristic pressure fluctuations, known as "buffet cells", were observed under the off-design conditions at a bump Strouhal number of 0.2-0.5. The PSP results showed that the buffet cells arise at the mid-span wing at eta approximate to 0.45, where a strong shock wave causes an initial boundary-layer separation. Phase shift distributions indicated that a pressure perturbation propagates from the inboard wing toward the outboard wing. The convection velocity and spanwise wavelength were approximately 0.5U(infinity) and 1.3c(MAC), respectively. The angle of attack at which buffet cells first appear was found to be approximately equal to the buffet onset determined by the global criteria, indicating that the occurrence of the buffet cells is deeply related to the buffet onset for the present wing geometry.[GRAPHICS].

DOI： 10.1007/s00348-020-03118-y

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media {LLC}  

Abstract: The feasibility of skin-friction field measurements using the global luminescent oil-film skin-friction field estimation method was evaluated for a challenging case of a supercritical airfoil model under transonic wind-tunnel conditions (freestream Mach number of 0.72) at a high Reynolds number (10 million, based on the model chord length). The oil-film thickness and skin-friction coefficient distributions were estimated over the airfoil model upper surface for a range of angles of attack (from - 0. 4 to 2. 0 ), thus enabling the study of different boundary-layer stability situations with laminar–turbulent transition, including cases with shock-wave/boundary-layer interaction. Conventional pressure measurements on the surface and in the wake of the model as well as Schlieren flow visualizations were conducted to support the oil-film based investigations. In the laminar-flow regions, the oil-film thickness could be generally kept below the critical limit of roughness that would induce premature boundary-layer transition. The skin friction in this region could be estimated with a moderate confidence level, as confirmed for portions of the chord by the reasonable agreement with numerical data obtained via laminar boundary-layer computations. Moreover, the location of transition onset was evaluated from the skin-friction estimations with relatively low uncertainty, thus enabling the examination of the transition location evolution with varying angle of attack. The estimated locations of transition onset were shown to be in general agreement with reference transition locations measured via temperature-sensitive paint. On the other hand, the oil-film thickness in the turbulent-flow regions was larger than the height of the viscous sublayer, which led to an hydraulically rough surface with increased skin friction, as compared to the clean configuration. For this reason, quantitative skin-friction estimations were not feasible in the turbulent-flow regions. The global effects of the oil-film setup on the flow around the airfoil were evaluated from the estimations of the aerodynamic coefficients. In particular, it was shown that the presence of the specific base coat used for the application of the oil film already induced a significant increase in airfoil drag, as compared to the clean configuration, whereas a thin oil film led to negligible or small additional increases in drag. Based on the present observations, considerations for the further improvement of the global luminescent oil-film skin-friction field estimation method in transonic flow experiments at high Reynolds numbers are elucidated. Graphic abstract: [Figure not available: see fulltext.] ∘ ∘

DOI： 10.1007/s00348-020-03109-z

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The aeroacoustic fields of twin jets and the equivalent single jet were experimentally investigated using particle image velocimetry (PIV), schlieren visualization, and acoustic measurement. The present study focuses on the aeroacoustic fields of the twin jets, and the effect of the interaction between each jet was investigated using various nozzle spacing. The PIV results indicated that strong interaction causes elliptical jet growth on a cross-stream plane and a decrease in the Reynolds stress of the inner shear layer on a plane containing both jet axes. The dynamic mode decomposition of the double-pulsed schlieren images extracted the interaction of each jet, which relates to the Mach wave generation. The noise of the twin jets was basically quieter than the noise of an equivalent single jet because of a shielding effect and a reduction in the Reynolds stress resulting in a decrease in the overall sound pressure level.

DOI： 10.2322/tjsass.64.312

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A vector-measurement-sensor-selection problem in the undersampled and oversampled cases is considered by extending the previous novel approaches: a greedy method based on D-optimality and a noise-robust greedy method in this paper. Extensions of the vector-measurement-sensor selection of the greedy algorithms are proposed and applied to randomly generated systems and practical datasets of flowfields around the airfoil and global climates to reconstruct the full state given by the vector-sensor measurement.

DOI： 10.32604/cmes.2021.016603

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Publishing type：Research paper (scientific journal)   Publisher：Japan Society for Aeronautical and Space Sciences  

DOI： 10.2322/tjsass.64.242

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers ({IEEE})  

The problem of selecting an optimal set of sensors estimating a high-dimensional data is considered. Objective functions based on D-, A-, and E-optimality criteria of optimal design are adopted to greedy methods, that maximize the determinant, minimize the trace of the inverse, and maximize the minimum eigenvalue of the Fisher information matrix, respectively. First, the Fisher information matrix is derived depending on the numbers of latent state variables and sensors. Then, a unified formulation of the objective function based on A-optimality is introduced and proved to be submodular, which provides the lower bound on the performance of the greedy method. Next, greedy methods based on D-, A-, and E-optimality are applied to randomly generated systems and a practical dataset concerning the global climate; these correspond to an almost ideal and a practical case in terms of statistics, respectively. The D- and A-optimality-based greedy methods select better sensors. The E-optimality-based greedy method does not select better sensors in terms of the index of E-optimality in the oversample case, while the A-optimality-based greedy method unexpectedly does so in terms of the index of E-optimality. The poor performance of the E-optimality-based greedy method is due to the lack of submodularity in the E-optimality index and the better performance of the A-optimality-based greedy method is due to the relation between A- and E-optimality. Indices of D- and A-optimality seem to be important in the ideal case where the statistics for the system are well known, and therefore, the D- and A-optimality-based greedy methods are suitable for accurate reconstruction. On the other hand, the index of E-optimality seems to be critical in the more practical case where the statistics for the system are not well known, and therefore, the A-optimality-based greedy method performs best because of its superiority in terms of the index of E-optimality.

DOI： 10.1109/ACCESS.2021.3067712

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers ({IEEE})  

The sparse sensor placement problem for the least square estimation is
considered, by extending the previous novel approach in this paper. First, the
objective function of the problem is redefined to be the maximization of the
determinant of the matrix appearing in pseudo inverse matrix operations,
leading to the maximization of the corresponding confidence intervals. The
procedure for the maximization of the determinant of the corresponding matrix
is proved to be the same as that of the previous QR method in the case of the
number of sensors less than that of state variables. On the other hand, the
authors have developed a new algorithm in the case of the number of sensors
greater than that of state variables. Then, the unified formulation of both
algorithms is derived, and the lower bound of the objective function given by
this algorithm is shown using the monotone submodularity. In the proposed
algorithm, optimal sensors are obtained by the QR method until the number of
sensors is the same as that of the state variables, and, after that, new
sensors are calculated by a proposed determinant-based greedy method which is
accelerated by both determinant formula and matrix inversion lemma. The
effectiveness of this algorithm on the dataset related to the global climate is
demonstrated by comparing it with the results by other algorithms. The
calculation results show that the computational time of the proposed extended
determinant-based greedy algorithm is shorter than that of other methods with
almost the minimum level of estimation errors.

DOI： 10.1109/ACCESS.2021.3076186

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Publishing type：Research paper (scientific journal)   Publisher：Cambridge University Press ({CUP})  

In the present study, compressible low-Reynolds-number flow past a stationary isolated sphere was investigated by direct numerical simulations of the Navier-Stokes equations using a body-fitted grid with high-order schemes. The Reynolds number based on free-stream quantities and the diameter of the sphere was set to be between 250 and 1000, and the free-stream Mach number was set to be between 0.3 and 2.0. As a result, it was clarified that the wake of the sphere is significantly stabilized as the Mach number increases, particularly at the Mach number greater than or equal to 0.95, but turbulent kinetic energy at the higher Mach numbers conditions is higher than that at the lower Mach numbers conditions of similar flow regimes. A rapid extension of the length of the recirculation region was observed under the transitional condition between the steady and unsteady flows. The drag coefficient increases as the Mach number increases mainly in the transonic regime and its increment is almost due to the increment in the pressure component. In addition, the increment in the drag coefficient is approximately a function of the Mach number and independent of the Reynolds number in the continuum regime. Moreover, the effect of the Mach and Reynolds numbers on the flow properties such as the drag coefficient and flow regime can approximately be characterized by the position of the separation point.

DOI： 10.1017/jfm.2020.629

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The resolution and the robustness of the weighted essentially non-oscillatory (WENO) scheme and two-step finite-difference WENO (TSFDWENO) schemes are compared by strictly using the same flux evaluation method and smoothness indicators. TSFDWENO schemes are defined to include a family of weighted compact nonlinear scheme (WCNS) and an alternativeWENO scheme. Comparison results indicate that WCNS has a higher resolution than the WENO scheme, while the WENO scheme is more robust than WCNS. Additionally, various flux evaluation methods are combined with TSFDWENO schemes, and they are evaluated. Then, the effects of the flux evaluation methods on the resolution and robustness of the scheme are investigated, and the results show that the robustness and the resolution can be significantly altered by changing the flux evaluation method. This study reveals the advantage of being able to use various flux evaluation methods in the TSFDWENO scheme as well as the fair comparison of the WENO schemes and WCNS. On the other hand, these effects are marginalized when changing the interpolation and differencing method. Such knowledge can be important when selecting schemes for actual simulation and developing guidelines for scheme improvement.

DOI： 10.4208/NMTMA.OA-2019-0033

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST AERONAUTICS ASTRONAUTICS  

DOI： 10.2514/1.J059239

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：{IOP} Publishing  

Successful operation of a multi-stage dielectric-barrier-discharge (DBD) plasma actuator is demonstrated by operating it on voltage one order of magnitude lower than that of a conventional single-stage DBD plasma actuator. An applied voltage waveform of direct current (DC) voltage combined with high-frequency repetitive pulses is generated by a simple power system consisting of a DC power supply and silicon carbide metal-oxide-semiconductor field-effect transistors. The time-averaged flow field obtained by particle image velocimetry indicates that a successively accelerated ionic wind is obtained by the eight-stage DBD plasma actuator. The velocity of the induced ionic wind increases with increasing DC voltage and repetitive pulse frequency. The maximum velocity of approximately 4.5 m s-1 is achieved when the DC voltage of 1500∼ V is applied with the switching frequency of 150∼ kHz, suggesting that the proposed multi-stage DBD plasma actuator induces the same level of ionic wind as a conventional high-voltage-operated single-stage DBD plasma actuator, even with a low voltage.

DOI： 10.1088/1361-6463/aba0e1

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Other Link： https://iopscience.iop.org/article/10.1088/1361-6463/aba0e1/pdf

Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER  

Abstract: The phase-averaged skin-friction analysis based on global luminescent oil film (GLOF) was conducted for periodically fluctuating unsteady phenomena at the frequency of approximately 150 Hz which is estimated based on Karman vortex shedding. An unsteady pressure transducer and a camera were synchronized, and the time-averaged and phase-averaged skin-friction fields were investigated. The time-series image pair data obtained by the camera were decomposed into eight intervals of a phase angle of π/ 4 with synchronizing the signal of the unsteady pressure. The phase-averaged result shows the periodical pattern corresponding to the vortices structure generated from the edge of the test model which was not resolved by the time-averaged result. The phase-averaged processing was successfully applied to the GLOF measurement, and the results showed the detail information of skin friction at each phase. Graphic abstract: [Figure not available: see fulltext.].

DOI： 10.1007/s12650-020-00667-6

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Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media {LLC}  

Abstract: A global luminescent oil-film (GLOF) image analysis method to estimate unsteady skin-friction fields in an unsteady flow field is proposed and demonstrated. A governing equation describing the dynamics of the oil film (the thin-oil-film equation) is employed for the unsteady oil-film images. The frequency response of the oil-film movement is analyzed, and a cutoff frequency is defined as a function of the oil-film thickness and the kinematic oil viscosity. The estimating skin-friction vector is defined along with a spatiotemporal weighted window and obtained by solving the overdetermined system of the thin-oil-film equation. The system can be solved by using the weighted linear least-squares method, and the time-resolved skin-friction field can be estimated. The time-resolved GLOF image analysis method is demonstrated on an experiment of a junction flow on a flat surface with a square cylinder. The GLOF images in the Kármán vortex shedding bounding the flat surface were acquired, and the time-resolved skin-friction fields were obtained. The results showed that fluctuation in the skin-friction vectors corresponds to the shedding frequency, and the vortices bounding the surface were extracted. The averaged skin-friction field is compared with the result of the previous study based on the time-independent model. The normalized skin friction from both methods showed good agreement, which indicates that the quantitative value will be obtained when a calibration process is involved in a future study. Graphic abstract: [Figure not available: see fulltext.].

DOI： 10.1007/s12650-020-00661-y

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Publishing type：Research paper (scientific journal)   Publisher：{IOP} Publishing  

In the present study, new performance evaluation parameters for pressure-sensitive paint (PSP) were proposed, and the effects of ambient pressure on the characteristics of the PSP were evaluated. The proposed parameters allow us to determine the optimal pressure range on the basis of the following quantities: (1) the Stern-Volmer coefficient, (2) the normalized pressure sensitivity to intensity changes due to flow-induced pressure fluctuations, (3) the change in the intensity of PSP emissions in response to the given change in pressure, and (4) the signal-to-noise ratio of the change in the PSP emission intensity due to flow-induced pressure fluctuations. The characteristics of several types of PSP were evaluated using the proposed parameters. It was demonstrated that the proposed parameters enable a comparison of the effects of ambient pressure on the characteristics of PSP, and the optimal pressure range for aerodynamic measurements could be successfully identified for the different PSP.

DOI： 10.1088/1361-6501/ab81bb

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers ({IEEE})  

A vector-measurement-sensor problem for the least squares estimation is considered, by extending a previous novel approach in this letter. An extension of the vector-measurement-sensor selection of the greedy algorithm is proposed and is applied to particle-image-velocimetry data to reconstruct the full state based on the information given by sparse vector-measurement sensors.

DOI： 10.1109/LSENS.2020.2999186

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Luminescent molecular sensors, which can be used as Temperature Sensitive Paint (TSP) or Pressure Sensitive Paint (PSP) in respective temperature or pressure measurements, has been proved to be one promising surface quantity measurement technology in recent years. It is advantageous in the experiments with complicated surface and is able to give flow field information such as surface flux and wall shear-stress. The current study is focused on the dynamic heat transfer measurement of a backward-facing step model, using luminescent molecular sensors as temperature probe. The effects of flow separation and re-attachment after a back-step model were experimentally discussed in wind tunnel tests. The experimental system was consisted of a molecular sensor calibration system, a dynamic data recording system and a data processing system. It is found that the reattachment process will form a low temperature region, which then gives the clear temperature field of the flow. Dynamic temperature field data show a re-attachment position around x/h = 5.7, which agrees well with oil-flow measurements as well as previous experiments. The dynamic temperature fluctuation data is discussed with the vibrations of flow and transient heat transfer behaviors after the backward step, which then is used in the analysis of surface wall shear-stress variations. It is concluded that the current luminescent molecular sensor method is capable of quantitative measurement for surface heat transfer and fluid flows.

DOI： 10.1016/j.ijheatmasstransfer.2020.119684

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© The Author(s), 2020. Published by Cambridge University Press. In the present study, a compressible low-Reynolds-number flow over a circular cylinder was investigated using a low-density wind tunnel with time-resolved schlieren visualizations and pressure and force measurements. The Reynolds number based on freestream quantities and the diameter of a circular cylinder was set to be between 1000 and 5000, and the freestream Mach number between 0.1 and 0.5. As a result, we have clarified the effect of on the aerodynamic characteristics of flow over a circular cylinder at. The results of the schlieren visualization showed that the trend of effect on the flow field, that are the release location of the Kármán vortices, the Strouhal number of vortex shedding and the maximum width of the recirculation, is changed at approximately. In addition, the spanwise phase difference of the surface pressure fluctuation was captured by the measurement using pressure-sensitive paint at approximately of higher- cases. The observed spanwise phase difference is considered to relate to the spanwise phase difference of the vortex shedding due to the oblique instability wave on the separated shear layer caused by the compressibility effects. The Strouhal number of the vortex shedding is influenced by and , and those effects are nonlinear. However, the effects of and can approximately be characterized by the maximum width of the recirculation. In addition, the effect on the drag coefficient can be characterized by the maximum width of the recirculation region and the Prandtl-Glauert transformation.

DOI： 10.1017/jfm.2020.221

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER  

There are typographical errors in the original publication of this article.

DOI： 10.1007/s00348-020-02980-0

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Abstract: A single-pixel ensemble correlation method was applied to the schlieren and shadowgraph image velocimetry (SIV) and a velocimetry method that can obtain the convection velocity distribution of high spatial resolution without an expensive pulsed laser system being achieved for a laboratory-scale supersonic jet flow. A cold axisymmetric supersonic jet was employed, and the basic characteristics of the convection velocity fields are measured by SIV as well as those of the velocity fields by the particle image velocimetry (PIV) in the single-pixel resolution. The Mach number of a supersonic jet was 2.0, and the Reynolds number based on the diameter of the nozzle exit was 1.0 × 10 . A pulsed light-emitting-diode light source was used for SIV as a less expensive light source. The single-pixel ensemble correlation method applied to PIV clearly visualizes the potential core and the shear layer development with the high spatial resolution. The axial velocity of SIV at the jet centerline is approximately 0.7–0.8 times of that of PIV which seems to relate to the convection velocity. The velocity calculated from the shadowgraph images agrees well with the convection velocity estimated from the Mach wave emission angle. The comparison between the scale of the visualized turbulent structure and the length scale of large eddies implied that the quantitative discussion of the SIV measurement requires careful consideration of the scale of the visualized turbulent structure on the SIV image. Graphic abstract: [Figure not available: see fulltext.] 6

DOI： 10.1007/s00348-020-02963-1

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A temperature-sensitive paint (TSP) using a chameleon luminophore [Tb Eu (hfa) (dpbp)] is proposed. The chameleon luminophore was dispersed in isobutyl methacrylate polymer in a toluene solvent to fix it on a sample coupon. Temperature and pressure sensitivities of the chameleon luminophore-based TSP were measured using a spectrofluorophotometer. The emission for each wavelength was confirmed to be dependent on the temperature and pressure. The temperature and pressure sensitivities of the TSP were 0.81–2.8%/K and 0.08–0.12%/kPa, respectively. Higher temperature sensitivity can be obtained using the ratio of emissions from the two lanthanide ions, Tb and Eu . The temperature sensitivity when using the ratio of the emission intensities at 616 nm derived from Eu and at 545 nm derived from Tb was 3.2%/K, which was the highest value in the present study. In addition, the pressure sensitivity for the case using the ratio of the emission intensities at 616 and 545 nm was 4.8 × 10 %/kPa. Higher temperature sensitivity and lower pressure sensitivity than that with a single wavelength can be achieved using the ratio of the emission intensities at the two peak wavelengths derived from Tb and Eu . 0.99 0.01 3 n III III III III −2 III III

DOI： 10.3390/s20092623

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DOI： 10.1063/1.5138195

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© 2020 by the authors. The flow control effects of a nanosecond-pulse-driven dielectric barrier discharge plasma actuator (ns-DBDPA) in dynamic stall flow were experimentally investigated. The ns-DBDPA was installed on the leading edge of an airfoil model designed in the form of a helicopter blade. The model was oscillated periodically around 25% of the chord length. Aerodynamic coefficients were calculated using the pressure distribution, which was obtained by the measurement of the unsteady pressure by sensors inside the model. The flow control effect and its sensitivity to pitching oscillation and ns-DBDPA control parameters are discussed using the aerodynamic coefficients. The freestream velocity, the mean of the angle of attack, and the reduced frequency were employed as the oscillation parameters. Moreover, the nondimensional frequency of the pulse voltage, the peak pulse voltage, and the type and position of the ns-DBDPA were adopted as the control parameters. The result shows that the ns-DBDPA can decrease the hysteresis of the aerodynamic coefficients and a flow control effect is obtained in all cases. The flow control effect can be maximized by adopting the low nondimensional frequency of the pulse voltage.

DOI： 10.3390/en13061376

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© 2019 Elsevier Ltd Luminescent mini-tufts method has been used for surface flow visualization for a long time. One major challenging point of this method is quantitative analysis of transient flows and the dynamic structures. This study is focused on the application of luminescent mini-tufts method in transient flows. A backward-facing step (BFS) is used in this analysis, which is one classic model that consists both flow separation and re-attachment processes. In this study, the instantaneous mini-tufts recognition, image averaging and tuft inclination angle/tuft angle estimation processes are introduced for the analysis of luminescent mini-tufts for the first time on backward-facing step flow (Rem = 2.0 × 105–7.9 × 105 and Reh = 1.3 × 104–5.3 × 104). Detailed transient features and characterization process for the backward-facing step model are explained in this study. The combination of optical-oil flow and hot-wire anemometry methods with luminescent mini-tufts are also shown useful to give comprehensive flow field information, including the surface flow behaviors, boundary layer, re-attachment position identification, etc. In addition, the decomposition of the luminescent mini-tufts visualization data is also conducted to give the power spectral density (PSD) and characteristic frequencies for the mini-tufts behaviors under transient fluctuating flow conditions.

DOI： 10.1016/j.flowmeasinst.2019.101657

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Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media {LLC}  

© 2019, Springer-Verlag GmbH Germany, part of Springer Nature. In this study, free-flight tests of a sphere for Reynolds numbers between 3.9 × 103 and 3.8 × 105 and free-flight Mach numbers between 0.9 and 1.6 were conducted using a ballistic range, and compressible low-Reynolds-number flows over an isolated sphere were investigated with the schlieren technique. The flow visualization was carried out under low-pressure conditions with a small sphere (minimum diameter of 1.5 mm) to produce compressible low-Reynolds-number flow. Also, time-averaged images of the flow near the sphere were obtained and compared to previous numerical results for Reynolds numbers between 50 and 1000. The experimental results clarified the structure of shock waves, recirculation region, and wake structures at the Reynolds number of 103–105 under transonic and supersonic flows. As a result, the following characteristics were clarified: (1) the amplitude of the wake oscillation was attenuated as the free-flight Mach number increased; (2) use of singular value decomposition permitted extraction of the mode of the wake structure even when schlieren images were unclear due to severe condition, and different modes in the wake structure were identified; (3) the Reynolds number had little effect on the separation point, but the length of the recirculation region increased as the Reynolds number decreased; and (4) the wake diameter at the end of the recirculation region decreased as the Mach number increased.

DOI： 10.1007/s00193-019-00924-0

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© 2020 by the authors. The flow control over the blades of a small horizontal-axis wind turbine (HAWT) model using a dielectric barrier discharge plasma actuator (DBD-PA) was studied based on large-eddy simulations. The numerical simulations were performed with a high-resolution computational method, and the effects of the DBD-PA on the flow fields around the blades were modeled as a spatial body force distribution. The DBD-PA was installed at the leading edge of the blades, and its impacts on the flow fields and axial torque generation were discussed. The increase in the ratios of the computed, cycle-averaged axial torque reasonably agreed with that of the available experimental data. In addition, the computed results presented a maximum of 19% increase in the cycle-averaged axial torque generation by modulating the operating parameters of the DBD-PA because of the suppression of the leading edge separation when the blade’s effective angles of attack were relatively high. Thus, the suppression of the leading edge separation by flow control can lead to a delay in the breakdown of the tip vortex as a secondary effect.

DOI： 10.3390/en13051218

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Publishing type：Research paper (scientific journal)   Publisher：{IOP} Publishing  

© 2019 IOP Publishing Ltd. Results from a skin-friction-field measurement method using global luminescent oil-film images, in which the oil film covers a large portion of the surface, were examined by comparing them with those obtained from conventional hot-wire measurements. The measurement processes including calibrations, oil-film image acquisition, and image processing are explained, and error sources are analyzed. The experimental results show that the skin frictions measured by both methods in a turbulent boundary layer on a flat surface agree when the oil-film thickness is less than the height of the viscous sublayer (corresponding to an oil-film thickness of less than five wall units). The uncertainty of the measurements is presented. When the measurement conditions were optimum (illumination, exposure time, frame rate, oil height, etc), the uncertainty was approximately 3%-4%.

DOI： 10.1088/1361-6501/ab512a

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© 2019 IOP Publishing Ltd. This paper proposes a lightweight, energy-efficient airflow control device that uses plasma discharge to realize stratospheric flight. Wind-tunnel experiments were performed in a reduced-pressure environment, and it was noted that plasma can suppress the flow separation around an airfoil, thereby dynamically changing the performance of the airfoil. The results demonstrate that even if the electron mean free path, reduced electric field for the plasma, and the aerodynamic Reynolds number for the stratospheric flight are very different from those at ground level, the plasma is effective in an airflow control device. Moreover, the proposed device operates by simply adhering thin tapes on a ready-made airfoil or hull of the airship and applying voltage to them, which contrasts with the conventionally developed plasma propulsion system. The airflow control technique using plasma will be a key technology in extending human activity to the stratosphere or regions at higher altitudes.

DOI： 10.1088/1361-6463/ab598b

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

We conduct large-eddy simulations of separated airfoil flows with control by a dielectric-barrier-discharge plasma actuator over a wide range of Reynolds numbers. The Reynolds numbers based on the chord length (Re) are set at Re = 5.0 x 10(3), 1.0 x 10(4), 6.3 x 10(4), 2.6 x 10(5), and 1.6 x 10(6). These Reynolds numbers cover most of the conditions used in the previous studies on separation control by a plasma actuator. The burst frequency nondimensionalized by the chord length and freestream velocity (F+) is used as the computational parameter, and the effective burst actuation and control mechanisms at each Reynolds number condition are investigated. With regard to cases without the control, the flows separate near the leading edge in the laminar state at the Reynolds number range of 10(3)-10(5), and a substantial turbulent separation occurs at the Reynolds number of 1.6 x 10(6). Separation control with a high burst frequency [F+ similar or equal to O(10)] can cause early flow reattachment through the promotion of turbulent transition of a separation shear-layer for Re = 6.3 x 10(4) and 2.6 x 10(5). Flow reattachment is mainly caused by momentum entrainment into the boundary layer by fine-scale turbulent vortices. On the other hand, the large-scale spanwise vortices play an important role at F+ = 1 for Re = 1.0 x 10(4) and 1.6 x 10(6). In these cases, the dynamics of the spanwise vortices show similar behavior and the pairing of these vortices significantly contributes to the separation control by increasing the momentum entrainment. The optimum value of F+ changes with a Reynolds number. In contrast, when a nondimensional burst frequency based on the characteristics of the separation shear-layer (F-theta s) is considered, a high lift-to-drag ratio is found at F-theta s similar or equal to O(10(-2) ) for all Reynolds numbers. This demonstrates that one of the effective burst frequencies is closely related to the scale of the separation shear-layer, especially for the spanwise vortex shed from the separation shear-layer. Published under license by AIP Publishing.

DOI： 10.1063/1.5136072

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DOI： 10.2514/1.J058925

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DOI： 10.2322/tjsass.63.8

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© 2020 The Japan Society for Aeronautical and Space Sciences Aerodynamics of an owl-like wing model at low Reynolds numbers (Re = O(104-5)) are investigated using large-eddy simulations with high-resolution computational schemes. The airfoil shape of the owl-like wing model is constructed based on a cross-sectional geometry of the owl wing at 40% wingspan from the root. The chord-based Re ranges from 1.0 © 104 to 5.0 © 104 and the angle of attack (¡) varies from 0 to 14 deg. The time-averaged lift (Cl) and drag coefficients computed are in reasonable agreement with the results of force measurement. The results computed clarify a nonlinear change in the Cl curve slope, which is due to an increase in the suction peaks in conjunction with the change in type of separation, the formation of a laminar separation bubble (LSB), and pressure recovery on the pressure side. The generation of the LSB on the suction and/or pressure sides at the Re of 2.3 © 104 and 4.6 © 104 are seen, while reattachments are observed only on the pressure side at the Re of 1.0 © 104 due to the camber of the wing. Furthermore, the owl-like wing model demonstrates favorable aerodynamic performance in terms of a maximum lift-to-drag ratio in comparison with several airfoils at the Re range considered. This is due to the strong suction peaks and distribution of surface pressure on the pressure side. It is emphasized that the concave lower surface enhances the time-averaged aerodynamic performance at all of the ¡ even though the LSB is generated and fluctuation in lift history is induced at low ¡.

DOI： 10.2322/tjsass.63.28

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© 2019, Springer-Verlag GmbH Germany, part of Springer Nature. The effects of jet temperature on acoustic waves generated by a supersonic jet are investigated using large eddy simulation (LES) based on a high-fidelity computational code. The sixth-order compact scheme and the fourth-order Runge–Kutta scheme are employed for spatial derivatives and time integration, respectively. First, a verification and validation study is conducted using simulations of a cold supersonic jet with a jet Mach number of 2.0 and Reynolds number of 9.0 × 10 5, and the effects of grid resolution and disturbance strength are evaluated. The verification and validation study shows that 6.5 × 10 8 grid points are sufficient for qualitative discussion of acoustic wave generation phenomena and that the addition of disturbances is important for suppressing the acoustic waves caused by the turbulent transition at the nozzle exit, as seen in previous studies for a subsonic jet. Then, LESs of supersonic jets with a jet Mach number of 2.0 and Reynolds number of 9.0 × 10 5 are performed for three temperature cases where the ratios of chamber to atmospheric temperature are 1.0, 2.7, and 4.0. The present results illustrate that different jet temperatures do not change the shear layer thickness, but the shear layer develops more inside the jet as the jet temperature increases, resulting in a shorter potential core for the hot jet. With regard to the acoustic fields, as the jet temperature increases, stronger Mach waves are emitted from a wider source region at wider radiation angles. We observe multiple Mach waves with different angles in the hot jet cases.

DOI： 10.1007/s00193-019-00895-2

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

We have conducted large-eddy simulations of turbulent separated flows over a NACA0015 airfoil with control by a plasma actuator. The Reynolds number based on the chord length is 1 600 000, and the angle of attack is 20.1 degrees. At this angle of attack, the flow around the airfoil is fully separated. The effects of the location and operating conditions of the plasma actuator on the separation control are investigated. The plasma actuator is set at the leading edge, the turbulent reattachment point, or near the turbulent separation point. The nondimensional burst frequency (F+) is set to 1, 4, or 100. These frequencies are determined based on the dominant frequencies of the turbulent separated flow field of the no control case. A continuous actuation case has also been conducted. The location of the actuator where it most effectively suppresses the separation is the one closest to the turbulent separation point. In the burst mode case, the nondimensional burst frequency of unity is most effective in terms of the increase in the lift. To clarify the effective control mechanism, five objectives for turbulent separation control are compared. The results show that it is difficult to suppress the turbulent separation using the same strategies as in laminar separation control. The effective mechanism for turbulent separation control by burst actuation is found to be inducing the pairing of large-scale vortices near the airfoil surface. This large-scale vortex pairing induces freestream momentum into the boundary layer, leading to separation suppression. In addition, three other control effects can be achieved by varying the operating settings of the plasma actuator. The drag is slightly improved by reducing the length of the laminar separation bubble through high-frequency actuation from the leading edge.

DOI： 10.1063/1.5110451

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER  

We proposed applying the cross-spectrum analysis to the unsteady pressure-sensitive paint (PSP) images to eliminate the electronic shot noise component from the pressure fluctuation spectrum. The data of surface pressure fluctuation behind a square cylinder which measured by means of the polymer/ceramic PSP with a frequency response of approximately 5kHz which is sufficiently high for studying the Karman vortex shedding was used for the validation of this analysis method. The cross spectrum is compared with the auto-spectrum of PSP images and that of the Kulite pressure transducer that corresponds to the reference signal. The cross spectrum can drastically reduce the electronic shot noise component in the pressure fluctuation spectrum. The second peak of vortex shedding which is not observed in the result of auto-spectrum can be observed thanks to the noise reduction. The convergence of calculation is faster as the size of the spatial area for calculating cross spectrum increases.[GRAPHICS].

DOI： 10.1007/s00348-019-2776-4

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Pressure-sensitive-paint (PSP) measurement was conducted for unsteady phenomena at various frequencies up to the order of kHz in low-speed flow to evaluate measurement accuracy of PSP. Pressure fluctuations on the floor surface induced by the Karman vortex were measured by PSP and unsteady pressure transducer. The dominant frequency of the pressure fluctuations is varied from 0.15 to 1.7kHz by changing the size of the square cylinder. While regions with large pressure fluctuations could be visualized by calculating root mean square of pressure fluctuations from PSP images, the values significantly differed from those measured by pressure transducer. By applying Fast Fourier Transform (FFT), the power spectral density (PSD) at peak frequencies could be obtained within an error of 20%. Singular-value decomposition (SVD) yields a remarkable improvement in signal-to-noise ratio. However, amplitude of pressure fluctuations is changed depending on the way how to select modes. Three mode-selection methods for SVD filtering/reconstruction analysis are proposed in this study which show good improvement compared with convection method and are proved capable of extracting characteristic behaviors of the flow phenomena even below the noise floor.[GRAPHICS].

DOI： 10.1007/s00348-019-2755-9

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© 2019 IOP Publishing Ltd. Wind tunnel experiments at a flow velocity of 40 m s-1 with a nanosecond-pulse-driven plasma actuator (ns-DBDPA) on an airfoil have been performed (i) to study discharge parameters inducing the ns-DBDPA flow control effect and (ii) to investigate discharge-mediating flow parameters representing the induced discharge-flow interactions. The lift and drag forces' measurements demonstrate that, in addition to the well-known frequency effect, the discharge energy per pulse can be the key discharge parameter representing the ns-DBDPA effect on the forces rather than the discharge power under various discharge energy per pulse raised up to 80 mJ m-1 and discharge frequencies ranged from 10 to 1600 Hz. In a single pulse operation free from the discharge frequency effect, Schlieren imaging and particle image velocimetry show that the dynamic of two heated zones generated by ns-DBDPA is identical to those of the induced two vortices. This discharge-flow interaction observed under the frequency-free condition implies that the key discharge mediating flow parameter can lie in the identical dynamics of the heated zones. This study suggests that the discharge-mediating flow parameters for the discharge-flow interaction leading to the flow control effect on the forces can be a statistical variation in the Schlieren image intensity or the angles of the heated zones' trajectories.

DOI： 10.1088/1361-6595/ab1daf

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© 2019 Elsevier Inc. As a widely used surface flow visualization method, luminescent mini-tuft has become one challenging topic with its practical advantages in quantitative flow measurement. The luminescent mini-tufts method is preferred with its reduced size and increased luminescence, which is suitable for surface visualization measurement. To provide a standard method/procedure in quantitative analysis for luminescent mini-tuft measurement, the current study established an experimental characterization platform of luminescent mini-tufts method and conducted flat-pate model for flow analysis. The experimental system is consisted of wind tunnel and model section, high-speed image data recording system, digital image processing as well as the control system. The digital imaging processing method for result analysis is also explained, which includes the dark current image extraction, averaging, mini-tufts recognition, and tuft inclination angle/tuft angle estimation process. In this study, the steady flow characterization and quantitative flow analysis is conducted on a flat plate model (Re = 1.6 × 105–6.6 × 105), which is combined with hot-wire anemometry to investigate the basic surface flow topology and boundary layer behaviors. The method is shown capable of capturing both the steady and transient behaviors of a surface flow. Luminescent mini-tufts physical model is also established and found good agreement with the experimental results in this study, which in turn support the mini-tufts characterization and selection in practical applications.

DOI： 10.1016/j.expthermflusci.2019.02.002

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© 2019 IOP Publishing Ltd. Gas density perturbations generated by an alternating-current dielectric-barrier-discharge plasma actuator (ac-DBDPA) are quantitatively visualised using the background-oriented schlieren (BOS) method. A method of setting the optimum boundary condition for solving the Poisson equation in the BOS method is studied, and an integration method for the boundary condition in the vicinity of the plasma where the density change is steep is proposed. The BOS method is applied in two cases with different voltage amplitudes, and the variation in the absolute value of the density is discussed with the discharge properties. The results show a decrease in density in the synthetic jet induced by the ac-DBDPA and a spatiotemporal variation indicating a step-wise gas-heating phenomenon due to plasma discharge.

DOI： 10.1088/1361-6595/ab1465

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The effect of Reynolds number on flow behaviors and pressure drag around axisymmetric conical boattails was experimentally investigated at low-speed conditions. Four conical boattails with slant angles of 12 degrees, 16 degrees, 20 degrees, and 22 degrees were studied. The Reynolds number ranged from 4.34x10(4) to 8.89x10(4) based on the model diameter. The global-luminescent-oil-film skin-friction measurement was employed to analyze the surface skin-friction topology. Quantitative skin-friction values at the centerline were obtained in this study. The results show that a separation bubble can be formed on boattail surfaces at angles from 12 degrees to 20 degrees. However, at a boattail angle of 22 degrees, flow is fully separated near the boattail shoulder. The integrated afterbody pressure drag indicated that, at angles of 12 degrees, 16 degrees, and 22 degrees, the Reynolds number has very small effect on the afterbody drag, while, at 20 degrees the drag coefficient decrease was relatively large with increasing Reynolds number. We believe that this study provided the first results for a boattail angle of 20 degrees and we observed that the size of the separation bubble decreased as the Reynolds number increased. The effect of the separation bubble on the pressure distribution was also examined in detail.[GRAPHICS].

DOI： 10.1007/s00348-019-2680-y

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A new dynamic mode decomposition (DMD) method is introduced for simultaneous system identification and denoising in conjunction with the adoption of an extended Kalman filter algorithm. The present paper explains the extended-Kalman-filter-based DMD (EKFDMD) algorithm which is an online algorithm for dataset for a small number of degree of freedom (DoF). It also illustrates that EKFDMD requires significant numerical resources for many-degree-of-freedom (many-DoF) problems and that the combination with truncated proper orthogonal decomposition (trPOD) helps us to apply the EKFDMD algorithm to many-DoF problems, though it prevents the algorithm from being fully online. The numerical experiments of a noisy dataset with a small number of DoFs illustrate that EKFDMD can estimate eigenvalues better than or as well as the existing algorithms, whereas EKFDMD can also denoise the original dataset online. In particular, EKFDMD performs better than existing algorithms for the case in which system noise is present. The EKFDMD with trPOD, which unfortunately is not fully online, can be successfully applied to many-DoF problems, including a fluid-problem example, and the results reveal the superior performance of system identification and denoising.

DOI： 10.1371/journal.pone.0209836

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© 2019 The Japan Society of Mechanical Engineers. Unsteady pressure distributions around a simplified sedan automobile model were investigated by conducting dynamic wind-tunnel testing using the newly developed forced oscillating apparatus, HEXA-X3, which can produce 6-degrees-of-freedom motion. The effects of heaving and pitching oscillation were investigated as the model simulated a vehicle running on a flat road at approximately 40 m/s and 1 Hz oscillation. The effects of the ground plate on unsteady pressure distributions over the model surfaces were measured while simulating heaving and pitching motion at Strouhal-number conditions similar to those for actual vehicles. The influence of the tubing on the frequency response of the pressure sensor was evaluated to be negligible by conducting a calibration experiment first. In the static case, the overall pressure distribution was consistent with that for a typical sedan, and the influence of the local relative flow velocity changes due to the contraction effect was observed in the underside of the model. In the forced oscillation tests, the effect of heaving and pitching motions on the flow around the underside was investigated. Effects of oscillation parameters, specifically amplitude and frequency, were investigated using the gain and phase-lag normalized by data from the steady model. Results of the test indicate that there is a characteristic distribution in pressure fluctuation, and the phenomena that become dominant in the flow around the underside vary according to location. The dynamic heaving motion was shown to change the pressure distribution, possibly due to changes in the effective angle of attack in addition to the static effect. The pitching test showed that a dynamic camber effect works in addition to those effects.

DOI： 10.1299/jfst.2019jfst0012

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：JAPAN SOC AERONAUT SPACE SCI  

This study is focused on the effect of boattail angle on the pressure distribution and drag force of axisymmetric afterbodies under low-speed conditions. Experiments were conducted on three conical boattails with angles of 10 degrees, 14 degrees and 20 degrees. The diameter-based Reynolds number is approximately 4.3 x 10(4) under the experimental conditions. Two types of flows, a fully-attached flow (beta = 10 degrees) and a flow with a separation bubble (beta = 14 degrees, 20 degrees), were observed. The aerodynamic drag measurements were conducted using both a strut-supported model and a support-free (levitated) model. The results show that boattail model with the angle of 20 degrees has a relatively large effect on the pressure distribution. The pressure drag resulting from pressure distribution on the vertical plane indicates that the model with a boattail angle of 14 degrees has the lowest drag. A good trend in agreement between afterbody drag (measured using pressure taps) and total aerodynamic drag (measured using the levitated system) was obtained. The effect of strut support on pressure distribution at different polar angles is also explained in this study.

DOI： 10.2322/tjsass.62.219

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© 2019 The Japan Society for Aeronautical and Space Sciences. In this study, a linear reduced-order model of flow fields around a NACA0015 is constructed based on time-resolved particle image velocimetry (PIV) data. The PIV data were obtained at the chord Reynolds number of 6.4 © 104, and angles of attack from 11° to 20°. Proper orthogonal decomposition (POD) analysis is employed for the PIV data and the degrees of freedom are reduced by truncating the POD modes. Next, a linear model of the POD modes is constructed using the least-squares method based on the POD-mode time histories. Although the estimated POD modes initially reproduce original data, they gradually attenuate and converge to zero. This behavior is also supported by the eigenvalue analysis results of the model’s coefficient matrices. In addition, the behavior of the low-order (more energetic) POD modes was reproduced better than that of the high-order (less energetic) POD modes. These results imply that the temporal fluctuation of large vortex structures has strong linearity and is not significantly affected by noise included in data. The former insight is also supported by the fact that the behaviors of POD modes were reproduced well in the case of high angle of attack.

DOI： 10.2322/tjsass.62.227

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：JAPAN SOC AERONAUT SPACE SCI  

DOI： 10.2322/tjsass.62.112

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© 2018 Cambridge University Press. In this study, direct numerical simulation of the flow around a rotating sphere at high Mach and low Reynolds numbers is conducted to investigate the effects of rotation rate and Mach number upon aerodynamic force coefficients and wake structures. The simulation is carried out by solving the three-dimensional compressible Navier-Stokes equations. A free-stream Reynolds number (based on the free-stream velocity, density and viscosity coefficient and the diameter of the sphere) is set to be between 100 and 300, the free-stream Mach number is set to be between 0.2 and 2.0, and the dimensionless rotation rate defined by the ratio of the free-stream and surface velocities above the equator is set between 0.0 and 1.0. Thus, we have clarified the following points: (1) as free-stream Mach number increased, the increment of the lift coefficient due to rotation was reduced; (2) under subsonic conditions, the drag coefficient increased with increase of the rotation rate, whereas under supersonic conditions, the increment of the drag coefficient was reduced with increasing Mach number; and (3) the mode of the wake structure becomes low-Reynolds-number-like as the Mach number is increased.

DOI： 10.1017/jfm.2018.756

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© 2018, Springer-Verlag GmbH Germany, part of Springer Nature. This study investigates the relationship between body-force fields and maximum velocity induced in quiescent air for development of a simple body-force model of a plasma actuator. Numerical simulations are conducted with the body force near a wall. The spatial distribution and temporal variation of the body force are a Gaussian distribution and steady actuation, respectively. The dimensional analysis is performed to derive a reference velocity and Reynolds number based on the body-force distribution. It is found that the derived Reynolds number correlates well with the nondimensional maximum velocity induced in quiescent conditions when the center of the Gaussian distribution is fixed at the wall. Additionally, two flow regimes are identified in terms of the Reynolds number. Considering the variation of the center of gravity of force fields, another Reynolds number is defined by introducing a new reference length. The nondimensional maximum velocity is found to be scaled with the latter Reynolds number, i.e., the maximum induced velocity in quiescent conditions is determined from three key parameters of the force field: the total induced momentum per unit time, the height of the center of gravity, and the standard deviation from it. This scaling turns out to be applicable to existing body-force models of the plasma actuator, despite the force distributions different from the Gaussian distribution. Comparisons of velocity profiles with experimental data validate the results and show that the flow induced by a plasma actuator can be simulated with simple force distributions by adjustment of the key body-force parameters.

DOI： 10.1007/s00162-018-0478-9

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE INC  

The surface flow pattern over a conical boattail on an axisymmetric body was investigated experimentally under low-speed and turbulent-boundary-layer conditions. Seven conical boattails with the same length but different angles from 10 degrees to 22 degrees were tested at a Reynolds number around 4.3 x 10(4), based on the model diameter. The study used the global luminescent oil-film (GLOF) skin friction measurement technique. The skin friction fields were measured and the corresponding flow topologies were extracted from the GLOF measurements. The effect of oil-film thickness on the separation position was also evaluated. Experimental results showed three different flow types on the boattail surface: (1) flow without separation, (2) flow with a separation bubble, and (3) fully separated flow. The critical angles for the transitions are discussed and compared with classic results for similar boattail models. The separation bubble generated at moderate boattail angles was observed for what we believe to be the first time under low-speed conditions, and the flow topology was clearly shown by the GLOF results. The azimuthally-averaged skin friction projected on the centerline showed different trends inside and behind the reattachment position when the boattail angle increased.

DOI： 10.1016/j.expthermflusci.2018.07.034

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© 2018 Elsevier Inc. We present a detailed numerical investigation of unsteady shear layer dynamics in downstream under an excited local body force, based on the assumption that a plasma actuator is positioned near the top of a two-dimensional hump for flow-separation control. A local body force works in temporal burst mode, which is homogeneous in the spanwise direction. In our previous report (Yakeno et al., 2015), the most effective frequency to cause early reattachment is fh=0.2, which corresponds to what Hasan (1992) and many other past studies referred to as the step mode. A periodic excitation generates two-dimensional roll vortices and other three-dimensional turbulence between downstream rolls, such as rib structures. These vortex characteristics significantly depend on the excitation frequency. In the study, we discuss these multi-scale turbulence motion separately by considering decomposition of temporal phase-locked periodic statistics of the excitation frequency and non-periodic turbulence fluctuation. At first, we found that non-periodic turbulence kinetic energy due to three-dimensional rib structure increases the most at the optimal frequency fh=0.2, although that frequency corresponds to the time scale that a hump-height vortex grows. It seems that non-periodic turbulence energy growth near separation point correlates with the control performance more than two-dimensional roll vortex increase. We operated linear hydrodynamic stability analysis on a free shear layer and confirmed that periodic phase fluctuation at high frequency grew on the Kelvin-Helmholtz instability. At low-frequency, periodic turbulence fluctuation is not reproduced with the exponential assumption, while its magnitude is large. From those results, we consider that the time and spanwise-averaged non-periodic turbulence energy becomes strong near the separation point the most at fh=0.2 because a hump-height vortex occurs the most times at this frequency, which is associated with a generation of the rib structure around it. Temporal-periodic momentum balance based on the decomposition is also investigated. A difference of terms contribution at high and low frequencies to the term of a pressure gradient in the wall-normal direction is discussed. Finally, we investigated how excitation position affects a total drag around a hump and found that, in some cases, two recirculation regions separately emerge in the downstream of the hump, and thus the control performance is degraded. At fh=0.2, one recirculation occurs regardless of the excitation position, while the most effective position is near the inflection point of the mean velocity of the uncontrolled flow near the wall.

DOI： 10.1016/j.ijheatfluidflow.2018.08.008

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：MDPI  

Polymer/ceramic pressure-sensitive paint (PC-PSP), which incorporates a high percentage of particles in the binder layer, is proposed in order to improve the characteristics of PSP. The procedure for embedding particles into the binder layer was modified. In the conventional procedure, dye is adsorbed onto a polymer/ceramic coating film (denoted herein as a dye-adsorbed (D-adsorbed) PSP). In the new procedure, the mixture of a dye and particles is adsorbed onto a polymer coating film (denoted herein as the particle/dye-adsorbed (PD-adsorbed) PSP). The effect of particle mass content on PSP characteristics was investigated. In addition, the effect of solvent on PSP characteristics and film structure were evaluated for the PD-adsorbed PSP. As a result, the difference in the PSP characteristics between the two types of PSP was clarified. Although surface roughness and time response increase with increased mass content of particles for both D-and PD-adsorbed PSPs, the critical pigment volume concentration (CPVC) for the PD-adsorbed PSP is smaller than that of the D-adsorbed PSP (88 wt% and 93 wt%, respectively). The PD-adsorbed PSP has a higher frequency response comparing with the D-adsorbed PSP while maintaining the same surface roughness. Observation by scanning electron microscope showed that the CPVC of the PC-PSP is governed primarily by surface structure. The coating film structure can be roughly classified into two states depending on the particle mass content. One is a state in which the coating film consisted of two layers: a lower particle-rich layer and an upper polymer-rich layer. This type of structure was observed in the PD-adsorbed PSP as well as in the D-adsorbed PSP. In the other state, polymer and particles are homogeneously distributed in the film, and pores are formed. This difference in the coating structure results in a change in the time response.

DOI： 10.3390/s18114041

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Other Link： http://orcid.org/0000-0001-7739-7104

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

A novel dynamic mode decomposition (DMD) method based on a Kalman filter is proposed. This paper explains the fast algorithm of the proposed Kalman filter DMD (KFDMD) in combination with truncated proper orthogonal decomposition for many-degree-of-freedom problems. Numerical experiments reveal that KFDMD can estimate eigenmodes more precisely compared with standard DMD or total least-squares DMD (tlsDMD) methods for the severe noise condition if the nature of the observation noise is known, though tlsDMD works better than KFDMD in the low and medium noise level. Moreover, KFDMD can track the eigenmodes precisely even when the system matrix varies with time similar to online DMD, and this extension is naturally conducted owing to the characteristics of the Kalman filter. In summary, the KFDMD is a promising tool with strong antinoise characteristics for analyzing sequential datasets. (c) 2018 Author(s).

DOI： 10.1063/1.5031816

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© 2018 IOP Publishing Ltd. The interaction between the gas-heating phenomenon in a pulsed discharge in atmospheric-pressure air and the separated shear layer in the flow around the airfoil is discussed. The first half of the paper details the development of the modeling for gas heating in a pulsed discharge in atmospheric-pressure air and reviews recent research results. Particular attention is paid to the processes of fast and slow gas heating. In the latter half of the paper, the experimental results of the high-speed Schlieren visualization are presented and the interaction between the nanosecond-pulse-driven dielectric-barrier-discharge plasma actuator (ns-DBDPA) actuation and the density field is discussed, based on the periodic and time-averaged components of the Schlieren signal intensity. The time-averaged intensity of the contrast of the Schlieren signal that originates in the separated shear layer changes according to the normalized actuation frequency of ns-DBDPA, F +. As F + increases from 0.1 to 2, the periodic component of the Schlieren signal intensity increases, resulting in a decrease in the time-averaged contrast of the Schlieren signal. When F + > 2, the heated air caused by ns-DBDPA actuation is accumulated along the separated shear layer, resulting in an increase in the time-averaged contrast of the Schlieren signal.

DOI： 10.1088/1361-6595/aae23c

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© 2018, Springer-Verlag GmbH Germany, part of Springer Nature. The control authority of a nanosecond-pulse-driven dielectric-barrier discharge plasma actuator (ns-DBDPA) was evaluated via wind tunnel experiments with the simultaneous measurement of lift and drag forces, pressure on the airfoil surface, and particle image velocimetry (PIV) measurements. In these experiments, a Reynolds number of Re = 2.6 × 105 was applied with a freestream velocity of 40 m/s under atmospheric pressure. The force measurements revealed multiple peaks of lift force recovery and drag force modulation depending on the angle of attack, α, and non-dimensional frequency, F+. At the positive post-stall α close to stall α of approximately 16°, F+ values around 2.0 were effective for lift recovery and drag reduction. When the deep-stall angle α is larger than 20° (either positive or negative), relatively low F+ values around 0.25 were effective for lift recovery. When actuating at a deep-stall angle corresponding to F+ = 0.25, the surface pressure measurements showed that a near flat pressure distribution is formed on the suction side, and the PIV measurement showed that this near flat distribution is caused by the increase in backflow velocity near the surface of the airfoil. This backflow enhancement near the suction side surface leads to the reduction in pressure in separated flow, resulting in significant increases in the lift and drag coefficients. Thus, this simultaneous measurement of force, pressure, and PIV is capable of evaluating the multiple control modes underlying lift and drag control by ns-DBDPA. Graphical abstract: [Figure not available: see fulltext.].

DOI： 10.1007/s00348-018-2584-2

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© 2018 Elsevier Ltd Backward-Facing Step (BFS) flow is one representative model for separation flows, which can be widely seen in aerodynamic flows (airfoil, spoiler, high attack angle process), engine flows, condensers, vehicles (cars, boat), heat transfer systems, and even the flow around buildings, etc. The flow separation after a simple stage will introduce separation bubble formation, evolution and re-attachment process, which is dependent on the BFS geometric design, the inlet and outlet conditions, turbulent intensity, as well as heat transfer conditions. In the past decades, it has been widely studied by various theoretical, experimental and numerical methods. Considering the importance of BFS flow in both theoretical and engineering aspects, this paper is focused on a review study of BFS flows from fundamental understandings to various experimental and numerical developments in a historical viewpoint. Basic models and the parameter-based after-step flow laws are summarized and categorized in this study. It is shown that the step size (duct expansion ratio) will define the basic re-circulation and re-attachment process, while the coupled effects of inflow parameters and the perturbation designs also help shape the flow behaviors after BFS. The review is also extended with model generalizations and the implications on system design, especially the heat transfer effects and the representative control designs are discussed in detail. Future trends and prospects in BFS studies are also included in this study.

DOI： 10.1016/j.tsep.2018.04.004

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER  

Transonic buffeting phenomena on a three-dimensional swept wing were experimentally analyzed using a fast-response pressure-sensitive paint (PSP). The experiment was conducted using an 80%-scaled NASA Common Research Model in the Japan Aerospace Exploration Agency (JAXA) 2 m x 2 m Transonic Wind Tunnel at a Mach number of 0.85 and a chord Reynolds number of 1.54 x 10(6). The angle of attack was varied between 2.82A degrees and 6.52A degrees. The calculation of root-mean-square (RMS) pressure fluctuations and spectral analysis were performed on measured unsteady PSP images to analyze the phenomena under off-design buffet conditions. We found that two types of shock behavior exist. The first is a shock oscillation characterized by the presence of "buffet cells" formed at a bump Strouhal number St of 0.3-0.5, which is observed under all off-design conditions. This phenomenon arises at the mid-span wing and is propagated spanwise from inboard to outboard. The other is a large spatial amplitude shock oscillation characterized by low-frequency broadband components at St < 0.1, which appears at higher angles of attack (alpha >6.0A degrees) and behaves more like two-dimensional buffet. The transition between these two shock behaviors correlates well with the rapid increase of the wing-root strain fluctuation RMS.

DOI： 10.1007/s00348-018-2565-5

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© 2018 Author(s). A data analysis method based on the linear least-squares (LLS) method was developed for the extraction of high-resolution skin friction fields from global luminescent oil film (GLOF) visualization images of a surface in an aerodynamic flow. In this method, the oil film thickness distribution and its spatiotemporal development are measured by detecting the luminescence intensity of the thin oil film. From the resulting set of GLOF images, the thin oil film equation is solved to obtain an ensemble-averaged (steady) skin friction field as an inverse problem. In this paper, the formulation of a discrete linear system of equations for the LLS method is described, and an error analysis is given to identify the main error sources and the relevant parameters. Simulations were conducted to evaluate the accuracy of the LLS method and the effects of the image patterns, image noise, and sample numbers on the results in comparison with the previous snapshot-solution-averaging (SSA) method. An experimental case is shown to enable the comparison of the results obtained using conventional oil flow visualization and those obtained using both the LLS and SSA methods. The overall results show that the LLS method is more reliable than the SSA method and the LLS method can yield a more detailed skin friction topology in an objective way.

DOI： 10.1063/1.5001388

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© 2017 In this study, an analysis of the flow properties around an isolated sphere under isothermal conditions for flows with high Mach numbers and low Reynolds numbers is conducted via direct numerical simulation (DNS) of the three-dimensional compressible Navier–Stokes equations. The calculations are performed with a boundary-fitted coordinate system. The Reynolds number based on the diameter of the sphere and the freestream quantities is varied from 100 to 300, the freestream Mach number is varied between 0.3 and 2.0, and the temperature ratio between the sphere surface and the freestream is varied between 0.5 and 2.0. We focus on the effects of the Mach number and the temperature ratio on the flow properties. The results show the following characteristics: (1) unsteady vortex shedding from the sphere is promoted (suppressed) when the temperature ratio is less (greater) than unity; (2) the drag coefficient increases with the temperature ratio, but previous drag relations give poor prediction on effect of the temperature ratio on the drag coefficient in the continuum regime; (3) Nusselt number relations proposed in previous studies can be applied if the temperature ratio is close to unity under subsonic conditions; (4) the changes in several flow properties can be characterized by a separation point in the range investigated.

DOI： 10.1016/j.ijheatmasstransfer.2017.12.042

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The drag coefficients of freestream-aligned circular cylinders of fineness ratios of 0.75-2.0 were investigated with a magnetic suspension and balance system (MSBS). The objective was to find the critical geometry, that is, the fineness ratio at which the drag coefficient becomes the local maximum within this ratio range. The experiments were conducted using the 1-m MSBS at the low turbulence wind tunnel at the Institute of Fluid Science, Tohoku University. The drag and base pressure coefficients of various cylinders were measured. The freestream velocity was varied to produce flows with Reynolds numbers ranging from 0.6 x 10(5) to 1.0 x 10(5). The drag coefficient monotonically decreases as the fineness ratio increases and no critical geometry or local maximum of the drag coefficient is found in the range we investigated. The base pressure coefficient decreases as the fineness ratio increases. The temporal fluctuations of the base pressure of the models with fineness ratios of 0.75, 1.0, and 1.2 are approximately twice as large as that of the model with a ratio of 2.0. The relationship between the fineness ratio and the drag coefficient is similar to that between the fineness ratio and the base pressure coefficient, similar to the findings of previous studies of two-dimensional bodies.

DOI： 10.1007/s00348-018-2531-2

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© 2017 The Authors A stable, non-dissipative, and conservative flux-reconstruction (FR) scheme is constructed and demonstrated for the compressible Euler and Navier–Stokes equations. A proposed FR framework adopts a split form (also known as the skew-symmetric form) for convective terms. Sufficient conditions to satisfy both the primary conservation (PC) and kinetic energy preservation (KEP) properties are rigorously derived by polynomial-based analysis for a general FR framework. It is found that the split form needs to be expressed in the PC split form or KEP split form to satisfy each property in discrete sense. The PC split form is retrieved from existing general forms (Kennedy and Gruber [33]); in contrast, we have newly introduced the KEP split form as a comprehensive form constituting a KEP scheme in the FR framework. Furthermore, Gauss–Lobatto (GL) solution points and g2 correction function are required to satisfy the KEP property while any correction functions are available for the PC property. The split-form FR framework to satisfy the KEP property, eventually, is similar to the split-form DGSEM–GL method proposed by Gassner [23], but which, in this study, is derived solely by polynomial-based analysis without explicitly using the diagonal-norm SBP property. Based on a series of numerical tests (e.g., Sod shock tube), both the PC and KEP properties have been verified. We have also demonstrated that using a non-dissipative KEP flux, a sixteenth-order (p15) simulation of the viscous Taylor–Green vortex (Re=1,600) is stable and its results are free of unphysical oscillations on relatively coarse mesh (total number of degrees of freedom (DoFs) is 1283).

DOI： 10.1016/j.jcp.2017.10.007

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© 2018 The Japan Society for Aeronautical and Space Sciences. The effects of Mach number at Re = 3,000 for different airfoils (NACA0012, NACA0002, NACA4412, NACA4402) with thickness and camber geometries are investigated for the propeller blade design of a Mars airplane. The present study shows that thin and cambered airfoils have larger variations in Cl than symmetric airfoils. As for thin airfoils, Cl at higher ¡ has rapid increases when the M¨ is low. This is because the flow separation occurs at the leading edge, and the flow is reattached on the airfoil surface. However, the rapid increase in Cl disappear as M¨ increases because the flow reattachment does not occurs. As for cambered airfoils, the decrease in Cl becomes larger than that on the symmetric airfoils when M¨ is higher. This is because Cp near the leading edge on the lower surface is smaller than that on the upper surface and the high-speed region on the lower side of the leading edge is enlarged as M¨ increases. Then, the Mcr at Re = 3,000 tends to be larger than that predicted by linear theory.

DOI： 10.2322/tjsass.61.258

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：JAPAN SOC AERONAUT SPACE SCI  

DOI： 10.2322/tjsass.61.281

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER  

© 2017, Springer-Verlag GmbH Germany. This study evaluates the influence of a serrated leading edge on flat-plate aerodynamics at low-Reynolds-number and subsonic high-Mach-number conditions. Forces are measured for a Mach number ranging from 0.2 to 0.64 at a Reynolds number of (12,000 ± 1000). Pressure distributions are obtained under the same conditions using pressure sensitive paint (PSP) measurement. Three models are tested: a flat plate without serrations used as the baseline case and two flat plates with serrated leading edges of different wavelength-to-amplitude ratios. Results show that the aerodynamic performance of flat plates with serrations is slightly changed from the baseline case. The plate with short-wavelength serrations underperforms in terms of the lift-to-drag ratio under all the conditions compared to the baseline case while the plate with large-wavelength serrations slightly outperforms it at around the stall angle. The Mach number has little effect on the attached flow while the lift increases with the Mach number under deep stall conditions. Serrations maintain the lift even under high angle of attack conditions when Mach number varies. The two-dimensional pressure distributions and the analyses of local chordwise pressure coefficient distributions at different spanwise locations and of periodicity of spanwise pressure coefficients allow categorisation of the complex flow structures into three types. These configurations feature different types of low pressure regions downstream of troughs. The periodicity of the pattern depends not only on the angle of attack but also on the Mach number.

DOI： 10.1007/s00348-017-2443-6

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：AMER INST AERONAUTICS ASTRONAUTICS  

Copyright © 2017 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved. This paper investigates the control of leading-edge flow separation over an airfoil using a dielectric-barrier discharge plasma actuator. A chord-based Reynolds number and an angle of attack are 2.6 × 105 and 18.8 deg, respectively. The flow around the stalled airfoil is computed by using large-eddy simulations. The body-force distribution-based plasma actuator model is adopted and set near the leading edge of the airfoil. Effects of a nondimensional burst frequency with a constant duty cycle onthe performance of flow control are studied. It isfound that, for the cases of the nondimensional burst frequencies 5, 10, 25, and 50, lift-to-drag ratios increase in comparison with that in the cases of the burst frequencies 1 and 100 and without the control. Controlled flow separates near the 30% chord from the leading edge due to adverse pressure gradient associated with the angleofattack. Mechanisms of suppression of the leading-edge flow separation in those cases are discussed.

DOI： 10.2514/1.J055727

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ACADEMIC PRESS INC ELSEVIER SCIENCE  

The characteristic-interpolation-based finite-difference weighted essentially non-oscillatory (WENO) scheme, which maintains the equilibriums of velocity, pressure, and temperature, is implemented to simulate compressible multicomponent flow fields. We propose the overestimated quasi-conservative form of the characteristic-interpolation-based finite-difference WENO scheme. The proposed WENO scheme is written in the split form that has the consistent and dissipation parts of the numerical flux. The dissipation part of the numerical flux is in the conservative form to maintain the conservation of conservative variables. The scheme implemented in this study can maintain the equilibriums of velocity, pressure, and temperature in various one-and two-dimensional problems. The results of the present studies provide new insights into the vector form of numerical dissipation. (C) 2017 Elsevier Inc. All rights reserved.

DOI： 10.1016/j.jcp.2017.02.054

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

© 2017 IOP Publishing Ltd. Gas-density perturbations near an airfoil surface generated by a nanosecond dielectric-barrier-discharge plasma actuator (ns-DBDPA) are visualized using a high-speed Schlieren imaging method. Wind-tunnel experiments are conducted for a wind speed of 20 m s-1 with an NACA0015 airfoil whose chord length is 100 mm. The results show that the ns-DBDPA first generates a pressure wave and then stochastic perturbations of the gas density near the leading edge of the airfoil. Two structures with different characteristics are observed in the stochastic perturbations. One structure propagates along the boundary between the shear layer and the main flow at a speed close to that of the main flow. The other propagates more slowly on the surface of the airfoil and causes mixing between the main and shear flows. It is observed that these two heated structures interact with each other, resulting in a recovery in the negative pressure coefficient at the leading edge of the airfoil.

DOI： 10.1088/1361-6463/aa6a80

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：WILEY  

© 2016 The Authors. International Journal for Numerical Methods in Fluids published by John Wiley & Sons Ltd This paper proposes WCNS-CU-Z, a weighted compact nonlinear scheme, that incorporates adapted central difference and low-dissipative weights together with concepts of the adaptive central-upwind sixth-order weighted essentially non-oscillatory scheme (WENO-CU) and WENO-Z schemes. The newly developed WCNS-CU-Z is a high-resolution scheme, because interpolation of this scheme employs a central stencil constructed by upwind and downwind stencils. The smoothness indicator of the downwind stencil is calculated using the entire central stencil, and the downwind stencil is stopped around the discontinuity for stability. Moreover, interpolation of the sixth-order WCNS-CU-Z exhibits sufficient accuracy in the smooth region through use of low-dissipative weights. The sixth-order WCNS-CU-Zs are implemented with a robust linear difference formulation (R-WCNS-CU6-Z), and the resolution and robustness of this scheme were evaluated. These evaluations showed that R-WCNS-CU6-Z is capable of achieving a higher resolution than the seventh-order classical robust weighted compact nonlinear scheme and can provide a crisp result in terms of discontinuity. Among the schemes tested, R-WCNS-CU6-Z has been shown to be robust, and variable interpolation type R-WCNS-CU6-Z (R-WCNS-CU6-Z-V) provides a stable computation by modifying the first-order interpolation when negative density or negative pressure arises after nonlinear interpolation. © 2016 The Authors. International Journal for Numerical Methods in Fluids published by John Wiley & Sons Ltd.

DOI： 10.1002/fld.4343

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST AERONAUTICS ASTRONAUTICS  

The various separation control mechanisms of burst-mode actuation with a dielectric barrier discharge plasma actuator were experimentally investigated in this study. The control of the separated flow around a NACA 0015 airfoil at a Reynolds number of 6.3 x 10(4) was investigated using a plasma actuator mounted at a distance from the leading edge of 5% of the chord length. A parametric study on the nondimensionalized burst frequency was conducted at three poststall angles of attack and various input voltages using time-averaged pressure measurements and time-resolved particle imaging velocimetry (PIV) results. The measurement results of the trailing edge pressure, which was selected as the index of separation control, indicate that the optimal burst frequency varies with the angle of attack. Several flow fields are discussed in detail in this paper, and two flow control mechanisms were observed: the use of a large-scale vortex and the promotion of turbulent transition. With regard to the first mechanism, the phase-locked PIV results indicate that a vortex structure, the size of which increases with decreasing burst frequency in the experimental range, is shed from the shear layer for each burst actuation. With regard to the second mechanism, time-averaged pressure and PIV measurements reveal that a burst frequency of F+ = 6-10 is able to promote turbulent transition. Among these two mechanisms, at higher angles of attack, the use of a large-scale vortex structure provides better separation control, whereas near the stall angle, the promotion of the turbulent transition provides better separation control.

DOI： 10.2514/1.J054678

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DOI： 10.2514/1.J054963

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE INC  

© 2016 Elsevier Inc. Flow characteristics under flow-separation control by the spanwise-modulated body force of a plasma actuator around a two-dimensional hump are numerically investigated. When the Reynolds number is set to Reh=16000, the incoming flow contains much disturbance and a near-wall streak structure appears around the hump. The local body force induces stable streamwise velocity to form a streak-like profile artificially. Under the forcing, turbulence fluctuations increase downstream, with resulting in early flow reattachment. These changes alter the momentum balance around the hump. Particularly, the wall-normal pressure gradient and velocity fluctuation terms in the momentum equation are modified. This somewhat suggests a relation between increases in wall-normal gradient terms and the flow reattachment profile under the control. In addition, the turbulence growth depends on the spanwise modulation wavelength of the forcing. The most effective wavelength among those tested here is λz=0.08, normalized by the hump height, which is λz+=150 in local viscous wall units. This viscous scale corresponds to that of a streak structure appearing around a two-dimensional hump.

DOI： 10.1016/j.ijheatfluidflow.2016.11.012

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：HINDAWI LTD  

We investigate the accuracy and the computational efficiency of the numerical schemes for evaluating fluid forces in Cartesian grid systems. A comparison is made between two different types of schemes, namely, polygon-based methods and mesh-based methods, which differ in the discretization of the surface of the object. The present assessment is intended to investigate the effects of the Reynolds number, the object motion, and the complexity of the object surface. The results show that themesh-based methods work as well as the polygon-based methods, even if the object surface is discretized in a staircase manner. In addition, the results also show that the accuracy of the mesh-based methods is strongly dependent on the evaluation of shear stresses, and thus they must be evaluated by using a reliable method, such as the ghost-cell or ghost-fluid method.

DOI： 10.1155/2017/8314615

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：TAYLOR & FRANCIS LTD  

© 2017 Informa UK Limited, trading as Taylor & Francis Group. This study investigates the predictability of the aerodynamic performance of some numerical methods at low Reynolds numbers and their dependency on the geometric shape of airfoil. We conducted three-dimensional large-eddy simulations (3-D LES), two-dimensional laminar simulations (2-D Lam), and Reynolds-averaged Navier–Stokes simulations with Baldwin–Lomax (2-D RANS(BL)) and Spalart–Allmaras (2-D RANS(SA)) turbulence models. Although there is little discrepancy between the 3-D LES, 2-D Lam, and 2-D RANS(SA) results in terms of the lift and drag characteristics, significant differences are observed in the predictability of the separation and reattachment points. The predicted lift, separation, and reattachment points of the 2-D Lam are qualitatively similar to those of the 3-D LES, except for high angles of attack at which a massive separation occurs. The 2-D RANS(SA) shows good predictability of the lift and separation points, but it does not estimate reattachment points accurately. The 2-D RANS(BL) fails to predict the precise separation points, which results in a poor lift predictability. These characteristics appear regardless of the airfoil geometry shapes. The results suggest that a 2-D Lam without any turbulence models can be used to estimate qualitative airfoil aerodynamic characteristics at the low Reynolds numbers.

DOI： 10.1080/10618562.2016.1274398

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：TAYLOR & FRANCIS LTD  

© 2017 Informa UK Limited, trading as Taylor & Francis Group. The two-fluid modelling based on an advection-upwind-splitting-method (AUSM)-family numerical flux function, AUSM+-up, following the work by Chang and Liou [Journal of Computational Physics 2007;225: 840–873], has been successfully extended to the fifth order by weighted-essentially-non-oscillatory (WENO) schemes. Then its performance is surveyed in several numerical tests. The results showed a desired performance in one-dimensional benchmark test problems: Without relying upon an anti-diffusion device, the higher-order two-fluid method captures the phase interface within a fewer grid points than the conventional second-order method, as well as a rarefaction wave and a very weak shock. At a high pressure ratio (e.g. 1,000), the interpolated variables appeared to affect the performance: the conservative-variable-based characteristic-wise WENO interpolation showed less sharper but more robust representations of the shocks and expansions than the primitive-variable-based counterpart did. In two-dimensional shock/droplet test case, however, only the primitive-variable-based WENO with a huge void fraction realised a stable computation.

DOI： 10.1080/10618562.2017.1311410

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Publisher：International Journal of Heat and Fluid Flow  

High-fidelity computations of the flow control around a pitching NACA633−618 airfoil by a plasma actuator are conducted. The effectiveness of the plasma actuator and the effects of its installation position are investigated. The plasma actuator is installed at x/c=0%, 10%, and 60% from the leading edge of the airfoil. The installation position of 60% is chosen based on the investigation of the uncontrolled flowfield; the case with this position successfully enhanced the aerodynamic performances of the airfoil. The results show the importance of a priori investigation of the separation and the reattachment points for an uncontrolled flowfield. In addition, the results illustrate that a properly installed and actuated plasma actuator is capable of controlling the dynamic flowfields and improving the aerodynamic performances of an airfoil.

DOI： 10.1016/j.ijheatfluidflow.2016.10.012

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DOI： 10.1016/j.ijheatfluidflow.2016.10.012

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

© 2016 The Authors An appropriate procedure to construct symmetric conservative metrics is presented for the high-order conservative flux-reconstruction scheme on three-dimensionally moving and deforming grids. The present framework enables direct discretization of the strong conservation form of governing equations without any errors in the freestream preservation and global conservation properties. We demonstrate that a straightforward implementation of the symmetric conservative metrics often fails to construct metric polynomials having the same order as a solution polynomial, which severely degrades the numerical accuracy. On the other hand, the symmetric conservative metrics constructed using an appropriate procedure can preserve the freestream solution regardless of the order of shape functions. Moreover, a convecting vortex is more accurately computed on deforming grids. The global conservation property is also demonstrated numerically for the convecting vortex on deforming grids.

DOI： 10.1016/j.compfluid.2016.03.024

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST AERONAUTICS ASTRONAUTICS  

This study explores possible design of an aeroacoustic flame deflector for a rocket launch pad. The design objectives are 1)minimization of the overall sound pressure level near the payload fairing, 2)minimization of the time-averaged maximum pressure on the flame-deflector surface, and 3)minimization of the shape difference from a flat plate inclined at 45deg. The acoustic wave characteristics associated with deflector shapes are identified by large-eddy simulations. To overcome difficulties of required computational time, the following are adopted: 1)a high-order scheme that reduces the computational cost of large-eddy simulations, 2)a multi-objective evolutionary algorithm for efficient parallelization, and 3)large-scale parallelization on the Japanese supercomputer K. Total computational time for optimization is approximately 350h with 6500 processors of the K computer. The analysis of nondominated (Pareto-optimal) solutions reveals a tradeoff relation and correlation among the objective functions. In the result, there appears a well-balanced solution that significantly reduces the overall sound pressure level. The shape difference is relatively minor, with a small bump located somewhat upstream of the impinging region. The result suggests that the local angle of the inclined deflector near the impinging region plays an important role for the reduction of overall sound pressure level near the rocket fairing.

DOI： 10.2514/1.A33420

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Publishing type：Research paper (international conference proceedings)  

© 2016 IEEE. PROCYON is a first full-scale, 50-kg-class probe featuring most of the key technologies for deep-space exploration. It was developed by the University of Tokyo and ISAS/JAXA and launched with Hayabusa 2 on 3 Dec 2014. PROCYON has a newly developed X-band telecommunication system fully compatible with the frequency range, up- and down-link turn-around ratio, modulation scheme, and DDOR tones following CCSDS-recommended standards, and it can establish X-band coherent two-way communication and ranging links with deep-space stations as larger deep-space probes have done. The total mass of the onboard telecommunication system is 7.3 kg excluding its RF coaxial harness, and total power consumption during two-way communication, 15 W of RF output power at SSPA, is 54.3 W. After launch, PROCYON's telecommunication system has been successfully working according to the system design. These achievements will provide core technologies for next-generation deep-space exploration by ultra-small probes.

DOI： 10.1109/AERO.2016.7500745

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：{AIP} Publishing  

© 2016 Author(s). In this study, analysis of flow properties around a sphere and its aerodynamic coefficients in the high-Mach-and-low-Reynolds-numbers conditions is carried out by direct numerical simulations solving the three-dimensional compressible NavierStokes equations. The calculation is performed on a boundary-fitted coordinate system with a high-order scheme of sufficient accuracy. The analysis is conducted by assuming a rigid sphere with a Reynolds number of between 50 and 300, based on the diameter of the sphere and the freestream velocity and a freestream Mach number of between 0.3 and 2.0, together with the adiabatic wall boundary condition. The calculation shows the following yields: (1) unsteady fluctuation of hydrodynamic forces become smaller as the Mach number increases under the same Reynolds number condition, (2) the drag coefficient increases with the Mach number due to an increase in the pressure drag by the shock wave, and (3) an accurate prediction of the drag coefficient in the supersonic regime using traditional models might be difficult.

DOI： 10.1063/1.4947244

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Discharge plasma evolution of DBD plasma actuator was numerically simulated to analyze the electrohydrodynamic (EHD) force field which is the source of the wall-surface jet generation of DBD plasma actuator. The simulation was conducted in the three-dimensional space, and three-dimensionality of the discharge and EHD force field was discussed in detail. As a result, many micro-discharges repetitively appear in the three-dimensional simulation. Although many of them appears at random position on the electrode edge, some of them appear at the same position. These discharge characteristics are in good agreement with observations in experiments. The EHD force field is also spanwise non-uniform corresponding to the plasma structure. To analyze the effects of the three-dimensional non-uniformity on the fluid, three-dimensional CFD simulation of the induced wall-surface jet was also conducted using the EHD force field obtained in the plasma simulation. The result indicated that not only transverse but also longitudinal vortex structures are generated, and the transverse vortex breaks into many small vortices with downstream advection.

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST AERONAUTICS ASTRONAUTICS  

The effects of plate angle on acoustic waves from a supersonic jet impinging on an inclined flat plate at angles of 30, 45, and 60 deg are numerically investigated. Three-dimensional compressible Navier-Stokes equations are solved using the modified weighted compact nonlinear scheme. Similar to previous studies, the acoustic fields indicate that there are at least three types of acoustic waves in all of the cases considered herein: 1) Mach waves generated from the shear layer of the main jet, 2) acoustic waves generated from the impingement region, and 3) Mach waves generated from the shear layer of the supersonic flow downstream of the jet impingement. Acoustic waves (2) are generated from two different acoustic sources: 1) the interaction between the plate shock wave and the shear layer, and 2) the interaction between the bubble-induced shock waves and the shear layer. The frequency characteristics of acoustic waves are related to the thickness of the shear layer in the impingement region. The results of the present study indicate the source location and the characteristics of acoustic waves (2) for various flat-plate angles.

DOI： 10.2514/1.J054152

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Language：Japanese  

超音速燃焼であるデトネーションの数値解析では,デトネーション波面を構成する衝撃波を厳密に解く必要がある.しかし,デトネーションの数値解析に使用される離散化手法には,数値散逸の大きい古い手法が多く,数値流体力学分野で発展が目覚ましい高次精度解析手法の導入例は少ない.そこで,9化学種の質量保存式を含む多成分流体の基礎方程式に,高次精度解析手法の一つであるrobust weighted compact nonlinear scheme(RWCNS)を実装し,研究例の多いH_2-Airデトネーションの数値解析を行った.その結果,従来の手法よりもデトネーション波面構造を詳細に捉えることができた.本稿では,反応性多成分流体にRWCNSを実装するための方法と得られたデトネーション波面構造について報告する.In order to simulate detonation which is a supersonic combustion, it is necessary to exactly solve a shock wave which is a part of detonation front. However, most of discretization methods used in earlier studies of detonation have strong numerical dissipation at discontinuity, and high order schemes developed in the field of computational fluid dynamics are hardly used. We calculate detonations by implementation of robust weighted compact nonlinear scheme (RWCNS). The present results using RWCNS show the detailed detonation wave structure. In this paper, we propose an efficient implementation of RWCNS to reactive multi-component fluid, and present the detonation wave structure.

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST AERONAUTICS ASTRONAUTICS  

The discharge plasma evolution of a dielectric barrier discharge plasma actuator is numerically simulated in the three-dimensional space. The spanwise nonuniformity in the body force field is analyzed, and the validity of the two-dimensional analysis is discussed. A sinusoidal voltage at the electrode is simulated, and the simulation successfully reproduces the characteristics of microdischarge as reported by previous experimental studies. The body force field obtained in the simulation is nonuniform in the spanwise direction, and a strong spanwise force is generated, even from the time-averaged viewpoint. The spanwise-averaged body force field is compared with the two-dimensional simulation result. Although the qualitative characteristics of the body force field in the two-dimensional simulation are the same as in the three-dimensional simulation, the two-dimensional simulation underestimates the chordwise extension of the force field and the force amplitude due to the weaker electric field concentration in the two-dimensional plasma structure. It can be expected that the two-dimensional simulation is useful for preliminary study on the dielectric barrier discharge plasma actuator. However, the three-dimensional simulation is indispensable to reproduce the spatial structure of the dielectric barrier discharge plasma, and understanding the interplay between plasma structure and actuator performance is an important aspect of plasma actuator research.

DOI： 10.2514/1.J054315

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：ELSEVIER SCIENCE INC  

© 2016, American Institute of Aeronautics and Astronautics Inc, AIAA. All rights reserved. Effects of plasma actuator on the dynamically stalled flowfield with Re = 2.56×105 around a pitching NACA0012 airfoil have been investigated by large-eddy simulations (LESs) with sufficient computational spanwise domain length. The dynamically stalled flowfield is characterized by laminar separation bubble, dynamic stall vortex and fully separated flowfield and the current LESs show good agreements with the experimental results with and without plasma actuator. With plasma actuator, the lift coefficient CL during the pitching down is significantly enhanced by the induced large leading edge vortex, whereas the drag coefficient CD is also increased.

DOI： 10.2514/6.2016-4239

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：THE JAPAN SOCIETY FOR AERONAUTICAL AND SPACE SCIENCES  

<p>A cooling system using oscillating heat pipe (OHP) has been developed for a balloon-borne astrophysics project GAPS (General Anti-Particle Spectrometer). Taking advantages of OHP, such as high conductivity, low-power, and suitability for spread heat source, OHP is planned to be used to cool the GAPS core detectors. OHP is a novel technique and it has never been utilized in practical use neither for a spacecraft nor for a balloon-craft, regardless of its many advantages. In these several years, we have investigated OHP's suitability for GAPS step by step. At first, we have succeeded in developing a scaleddown OHP model with a three-dimensional routing, which can operate in a wide temperature range around between 230 K and 300 K. We also succeeded in the first OHP flight demonstration with a prototype GAPS balloon experiment. Subsequently, we developed actual-sized OHP models with various routings. Numerical simulation models have been developed in parallel to further optimize the OHP design by understanding the OHP performance both macroscopically and microscopically. The design of the OHP check valve has been improved as well. This paper discusses the latest status of the GAPS-OHP development.</p>

DOI： 10.2322/tastj.14.Pi_17

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DOI： 10.2322/tastj.14.Pk_47

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Publisher：International Journal of Acoustics and Vibrations  

The generation of acoustic disturbances in supersonic laminar cavity flows is investigated by large-eddy simulations of supersonic laminar flow (M = 1:2, 2:0, and 3:0) past a rectangular cavity with a length-to-depth ratio of 2. Results suggest that well-originated large-scale vortical structures with strong spanwise coherence are present in the shear layer. Compressibility effects have significant impacts on the shear-layer development and the fluctuation properties. The dominant mechanism for the acoustic radiation in supersonic laminar cavity flows is shown to be associated with the successive passage of large-scale vortices over the cavity trailing edge. It is found that Mach waves radiated from the cavity shear layer may have significant contributions for the noiseradiation in terms of enhancing the strength of the feedback compression waves.

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

Large-eddy simulations have been conducted to investigate the mechanisms of separated-flow control using a dielectric barrier discharge plasma actuator at a low Reynolds number. In the present study, the mechanisms are classified according to the means of momentum injection to the boundary layer. The separated flow around the NACA 0015 airfoil at a Reynolds number of 63 000 is used as the base flow for separation control. Both normal and burst mode actuations are adopted in separation control. The burst frequency non-dimensionalized by the freestream velocity and the chord length (F+) is varied from 0.25 to 25, and we discuss the control mechanism through the comparison of the aerodynamic performance and controlled flow-fields in each normal and burst case. Lift and drag coefficients are significantly improved for the cases of F+ = 1, 5, and 15 due to flow reattachment associated with a laminar-separation bubble. Frequency and linear stability analyses indicate that the F+ = 5 and 15 cases effectively excite the natural unstable frequency at the separated shear layer, which is caused by the Kelvin-Helmholtz instability. This excitation results in earlier flow reattachment due to earlier turbulent transition. Furthermore, the Reynolds stress decomposition is conducted in order to identify the means of momentum entrainment resulted from large-scale spanwise vortical structure or small-scale turbulent vortices. For the cases with flow reattachment, the large-scale spanwise vortices, which shed from the separated shear layer through plasma actuation, significantly increase the periodic component of the Reynolds stress near the leading edge. These large-scale vortices collapse to small-scale turbulent vortices, and the turbulent component of the Reynolds stress increases around the large-scale vortices. In these cases, although the combination of momentum entrainment by both Reynolds stress components results in flow reattachment, the dominant component is identified as the turbulent component. This indicates that one of the effective control mechanisms for laminar separation is momentum entrainment by turbulent vortices through turbulent transition. (C) 2015 AIP Publishing LLC.

DOI： 10.1063/1.4935357

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE INC  

© 2015 Elsevier Inc. Separation and reattachment around a two-dimensional hump controlled by a two-dimensional periodic excitations induced by a dielectric barrier discharge plasma actuator are investigated at Reynolds numbers of 4000 and 16, 000 based on the freestream velocity and the hump height. A two-dimensional excitation is adopted in the present study for promoting two-dimensional instability in the shear layer and a resulting laminar-turbulent transition. Note that the most effective frequency for reattachment is fh(=f*u∞*/h*)=0.20 among the presently considered cases at both Reynolds numbers, which is close to the reference value previously reported for the flow around a backward-facing step (Hasan, 1992).This frequency is the highest among the frequencies that provide sufficient time for the vortex scale to become the hump height. In addition, the momentum balance around the hump is examined by decomposing the averaged momentum equations. The velocity fluctuation terms are found to be considerable in size: they increase around the separation position under periodic excitation control. These terms are balanced with other terms: the gradient of the velocity fluctuation in the streamwise direction is comparable to the sum of convection terms of the time-averaged velocity and that in the wall-normal direction is comparable to the pressure gradient in the wall-normal direction. The control performance for reattachment is correlated with these velocity fluctuation effects. There is a possibility that the excitation control enforces reattachment by increasing turbulence fluctuation and modifying the momentum transfer balance.

DOI： 10.1016/j.ijheatfluidflow.2015.07.014

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The generation of acoustic disturbances in supersonic laminar cavity flows is investigated by large-eddy simulations of supersonic laminar flow (M = 1:2, 2:0, and 3:0) past a rectangular cavity with a length-To-depth ratio of 2. Results suggest that well-originated large-scale vortical structures with strong spanwise coherence are present in the shear layer. Compressibility effects have significant impacts on the shear-layer development and the fluctuation properties. The dominant mechanism for the acoustic radiation in supersonic laminar cavity flows is shown to be associated with the successive passage of large-scale vortices over the cavity trailing edge. It is found that Mach waves radiated from the cavity shear layer may have significant contributions for the noiseradiation in terms of enhancing the strength of the feedback compression waves.

DOI： 10.20855/ijav.2015.20.3377

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST AERONAUTICS ASTRONAUTICS  

A substantial number of large-eddy simulations are conducted on separated flow controlled by a dielectric barrier discharge plasma actuator at a Reynolds number of 63,000. In the present paper, the separated flow over a NACA 0015 airfoil at an angle of attack of 12deg, which is just poststall, is used as the base flow for separation control. The effects of the location and operating conditions of the plasma actuator on the separation control are investigated by a parametric study. The control effect is evaluated based on the improvement of not only the lift coefficient but also the drag coefficient over an airfoil. The most effective location of the plasma actuator for both lift and drag improvement is precisely confirmed to be upstream of the natural separation point. Even a low burst ratio is found to be sufficient to obtain the same improvements as the cases with a high burst ratio. The effective nondimensional burst frequency F+ is observed at 4F+6 for the improvement in the lift coefficient and at 6F+20 for that in the drag coefficient. The lift/drag ratio shows a clear peak at 6F+10. To clarify the mechanism of the laminar-separation control, the effect of a turbulent transition is investigated. There is a clear relationship between the separation control effect and the turbulent transition at the shear layer. An earlier and smoother transition case shows greater improvements in the lift and drag coefficients. Flow analyses show that the cases with early and smooth turbulent transition can attach the separated flow further upstream, resulting in a higher suction peak of the pressure coefficient. In addition, another mechanism of the separation control is observed in which the lift coefficient is improved, not by the reattachment through the turbulent transition but by the large-scale vortex shedding induced by the actuation. It is possible to separate these two dominant mechanisms based on the effect of the turbulent transition on the separation control.

DOI： 10.2514/1.J053700

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：TAYLOR & FRANCIS LTD  

© 2015 Taylor & Francis. Large-eddy simulations (LES) are employed to understand the flow field over a NACA 0015 airfoil controlled by a dielectric barrier discharge (DBD) plasma actuator. The Suzen body force model is utilised to introduce the effect of the DBD plasma actuator. The Reynolds number is fixed at 63,000. Transient processes arising due to non-dimensional excitation frequencies of one and six are discussed. The time required to establish flow authority is between four and six characteristic times, independent of the excitation frequency. If the separation is suppressed, the initial flow conditions do not affect the quasi-steady state, and the lift coefficient of the higher frequency case converges very quickly. The transient states can be categorised into following three stages: (1) the lift and drag decreasing stage, (2) the lift recovery stage, and (3) the lift and drag converging stage. The development of vortices and their influence on control is delineated. The simulations show that in the initial transient state, separation of flow suppression is closely related to the development spanwise vortices while during the later, quasi-steady state, three-dimensional vortices become more important.

DOI： 10.1080/10618562.2015.1032271

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© 2015 AIP Publishing LLC. Mechanisms behind the pressure distribution and skin friction within a laminar separation bubble (LSB) are investigated by large-eddy simulations around a 5% thickness blunt flat plate at the chord length based Reynolds number 5.0 × 103, 6.1 × 103, 1.1 × 104, and 2.0 × 104. The characteristics inside the LSB change with the Reynolds number; a steady laminar separation bubble (LSB_S) at the Reynolds number 5.0 × 103 and 6.1 × 103, and a steady-fluctuating laminar separation bubble (LSB_SF) at the Reynolds number 1.1 × 104, and 2.0 × 104. Different characteristics of pressure and skin friction distributions are observed by increasing the Reynolds number, such that a gradual monotonous pressure recovery in the LSB_S and a plateau pressure distribution followed by a rapid pressure recovery region in the LSB_SF. The reasons behind the different characteristics of pressure distributions at different Reynolds numbers are discussed by deriving the Reynolds averaged pressure gradient equation. It is confirmed that the viscous stress distributions near the surface play an important role in determining the formation of different pressure distributions. Depending on the Reynolds numbers, the viscous stress distributions near the surface are affected by the development of a separated laminar shear layer or the Reynolds shear stress. In addition, we show that the same analyses can be applied to the flows around a NACA0012 airfoil.

DOI： 10.1063/1.4913500

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

A new technique for a finite-difference weighted essentially nonoscillatory scheme (WENO) on curvilinear grids to preserve freestream is introduced. This technique first divides the standard finite-difference WENO into two parts: (I) a consistent central difference part and (2) a numerical dissipation part. For the consistent central difference part, the conservative metric technique is directly adopted. For the numerical dissipation part, it is proposed that the metric term should be frozen for constructing the upwinding flux. This treatment only affects the numerical dissipation part, and the order of accuracy is maintained. With this technique, the freestream is perfectly preserved, and the flow fields are better resolved on wavy and random grids. (C) 2014 The Authors. Published by Elsevier Ltd.

DOI： 10.1016/j.compfluid.2014.09.025

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ACADEMIC PRESS INC ELSEVIER SCIENCE  

© 2014 Elsevier Inc. The appropriate procedure for constructing the symmetric conservative metric is presented with which both the freestream preservation and global conservation properties are satisfied in the high-order conservative flux-reconstruction scheme on a three-dimensional stationary-curvilinear grid. A freestream preservation test is conducted, and the symmetric conservative metric constructed by the appropriate procedure preserves the freestream regardless of the order of shape functions, while other metrics cannot always preserve the freestream. Also a convecting vortex is computed on three-dimensional wavy grids, and the formal order of accuracy is achieved when the symmetric conservative metric is appropriately constructed, while it is not when they are inappropriately constructed. In addition, although the sufficient condition for the freestream preservation with the nonconservative (cross product form) metric was reported in the previous study to be that the order of solution polynomial has to be greater than or equal to the twice of the order of a shape function, a special case is newly found in the present study: when the Radau polynomial is used for the correction function, the freestream is preserved even if the solution order is lower than the known condition. Using the properties of Legendre polynomials, the mechanism for this special case is analytically explained, considering the cancellation of aliasing errors.

DOI： 10.1016/j.jcp.2014.10.011

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST AERONAUTICS ASTRONAUTICS  

©2014 by DongHwi Lee, Taku Nonomura, Akira Oyama, and Kozo Fujii. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission. The flowfields around the NACA0012 airfoil at Reynolds numbers 1 × 104 , 3 × 104 , and 5 × 104 are studied, and the predictability of aerodynamic characteristics derived from various numerical methods is examined. Twodimensional laminar simulation, two-dimensional Reynolds-averaged Navier-Stokes simulation using the Baldwin-Lomax turbulence model, and three-dimensional implicit large-eddy simulation are employed in this study. The twodimensional laminar and three-dimensional implicit large-eddy simulations accurately predict the separation point, and capture the characteristics of a separation bubble for each Reynolds number and each angle of attack. Nonlinearity in the lift curve is also captured in the results of the two-dimensional laminar and three-dimensional implicit large-eddy simulations. The two-dimensional Reynolds-averaged Navier-Stokes simulation using the Baldwin-Lomax turbulence model predicts the separation point nearer the trailing edge than does the twodimensional laminar and three-dimensional implicit large-eddy simulations, and the separation bubble is not captured for any Reynolds number and angle of attack by this method. Nonlinearity of the lift curve does not appear in the results of the two-dimensional Reynolds-averaged Navier-Stokes simulation using the Baldwin-Lomax turbulence model. The two-dimensional laminar simulation can predict airfoil aerodynamic characteristics qualitatively, and it can be used as an appropriate numerical method at lower Reynolds numbers. The three-dimensional-implicit-large-eddy-simulation technique can be employed when more accurate qualitative characteristics are needed.

DOI： 10.2514/1.C032721

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Hindawi Publishing Corporation  

© 2015 Yusuke Mizuno et al. This study is devoted to investigating a flow around a stationary or moving sphere by using direct numerical simulation with immersed boundary method (IBM) for the three-dimensional compressible Navier-Stokes equations. A hybrid scheme developed to solve both shocks and turbulent flows is employed to solve the flow around a sphere in the equally spaced Cartesian mesh. Drag coefficients of the spheres are compared with reliable values obtained from highly accurate boundary-fitted coordinate (BFC) flow solver to clarify the applicability of the present method. As a result, good agreement was obtained between the present results and those from the BFC flow solver. Moreover, the effectiveness of the hybrid scheme was demonstrated to capture the wake structure of a sphere. Both advantages and disadvantages of the simple IBM were investigated in detail.

DOI： 10.1155/2015/438086

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：TAYLOR & FRANCIS INC  

Copyright © Taylor & Francis Group, LLC. In order to simulate detonation, a high-order shock-capturing scheme that models chemical reactions is implemented, and its resolution is examined by testing it on several numerical problems. A robust weighted compact nonlinear scheme (RWCNS) is adopted to take advantage of its robustness and ability to handle different flux types. This study shows the high resolution of the RWCNS compared with the conventional scheme. The results show that the RWCNS predicts the detailed vortex structure behind the detonation wavefront. ©

DOI： 10.1080/00102202.2014.935646

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST AERONAUTICS ASTRONAUTICS  

Copyright © 2013 by Masayuki Anyoji. A high-performance Ishii airfoil was analyzed using both a wind-tunnel and large-eddy simulations at a low- Reynolds-number condition (Re = 23,000). The design guidelines for an airfoil shape with a high lift-to-drag ratio under the aforementioned condition are described by analyses of flowfields and aerodynamic characteristics of the Ishii airfoil. Compared with conventional airfoils, such as the NACA 0012 and NACA 0002, the shape characteristic effects of the Ishii airfoil on its flowfield and aerodynamic characteristics are discussed. The shape on the suction side of the Ishii airfoil can cause delays in the flow separation at low angle of attacks. The separated flow reattaches, and a separation bubble forms even when trailing-edge separation changes to leading-edge separation. The separation bubble contributes to an increase in lift coefficient. In addition, the Ishii airfoil can gain a high positive pressure on the pressure side as compared with the other two symmetric airfoils due to the camber near the trailing edge.Onthe other hand, the pressure drag of the Ishii airfoil, which is a dominant factor of total drag, is considerably smaller than those of the other two airfoils. It was found that the shape on the suction side as well as that on the pressure side (such as the leading-edge roundness and the camber) are very significant in the low-Reynolds-number airfoil with a high lift-todrag ratio.

DOI： 10.2514/1.C032553

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

A two-fluid AUSM+-up numerical flux function with the exact (Godunov) Riemann solver for the stratified flow model concept by Chang et al. (2007) has been extended for simple and robust computations of compressible multiphase flows. The present method replaces the Godunov part with the HLLC approximate Riemann solver with no-iteration procedure in a very simple manner: This two-fluid HLLC has been inspired by the work by Hu et al. (2009), but used in a totally different way. Numerical tests demonstrate that the present two-fluid AUSM+-up is, if only velocity and pressure in the middle zone are computed by HLLC, as robust as the original, Godunov-combined AUSM+-up, despite being free from iterations and convergence criteria. © 2014 Elsevier Ltd.

DOI： 10.1016/j.compfluid.2014.05.019

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.2322/tastj.12.Pe_43

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Publishing type：Research paper (scientific journal)   Publisher：Japan Society for Aeronautical and Space Sciences  

DOI： 10.2322/tastj.12.Pk_7

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

To develop simulation techniques for reconstructing microdischarges in a dielectric barrier discharge (DBD) plasma actuator and analyze spanwise non-uniformity in a body force field, three-dimensional discharge plasma simulations of a DBD plasma actuator were conducted assuming step-like positive and negative applied voltages. Our study showed that to break the spanwise uniformity, some disturbances were required in the computational conditions to reconstruct the three-dimensional microdischarges, and the attachment of some minute bumps (several tens of micrometers in size) on the electrode edge allowed for the successful reconstruction of glow-type microdischarges and streamer-type filamentary discharges in the negative and positive applied voltage cases, respectively. The tentative body force field has strong spanwise non-uniformity corresponding to the plasma structure, and in addition, a spanwise directional body force also exists, especially in the streamer discharge. However, the spanwise averaged body force has the same spatial-distribution and time-evolution characteristics obtained with the two-dimensional simulation. (C) 2014 AIP Publishing LLC.

DOI： 10.1063/1.4870384

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ACADEMIC PRESS INC ELSEVIER SCIENCE  

The role of a geometric conservation law (GCL) on a finite-difference scheme is revisited for conservation laws, and the conservative forms of coordinate-transformation metrics are introduced in general dimensions. The sufficient condition of a linear high-order finite-difference scheme is arranged in detail, for which the discretized conservative coordinate-transformation metrics and Jacobian satisfy the GCL identities on three-dimensional moving and deforming grids. Subsequently, the geometric interpretation of the metrics and Jacobian discretized by a linear high-order finite-difference scheme is discussed, and only the symmetric conservative forms of the discretized metrics and Jacobian are shown to have the appropriate geometric structures. The symmetric and asymmetric conservative forms of the metrics and Jacobian are examined by the computation of an inviscid compressible fluid on highly-skewed stationary and deforming grids using sixth-order compact and fourth-order explicit central-difference schemes, respectively. The resolution of the isentropic vortex and the robustness of the computation are improved by employing symmetric conservative forms on the coordinate-transformation metrics and Jacobian that have an appropriate geometry background. An integrated conservation of conservative quantities is also attained on the deforming grid when symmetric conservative forms are adopted to the time metrics and Jacobian. © 2013 Elsevier Inc.

DOI： 10.1016/j.jcp.2013.12.019

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ACADEMIC PRESS INC ELSEVIER SCIENCE  

A simple interface sharpening technique based on hyperbolic tangent interpolation, which was proposed in the previous study [F. Xiao, Y. Honma, K. Kono, A simple algebraic interface capturing scheme using hyperbolic tangent function, Int. J. Numer. Methods Fluids 48 (2005) 1023-1040], is applied to the compressible two-fluid modeling. The implementation of this scheme is very simple: the interpolation of the volume fraction in the monotonicity-upwind-scheme-for-conservation-law (MUSCL) solver is just replaced by the hyperbolic tangent interpolation, while the MUSCL interpolations for other variables are maintained. This technique is limited for the region near the interface to prevent the spurious oscillations of a minor phase. The one-dimensional and two-dimensional problems are solved, and the results are compared with those of the original MUSCL solver. The results show that the interface is significantly sharpened with this technique, and its sharpness is well controlled by one parameter. In addition, the robustness of the scheme does not change with sharpening the interface in the range we investigated. (C) 2013 Elsevier Inc. All rights reserved.

DOI： 10.1016/j.jcp.2013.10.021

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：HINDAWI PUBLISHING CORPORATION  

A computational code adopting immersed boundary methods for compressible gas-particle multiphase turbulent flows is developed and validated through two-dimensional numerical experiments. The turbulent flow region is modeled by a second-order pseudo skew-symmetric form with minimum dissipation, while the monotone upstream-centered scheme for conservation laws (MUSCL) scheme is employed in the shock region. The present scheme is applied to the flow around a two-dimensional cylinder under various freestream Mach numbers. Compared with the original MUSCL scheme, the minimum dissipation enabled by the pseudo skew-symmetric form significantly improves the resolution of the vortex generated in the wake while retaining the shock capturing ability. In addition, the resulting aerodynamic force is significantly improved. Also, the present scheme is successfully applied to moving two-cylinder problems.

DOI： 10.1155/2014/252478

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DOI： 10.2322/tastj.12.Pe_11

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：THE JAPAN SOCIETY FOR AERONAUTICAL AND SPACE SCIENCES  

Aerodynamic characteristics and flow fields around an owl-like airfoil at a chord Reynolds number of 23,000 are investigated using two-dimensional laminar flow computations. Computed results demonstrate that the deeply concaved lower surface of the owl-like airfoil contributes to lift augmenting, and both a round leading edge and a flat upper surface lead to lift enhancement and drag reduction due to the suction peak and the presence of the thin laminar separation bubble near the leading edge. Subsequently, the owl-like airfoil has higher lift-to-drag ratio than the high lift-to-drag Ishii airfoil at low Reynolds number. However, when the minimum drag is presented, the Ishii airfoil gains lift coefficient of zero while lift coefficient of the owl-like airfoil does not becomes zero. Furthermore, a feature of unsteady flow structures around the owl-like airfoil at the maximum lift-to-drag ratio condition is highlighted.

DOI： 10.2322/tastj.12.Tk_35

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：THE JAPAN SOCIETY FOR AERONAUTICAL AND SPACE SCIENCES  

The aim of this study is to find the optimal airfoil for Mars exploration aircraft, which requires high-lift-to-drag ratio. However, existing airfoils for flying in the Earth's atmosphere do not have a high enough lift-to-drag ratio in Mars flight condition. The airfoil studied here was designed using a Genetic Algorithm (GA) and evaluated using two-dimensional Computational Fluid Dynamics (CFD) without turbulence model (laminar). The objectives in this optimization include the maximization of lift and minimization of drag coefficients at only angle of attack of 6 °. The Reynolds number is 2.3 × 10⁴under the aircraft cruising condition. B-spline curves that connect neighboring control points express the upper and lower surfaces of the airfoil. The results show that some typical types of airfoils excel in aerodynamic performance. Most optimal airfoils have a large upper surface curvature or a strong curvature at the center of the lower surface. The former feature generates a separation bubble that leads to a high negative pressure, and the latter character makes a high positive pressure. Both phenomena generate lift force, and yield higher lift coefficient and high lift-to-drag ratio. Furthermore, most airfoils on the Pareto front have a thickness less than 10 % of the chord length, which is suitable for the wing structure design of the Mars aircraft.

DOI： 10.2322/tastj.12.Pk_59

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Language：English   Publisher：THE JAPAN SOCIETY FOR AERONAUTICAL AND SPACE SCIENCES  

The thermal condition of ASTRO-H under air-cooled environment before launch is investigated with a thermal testing and a computational analysis. The thermal testing shows that the temperatures of devices are confirmed to be within the operating range if the additional fans are used. Moreover, the results of the thermal testing are compared with those of computational results. The computational results of temperature of the devices around the dewar with the additional fans are in good agreement with those of the thermal testing. The good agreement in the condition with the additional fans is because the forced convection, which is a dominant effect, is well captured in the computational analysis. Meanwhile, the computational results of temperature on the side panels are in very good agreement with thermal testing despite the difference in the flow outside satellite by air conditioner: computational analysis models the air flow from the air-conditioner while thermal testing does not. This is because the air-flow is very slow (0.1[m/s] at the side panel locations) and forced convection effects are very small.

DOI： 10.2322/tastj.12.To_4_1

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：THE JAPAN SOCIETY FOR AERONAUTICAL AND SPACE SCIENCES  

Flow field and aerodynamic performance of the Mars airplane with a complete aircraft configuration are analyzed by RANS simulations. At the Reynolds number of 3.3x10⁴, a flow field is solved by an unstructured three-dimensional compressible CFD solver (LS-FLOW). Here, the Mars airplane is assumed to have the Ishii airfoil as the main wing shape. The Ishii airfoil is known as its good performance at the low Reynolds number condition. An objective of the present study is to clarify flow structures around a complete aircraft, for optimization of design of the Mars airplane. The results show that the features of the aerodynamic coefficients correspond to those of experimental results and the contribution of the main wing is significant on the aerodynamic characteristics of the entire airplane.

DOI： 10.2322/tastj.12.Tk_1

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Language：English  

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

This study presents a new weighted compact nonlinear scheme (WCNS). In the WCNS procedure, the linear difference scheme is modified to use the flux on the computational nodes together with that on the midpoints. This modification makes the scheme more robust, but at the same time, more dissipative. We conduct the truncation error analysis and discuss the reasons why this modification makes the scheme robust and dissipative. The standard shock problems are solved with both original and modified WCNS, and the results show that the discontinuity thickness of the modified WCNS increases, and over/undershoots at the discontinuities are suppressed better in WCNS. In addition, the stiff shock tube problems, which cannot be solved by the original WCNS because of negative pressure, can be solved using the modified WCNS without a blow-up of computation. A series of numerical tests show the robustness of the modified WCNS. (C) 2012 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.compfluid.2012.09.001

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST AERONAUTICS ASTRONAUTICS  

The article examines the Mach-number effects on vortex breakdown in subsonic flows over delta wings. The governing equations are three-dimensional compressible Navier-Stokes equations. The length and velocity are non-dimensionalized by the chord length and sound speed of the free stream condition, respectively. The computational code used here is based on the well validated Navier-Stokes code with recent modifications to realize more efficient implicit time-integration schemes and a high-order accurate evaluation of space derivatives. The wing-surface contours show pressure distribution is observed for the cases with a Mach number greater than 0.5. Here, the mode is distinguished by the instantaneous shape of the iso-surface of the second invariant of the velocity-gradient tensor. First, the results of the implicit large-eddy simulation and Reynolds averaged Navier-Stokes hybrid simulation using the Baldwin-Lomax turbulence model with a Mach number of 0.065 were validated with the experimental study.

DOI： 10.2514/1.J052321

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

The mechanism of controlling supersonic cavity oscillations using upstream mass injection is investigated by implicit large-eddy simulations of a turbulent flow (M∞ = 2.0, ReD = 105) past a rectangular cavity with a length-to-depth ratio of 2. The mass injection is simulated by specifying a vertical velocity profile of a jet ejecting steadily through a slot placed at the upstream of the cavity leading edge. The results show that the steady upstream mass injection produces significant attenuation of the cavity oscillations, and two primary mechanisms are demonstrated to be directly responsible for the noise suppression: lifting up of the cavity shear layer, and damping of the shear-layer instability. It is found that the case of low mass flow injection investigated is more effective in stabilizing the cavity shear layer than the high mass flow injection. A transition stage might exist between two well-developed oscillating modes, but "mode-switching" is not observed. © 2013 AIP Publishing LLC.

DOI： 10.1063/1.4816650

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Language：English   Publishing type：Research paper (scientific journal)  

Self-sustained oscillations in supersonic cavity flows are investigated by implicit large-eddy simulations of a supersonic flow (M∞ = 2.0, ReD = 105) past a three-dimensional rectangular cavity with length-to-depth ratio of 2. Both turbulent and laminar inflows are considered, and a variation of boundary-layer thickness in the turbulent inflow case is conducted. An additional simulation of turbulent free shear layer is also performed to illustrate the relationship between shedding vortices and acoustic excitations. Feedback mechanism is identified as the dominant mechanism driving the self-sustained oscillations in supersonic open cavity flows, regardless of the upstream turbulent state and the boundary-layer thickness. The generation of discrete vortices in the cavity shear layer is shown to be highly associated with acoustic excitations rather than natural instabilities of the cavity shear layer. Simulation results support that the primary noise source arises from the successive passage of large-scale vortices over the cavity trailing edge. The effects of upstream boundary layer on the shear-layer characteristics and acoustic fields will also be discussed. © 2013 AIP Publishing LLC.

DOI： 10.1063/1.4804386

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Publisher：Advances in the Astronautical Sciences  

We have analyzed the asymmetric separation flow over a slender body at high angle of attack by numerical simulations aiming a control of the asymmetric vortices using a dielectric barrier discharge (DBD) plasma actuator. The Reynolds Averaged Navier Stokes/Large-Eddy Simulation hybrid method (RANS/LES) was adopted with the high-order compact spatial difference scheme. First, for investigating the characteristics of the asymmetric separation flow, the simulation of the flow field over the slender body was conducted for various angle of attack and bump height. Note that the bump is added near the body apex to simulate the symmetry-breaking imperfection. When the angle of attack or the bump becomes higher, the asymmetricity of vorticities becomes stronger. The side force has nonlinearity with the angle of attack or the bump height. Next, numerical simulations of the flow control using a plasma actuator were conducted. The side force coefficient can be continuously controlled in response to output power of the actuator within about ±1.0 on an average by the actuator's actuation at the aft body. However, the flow control effect is totally difference between starboard-side actuator's actuation and port-side actuator actuation. In addition, it is strongly influenced by the angle of attack.

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Publisher：Lecture Notes in Computer Science (including subseries Lecture Notes in Artificial Intelligence and Lecture Notes in Bioinformatics)  

We propose a new type of multi-objective genetic programming (MOGP) for multi-objective design exploration (MODE). The characteristic of the new MOGP is the simultaneous symbolic regression to multiple objective functions using correlation coefficients. This methodology is applied to non-dominated solutions of the multi-objective design optimization problem to extract information between objective functions and design parameters. The result of MOGP is symbolic equations that are highly correlated to each objective function through a single GP run. These equations are also highly correlated to several objective functions. The results indicate that the proposed MOGP is capable of finding new design parameters more closely related to the objective functions than the original design parameters. The proposed MOGP is applied to the test problem and the practical design problem to evaluate the capability. © 2013 Springer-Verlag.

DOI： 10.1007/978-3-642-37140-0_40

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ACADEMIC PRESS INC ELSEVIER SCIENCE  

DOI： 10.1016/j.jcp.2012.08.031

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST AERONAUTICS ASTRONAUTICS  

The mechanism driving the self-sustained oscillations in supersonic laminar cavity flows is studied to be a feedback-loop mechanism between the discrete vortices and acoustic disturbances. Implicit large eddy simulations (ILESs) are conducted. One typical feedback cycle is visualized with phase-averaged flowfields. The feedback compression waves are radiated from the region near the cavity trailing lip. Their generation is related to the passage of large-scale vortices over the trailing edge. In phase of acoustic excitation near the cavity leading edge, the incoming boundary layer rolls up into two well-originated vortices with highly two-dimensional characteristics and strong spanwise coherence. Vortex pairing seems to occur between these two discrete vortices. Phase averaging is shown to be a superior approach for the analysis of cavity oscillations.

DOI： 10.2514/1.J051422

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Language：English   Publishing type：Research paper (scientific journal)  

In this study, the flowfields around NACA0012 and NACA0002 airfoils at Reynolds number of 23,000 and the aerodynamic characteristics of these flowfields were analyzed using implicit large-eddy simulation and laminar-flow simulation. Around this Reynolds number, the flow over an airfoil separates, transits, and reattaches, resulting in the generation of a laminar separation bubble at the angle of attack in a certain degree range. Over an NACA0012 airfoil, the separation point moves toward its leading edge with an increasing angle of attack, and the separated flow may transit to create a short bubble. On the other hand, over an NACA0002 airfoil, the separation point is kept at its leading edge, and the separated flow may transit to create a long bubble. Moreover, nonlinearity appears in the lift curve of the NACA0012 airfoil, but not in that of NACA0002, despite the existence of a laminar separation bubble. Copyright © 2012 by Ryoji Kojima, Taku Nonomura, Akira Oyama, and Kozo Fujii.

DOI： 10.2514/1.C031849

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：THE JAPAN SOCIETY FOR AERONAUTICAL AND SPACE SCIENCES  

In this study, aerodynamic design exploration for reusable launch vehicle (RLV) is conducted using genetic algorithm with Navier-Stokes solver to understand the aerodynamic characteristics for various body configurations and find design information such as tradeoff information among objectives. The multi-objective aerodynamic design optimization for minimizing zero-lift drag at supersonic condition, maximizing maximum lift-to-drag ratio (<i>L/D</i>) at subsonic condition, maximizing maximum <i>L/D</i> at supersonic condition, and maximizing volume of shape is conducted for bi-conical shape RLV based on computational fluid dynamics (CFD). The total number of evaluation in multi-objective optimization is 400, and it is necessary for evaluating one body configuration to conduct 8 CFD runs. In total, 3200 CFD runs are conducted. The analysis of Pareto-optimal solutions shows that there are various trade-off relations among objectives clearly, and the analysis of flow fields shows that the shape for the minimum drag configuration is almost the same as that of the shape for the maximum <i>L/D</i> configuration at supersonic condition. The shape for the maximum <i>L/D</i> at subsonic condition obtains additional lift at the kink compared with the minimum drag configuration. It leads to enhancement of <i>L/D</i>.

DOI： 10.2322/tastj.10.Pe_55

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Language：English   Publishing type：Research paper (scientific journal)  

The scalar tuning of a compressible fluid solver for a supercomputer with a distributed memory architecture is conducted. We use the K computer which is one of the peta-scale supercomputers recently developed in Japan. A computational code "LANS3D" and its high-order compact differencing option are tuned. The original version of the code achieves approximately 4.5% of full performance of CPU for the simple test case. Scalar tuning based on combining do-loops works well, and the tuned code attains about 10% of full performance for the same case. The reasons are the improvement in the use of the cache, the suppression of the data transfer, and the efficient use of the data that once transferred to the cache from the memory that results in hiding the low speed of data transfer. The tuned code becomes twice faster than the original one in the wall-clock time and enables us to perform over-160-case parametric study about airfoil flow computation by large-eddy simulations with high-order accurate and high resolution numerical scheme. © 2013 All rights reserved. ISSR Journals.

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：UNIVELT INC  

We have analyzed the asymmetric separation flow over a slender body at high angle of attack by numerical simulations aiming a control of the asymmetric vortices using a dielectric barrier discharge (DBD) plasma actuator. The Reynolds Averaged Navier Stokes/Large-Eddy Simulation hybrid method (RANS/LES) was adopted with the high-order compact spatial difference scheme. First, for investigating the characteristics of the asymmetric separation flow, the simulation of the flow field over the slender body was conducted for various angle of attack and bump height. Note that the bump is added near the body apex to simulate the symmetry-breaking imperfection. When the angle of attack or the bump becomes higher, the asymmetricity of vorticities becomes stronger. The side force has nonlinearity with the angle of attack or the bump height. Next, numerical simulations of the flow control using a plasma actuator were conducted. The side force coefficient can be continuously controlled in response to output power of the actuator within about +/- 1.0 on an average by the actuator's actuation at the aft body. However, the flow control effect is totally difference between starboard-side actuator's actuation and port-side actuator actuation. In addition, it is strongly influenced by the angle of attack.

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Other Link： http://orcid.org/0000-0001-7739-7104

Language：English   Publisher：THE JAPAN SOCIETY FOR AERONAUTICAL AND SPACE SCIENCES  

We analyze the asymmetric vortices in the flow over a slender body at high angle of attack by numerical simulations aiming a proportional control of the side forces generated by vortices with a device such as dielectric barrier discharge (DBD) plasma actuator. With regard to the computational method, Reynolds averaged Navier Stokes/large-eddy simulation hybrid method is adopted with high-order compact spatial difference scheme. The grid convergence analysis is firstly conducted and the results show that the computational grid adopted in this study is fine enough for qualitative discussion. The total number of the grid point is 411 million points. Then, the effects of bump height on flow fields and aerodynamic characteristics are discussed. Note that bump is added near the body apex to simulate the symmetry-breaking imperfection. As a higher bump is adopted, stronger asymmetry is observed in the flow fields. On the other hand, side-force has nonlinearity with the bump height.

DOI： 10.2322/tastj.10.Pe_89

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ACADEMIC PRESS INC ELSEVIER SCIENCE  

A weighted compact nonlinear scheme (WCNS) is applied to numerical simulations of compressible multicomponent flows, and four different implementations (fully or quasi-conservative forms and conservative or primitive variables interpolations) are examined in order to investigate numerical oscillation generated in each implementation. The results show that the different types of numerical oscillation in pressure field are generated when fully conservative form or interpolation of conservative variables is selected, while quasi-conservative form generally has poor mass conservation property. The WCNS implementation with quasi-conservative form and interpolation of primitive variables can suppress these oscillations similar to previous finite volume WENO scheme, despite the present scheme is finite difference formulation and computationally cheaper for multi-dimensional problems. Series of analysis conducted in this study show that the numerical oscillation due to fully conservative form is generated only in initial flow fields, while the numerical oscillation due to interpolation of conservative variables exists during the computations, which leads to significant spurious numerical oscillations near interfaces of different component of fluids. The error due to fully conservative form can be greatly reduced by smoothing interface, while the numerical oscillation due to interpolation of conservative variables cannot be significantly reduced. The primitive variable interpolation is, therefore, considered to be better choice for compressible multicomponent flows in the framework of WCNS. Meanwhile better choice of fully or quasi-conservative form depends on a situation because the error due to fully conservative form can be suppressed by smoothed interface and because quasi-conservative form eliminates all the numerical oscillation but has poor mass conservation. (C) 2012 Elsevier Inc. All rights reserved.

DOI： 10.1016/j.jcp.2011.12.035

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Publisher：Civil-Comp Proceedings  

A numerical study of the effects of airfoil shape on low Reynolds number aerodynamics is presented. The large-eddy simulations are performed with 6 ^th-order compact finite difference scheme and 10^th-order low pass filter, and 2^nd-order backward implicit time integration with inner iterations. Systematic numerical excesses show the feasibility of the current simulations to predict flow fields around fixed-wing configurations involving a laminar separation and laminar-to-turbulence transition at low Reynolds number. At the Reynolds number of 2.3×10⁴, two types of thin and asymmetric airfoils as a target airfoil shape of micro-size air vehicle are considered. The results show that the airfoil cross section affects the formation of a laminar separation bubble and the transition to turbulence in the three-dimensional flow around the wings at low angle of attack and hence significant influence on the aerodynamic performance. © Civil-Comp Press, 2012.

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：HINDAWI PUBLISHING CORPORATION  

We conducted large eddy simulations of the control of separated flow over an airfoil using body forces and discuss the role of a three-dimensional vortex structure in separation control. Two types of cases are examined: (1) the body force is distributed in a spanwise uniform layout and (2) the body force is distributed in a spanwise intermittent layout, with three-dimensional vortices being expected to be generated in the latter cases. The flow fields in the latter cases have a shorter separation bubble than those in the former cases although the total momentum of the body force in the latter cases is the same as or half of the former cases. In the flow fields of the latter type, the three-dimensional vortices, which are not observed in the former cases, are generated by the body force downstream of the body force distributed. Thus, three-dimensional vortices are considered to be effective in controlling the separated flow. Copyright © 2012 Ittetsu Kaneda et al.

DOI： 10.1155/2012/786960

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Language：English   Publisher：THE JAPAN SOCIETY FOR AERONAUTICAL AND SPACE SCIENCES  

Asymmetric separation vortices over a slender body at a high angle of attack exert a strong side force on the body and lead to the loss of attitude stability. We investigated the active control of the separation flow over a slender body and addressed the proportional control of the side force and the pitching moment. A flow control experiment was conducted in a wind tunnel using a cone-cylinder test body and a Dielectric Barrier Discharge (DBD) plasma actuator as a flow control device. The free-stream velocity was 9 m/s and the Reynolds number was approximately 42000. The side force coefficient was proportionally controlled within approximately ±1.0 using the actuator at the aft body, and the static stability angle of attack was controlled from 25 to 40 degrees and 65 to 85 degrees by controlling the pitching moment when the center of gravity was at the 55% position from the body apex. We estimated that a higher actuator output power is required for the effective control of the aerodynamics in a real flight. In addition, we confirmed that the actuator burst operation mode could reduce the required output power.

DOI： 10.2322/tastj.10.Pe_97

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST AERONAUT ASTRONAUT  

For predicting acoustic waves emitted from a rocket plume, the overexpansion effects on Mach 3.0 supersonic jet acoustics are investigated using an implicit large eddy simulation. A Mach 2.0 supersonic free jet is computed for code validation, and the results show qualitatively good agreement with the experiments. Then, computations of three different jets (design Mach numbers 3.0, 3.5, and 4.0 with fully expanded jet Mach number 3.0) are conducted, and nondimensionalizations based on design parameters and fully expanded parameters are discussed. Acoustic far-field spectra show that nondimensionalization based on fully expanded parameters works well for the high-Mach-number overexpanded jets, as it does for the low-Mach-number underexpanded jets that were investigated in previous studies. This nondimensionalization improves the accuracy of prediction of the acoustic waves emitted from rocket plumes because one parameter, the design Mach number, can be neglected for acoustic far fields. In addition, actual overexpansion effects after nondimensionalization are discussed. A comparison of the near flowfields and acoustic fields shows that Mach wave sources move upstream because of the existence of Mach disks, which enhances shear-layer mixing. Meanwhile, the overexpanded jet, which possesses only a shock cell without Mach disks, exhibits the same Mach wave generation characteristics as an ideally expanded jet.

DOI： 10.2514/1.J051054

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SAGE PUBLICATIONS INC  

This paper presents a computational study of the flow and flow-induced acoustic fields of a supersonic jet impinging on an inclined flat plate. For the numerical simulations, we solved three-dimensional compressible Navier-Stokes equations with a modified weighted compact nonlinear scheme. We analyzed the simulation results mainly from the viewpoint of the acoustic emission and propagation mechanism, and we investigated the acoustic field characteristics such as directivity, their spectra, and acoustic wave source positions. The acoustic fields indicate that there are at least three types of acoustic waves in all the cases considered in the study: (i) Mach waves generated from the shear layer of the main jet, (ii) acoustic waves generated from the impingement region, and (iii) Mach waves generated from the shear layer of the supersonic flow downstream of the jet impingement. The indication of the second type of wave (ii) is important because the commonly used empirical method for the estimation of the acoustic waves from a rocket plume does not consider such acoustic waves. We also discussed the effects of nozzle-plate distance and temperature on the second type of acoustic waves (ii).

DOI： 10.1260/1475-472X.10.4.401

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：JOHN WILEY & SONS LTD  

This paper gives an overview of the numerical simulations for the analysis of strong nonlinear acoustic waves during rocket development, with the emphasis on recent ones carried out using large-scale supercomputers. After the discussion on the difficulties encountered in such simulations, a computational study of blast wave propagation conducted for estimating the safety distance of a rocket is presented. The study was conducted about 20 years ago and the result showed the advantages of the moving grid method as well as the importance of grid resolution studies. A recent study on rocket plume acoustics is then presented. The result shows that the generation and propagation of Mach waves from the plume shear layers are key features to be captured. Direct simulations of such flows have now become feasible owing to the developments in computers and numerical schemes. Then, problems that still remain unsolved are discussed. Our study so far has been limited to simulations using structured grids of high spatial resolution although direct simulations of strongly nonlinear acoustic waves are becoming feasible. More studies have to be carried out for developing highly accurate schemes for unstructured grid systems for the applications to flow configurations over complex geometries. With such improvements, computational fluid dynamics (CFD) would become a still better effective tool for the analysis and estimation of nonlinear acoustic phenomena, especially in aerospace applications. © 2010 John Wiley & Sons, Ltd.

DOI： 10.1002/fld.2446

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Publisher：Journal of Aircraft  

A new approach to extract useful design information from the shape data of Pareto-optimal solutions of an optimization problem is proposed and applied to the optimization of airfoil shapes for good aerodynamic performance at transonic speed. The proposed approach decomposes shape data into principal modes and corresponding base vectors, using proper orthogonal decomposition. The advantageofthe proposed approachisthat the knowledge one can obtain does not depend onhow the shape is parameterized for design optimization. Analysis of the airfoil shapes obtained as the Pareto-optimal solutions for aerodynamic performance at transonic speeds shows that the optimized airfoils can be categorized into three families (low-drag designs, high-lift-to-drag designs, and high-lift designs), where the lift is increased by changing the camber near the trailing edge among the low-drag designs, while the lift is increased by moving the lower surface upward among the high-lift designs. Copyright © 2010 bythe American Institute of Aeronautics and Astronautics, Inc.

DOI： 10.2514/1.C000264

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DOI： 10.2514/1.C000264

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST AERONAUT ASTRONAUT  

A study was conducted to investigate the computational analysis of Mach number effects on the Edgetone phenomenon. It was demonstrated that the feedback loop of the edgetone was verified by investigating the effect of Mach number. It was observed that the Strouhal number Sr of the peak frequency of the same mode decreased with increasing jet Mach number when the feedback-loop equation in non-dimensional form was correct. It was easy to change the jet Mach number in computational studies, while keeping the Reynolds number fixed. The computational approach was more appropriate to verify the feedback-loop mechanism. The edgetone phenomena of a two-dimensional laminar jet and a triangle wedge were also computed in the study at Mach numbers ranging from 0.087 to 0.435 at three fixed Reynolds numbers, such as 208, 416, and 624.

DOI： 10.2514/1.44849

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

Freestream and vortex preservation properties of a weighted essentially nonoscillatory, scheme (WENO) and a weighted compact nonlinear scheme (WCNS) on curvilinear grids are investigated. While the numerical technique used for the compact difference scheme can be applied to WCNS, applying it to WENO is difficult. This difference is caused by difference in the formulation of numerical fluxes. WENO computed in the generalized coordinate system does not work well for either freestream or vortex preservation, whereas WENO computed in the Cartesian coordinate system works well for both freestream and vortex preservation, but its resolution is lower than that of WCNS. In addition, WENO in the Cartesian coordinate system costs three times as much as WENO or WCNS in the generalized coordinate system. Therefore, WENO in the Cartesian coordinate system is not suitable for solving Euler equations on a curvilinear grid. On the other hand, WCNS computed in the generalized coordinate system works well for freestream and vortex preservation when used with the numerical technique proposed for the compact difference scheme. The results show that WCNS with this numerical technique can be used for an arbitrary grid system. In this paper, the excellent freestream and vortex preservation properties of WCNS when used with the numerical technique, compared with those of WENO, are shown for the first time. (C) 2009 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.compfluid.2009.08.005

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ACADEMIC PRESS INC ELSEVIER SCIENCE  

DOI： 10.1016/j.jcp.2009.02.018

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ACADEMIC PRESS INC ELSEVIER SCIENCE  

DOI： 10.1016/j.jcp.2009.01.032

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