Faculty Profiles - NAKAMURA Tomoaki

Papers - NAKAMURA Tomoaki

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Two-dimensional numerical analysis of wave-induced instability of artificial shallow composed of dredged soil Reviewed

Nakamura, T., Ishihara, R., and Mizutani, N.

Proc., Coastal Structures 2011, ASCE page： 12 2011.9

A nonlinear three-dimensional coupled fluid-sediment interaction model for large seabed deformation Reviewed

Nakamura, T. and Yim, S. C.

Journal of Offshore Mechanics and Arctic Engineering, ASME Vol. 133 ( 3 ) page： 14 2011.8

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

A nonlinear three-dimensional two-way coupled fluid-sediment interaction model is developed in this study. The model is composed of a generalized Navier-Stokes solver (GNS) with a volume of fluid module for air-water interface tracking and a sediment transport module (STM) for fluid-sediment interface tracking. The GNS model is based on the finite difference method with a turbulent stress model of large-eddy simulation to compute incompressible viscous multiphase flows. The STM is used to compute nonlinear sediment bed profile change due to bed-load sediment transport. A two-way coupling scheme connecting GNS with STM is implemented at each time step to ensure the fluid-sediment interaction. For validation, the fluid-sediment interaction model is applied to predict cross-shore profile change of a sloping beach due to breaking solitary waves, and the resulting predictions are examined and compared with the measured data from a set of hydraulic tests. It is found that the fluid-sediment interaction model predicts reasonably well the sediment transport and the resulting beach profile change. The sensitivity of model parameters involving the sediment transport to the beach profile change is analyzed. Finally, the fluid-sediment interaction model is applied to predict local scour in front of a quay wall due to a jet flow to demonstrate its applicability to general threedimensional problems.

Three-dimensional coupled fluid-sediment interaction numerical model for suspended sediment analysis Reviewed

Nakamura, T. and Mizutani, N.

Proc., 21th International Offshore and Polar Engineering Conference page： 1032-1039 2011.6

Authorship：Lead author Language：English

A three-dimensional two-way coupled fluid-sediment interaction model applicable to suspended sediment analysis is developed in this study. For validation, the developed model is applied to hydraulic experiments on suspended sediment in a steady uniform flow and a swash zone. For both flow conditions, the predictive capability of the developed model is demonstrated against experimental data in terms of time-averaged suspended sediment concentration. Numerical results for the swash zone show that complex suspended sediment transport relates with three-dimensional vortex motion due to plunging breaking waves, and demonstrate the usefulness of the developed model in investigating suspended sediment transport under breaking waves.

Field survey for 2010 Chilean earthquake and tsunami disaster in ports and coasts Reviewed

Takahashi, S., Sugano, T., Tomita, T., Arikawa, T., Tatsumi, D., Kashima, H.,Murata, S., Matsuoka, Y., and Nakamura, T.

Proc., 8th International Conference on Urban Earthquake Engineering page： 8 2011.3

Three-dimensional numerical analysis of jet-induced local scouring in front of quay walls and its countermeasure using filter units Reviewed

Nakamura, T., Shinoda, Y., Mizutani, N., and Koyama, H.

Coastal Engineering Journal, JSCE Vol. 53 ( 1 ) page： 41-62 2011.3

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

A bow thruster mounted in the bow of a vessel is generally used to improve its maneuverability in ports. When a bow thruster is used near a berth, a jet generated from the bow thruster can cause sediment transport, resulting in local scouring of the seabed in front of quay walls. Countermeasures against such scouring are essential to ensure the stability of the quay walls. In recent years, new protective units called filter units have been used to protect seawalls and river dikes. However, it remains to be determined whether filter units covering the seabed in front of quay walls are effective in preventing jet-induced local scouring. In this study, jet-induced local scouring in front of a quay wall and its countermeasure using filter units are investigated using a three-dimensional two-way coupled fluid-sediment interaction model. The predictive capability of the three-dimensional coupled model is verified against experimental data in terms of average jet flow velocity, pore-water pressure, and the final seabed profile. The results show that the filter units covering the seabed prevent negative effective stress fluctuations on the surface layer of the seabed, and demonstrate the effectiveness of the countermeasure using the filter units.

Development of three-dimensional coupled fluid-structure-sediment interaction model and its application to local scour around submerged object Reviewed

Nakamura, T., Yim, S. C., and Mizutani, N.

Journal of Coastal Engineering, JSCE Vol. 57 page： 406-410 2010.11

Authorship：Lead author Language：Japanese Publishing type：Research paper (scientific journal)

A three-dimensional coupled fluid-structure-sediment interaction model (FSSM) is developed in this study. FSSM consists of an extended Navier-Stokes solver (XNS) with a volume of fluid module (VOF) for air-water interface tracking, an immersed boundary module (IBM) for structure motion tracking, and a sediment transport module (STM) for sediment surface tracking and suspended sediment tracking. In FSSM, a two-way coupling procedure is implemented in each time step to connect XNS with VOF, IBM, and STM. FSSM is applied to wave-induced local scour around a submerged movable short cylinder and resulting burial of the cylinder. The computational capability of FSSM is demonstrated against experimental data in terms of seabed change around the cylinder.

Three-dimensional numerical analysis on stability of superficial sediments of artificial shallows in wave-induced pressure field Reviewed

Nakamura, T., Shinoda, Y., and Mizutani, N.

Journal of Coastal Engineering, JSCE Vol. 57 page： 911-915 2010.11

Authorship：Lead author Language：Japanese Publishing type：Research paper (scientific journal)

The stability of superficial sediments of an actual artificial shallow constructed in a pilot project is investigated by using a three-dimensional coupled fluid-sediment interaction model in terms of the Shields parameter (SP) and a relative vertical effective stress ratio (RESR) in the surface layer of the shallow. The wave action causes the erosion at the seaward edge of the crown, the upper part of the seaward ridge, and the center of the landward slope, and the deposition at their adjacent areas. These topographic changes are related to large values of the averaged SP, while the values of RESR around the eroded areas are larger than those at the other areas, resulting in the acceleration of the sediment transport. The topographic change of the shallow is evaluated more reasonably by using a SP modified with RESR.

Three-dimensional numerical analysis on deformation of run-up tsunami and tsunami force acting on square structures Reviewed

Nakamura, T., Mizutani, N., and Fujima, K.

Proc., International Conference on Coastal Engineering, ASCE Vol. 32 page： 10 2010.7

Authorship：Lead author Language：English

A three-dimensional two-way coupled fluid-sediment interaction model (FSM) is applied to investigate run-up tsunami deformation and tsunami force acting on square structures on land. The FSM consists of a generalized Navier-Stokes solver (GNS) for multi-phase flow including porous flow, a volume of fluid module (VFM) for air-water interface tracking, and a sediment transport module (STM) for fluid-sediment interface tracking. In the FSM, a two-way coupling procedure is implemented at each time step to connect the GNS with the VFM and the STM. The predictive capability of the FSM is demonstrated through comparison between numerical results and experimental data in terms of water surface elevation, inundation depth, and tsunami force. The process of tsunami run-up in the presence of square structures is investigated in terms of vortex structures. The result shows that the FSM is a useful tool providing detailed information in discussing run-up tsunami deformation and tsunami force.

Reflection and run-up of tsunami on quay wall and behavior of drifted vessels due to tsunami Reviewed

Mizutani, N., Nakamura, T., and Koike, T.

Proc., International Conference on Coastal Engineering, ASCE Vol. 32 page： 10 2010.7

Two-dimensional numerical analysis on effects of flow velocity and water pressure on stability of artificial shallows composed of dredged soils Reviewed

Nakamura, T., Ishihara, R., and Mizutani, N.

Annual Journal of Civil Engineering in the Ocean Vol. 26 page： 699-704 2010.7

Authorship：Lead author Language：Japanese Publishing type：Research paper (scientific journal)

The stability of superficial sediments of an artificial shallow composed of dredged soils is investigated using a three-dimensional coupled fluid-sediment interaction model in terms of the Shields parameter (SP), the relative effective stress ratio (RESR), and a SP modified with RESR (MSP). Topographic change reducing the stability of the shallow relates to large values of the averaged SP (ASP). In predicting ASP, the fluid-sediment interaction is essential because SP varies with the topographic change. In addition, the stability of the shallow can be evaluated appro-priately using MSP because it is possible that the sediment mobility increases with RESR. The effectiveness of a coarse-sand layer covering the shallow is demonstrated to improve the stability of the shallow.

Development of three-dimensional coupled fluid-sediment interaction model for suspended sediment analysis and its application Reviewed

Nakamura, T. and Mizutani, N.

Annual Journal of Civil Engineering in the Ocean Vol. 26 page： 1197-1202 2010.7

Authorship：Lead author Language：Japanese Publishing type：Research paper (scientific journal)

A three-dimensional two-way coupled fluid-sediment interaction model for suspended sediment analysis (FSM) is developed in this study. FSM is composed of a generalized Navier-Stokes solver with a volume of fluid module for air-water interface tracking and a sediment transport module (STM) for sediment surface tracking. STM is extended to analyze advection-diffusion problems of suspended sediment concentration. FSM is applied to predict vertical profiles of suspended sediment concentration in a steady uniform flow, and a model parameter involving suspended sediment is determined to minimize the root mean square error of the equilibrium concentration profiles. FSM is also applied to predict suspended sediment concentration due to plunging breaking waves, and the transport process of suspended sediment in the wave action is investigated in terms of three-dimensional vortex structures. The pre-dictive capability of FSM is demonstrated against experimental data for both phenomena.

Three-dimensional numerical simulation on deformation of run-up tsunami and tsunami force acting on structures Reviewed

Nakamura, T., Mizutani, N., and Fujima, K.

Nagare, JSFM Vol. 29 ( 2 ) page： 107-117 2010.4

Authorship：Lead author Language：Japanese Publishing type：Research paper (scientific journal)

A three-dimensional two-way coupled fluid-sediment interaction model (FSM) is applied to tsunami run-up around fixed impermeable square structures on a coastal area. FSM is composed of a generalized Navier-Stokes solver (GNS) to compute incompressible viscous air-water multi-phase flow including porous flow inside porous media, a volume of fluid module (VOF) to track air-water interface motion, and a sediment transport module (STM) to track fluid-sediment interface motion due to bed-load sediment transport. In FSM, STM is incorporated into GNS with VOF using a two-way coupling scheme to ensure the fluid-sediment interaction. To verify the validity of FSM, numerical results computed using FSM are analyzed and compared with experimental data measured in hydraulic tests. The predictive capability of FSM is demonstrated in terms of water surface elevations in the shallow water region, inundation depth on the land, and tsunami force acting on the structures. The vortex structure computed using FSM is visualized based on the 2 definition to investigate the process of the tsunami run-up.

Collision analysis of container drifted by runup tsunami using drift collision coupled model Reviewed

Yeom, G.-S., Nakamura, T., and Mizutani, N.

Journal of Disaster Research Vol. 4 ( 6 ) page： 441-449 2009.12

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

This study proposes analyzing tsunami generation and propagation of tsunamis, wave runup onto land, object drift, collision with other structures, deformation prediction of colliding and collided-with, and collision force estimation. To reduce calculation load in using the LS-DYNA collision model, we use a drift model using the immersed boundary (IB) method from wave generation to just before collision, and the collision model is used during the collision phase, in which numerical data calculated using the drift model are used as an initial condition of the collision model. Validity of the drift model for wave level, wave force, and drift behavior of a container were verified through comparison with experimental data measured in laboratory experiments. Collision model predictability was also confirmed in terms of drift collision force. Fluid-structure interaction (FSI) between the container and the runup wave is reproduced in numerical drift collision analysis. Through this analysis of a full-scale container based on the international standard with a concrete column, we confirmed the applicability of the collision analysis using the drift simulation as an initial condition for an actual field.

Three-dimensional numerical analysis on local scour in front of quay wall due to jet flow and its countermeasures Reviewed

Nakamura, T., Mizutani, N., Shinoda, Y., and Koyama, H.

Journal of Coastal Engineering, JSCE Vol. 56 page： 486-490 2009.11

Authorship：Lead author Language：Japanese Publishing type：Research paper (scientific journal)

Local scour in front of a quay wall due to a jet flow is investigated using a three-dimensional two-way coupled fluid-sediment interaction model (FSM). Numerical results computed using FSM are analyzed and compared with experimental data measured in hydraulic tests. A model parameter involving the sediment transport and the boundary condition of the wall are determined in such a way that the root mean square error of the final seabed profile is minimized for a reference case. The predictive capability of FSM is validated against experimental data in terms of average flow velocity along the jet flow, excess pore-water pressure in the seabed, and final seabed profiles. The effectiveness of filter units against local scour and resulting instability of the wall is demonstrated in terms of excess pore-water pressure predicted using FSM.

Study on reflection and run-up of tsunami due to quay and behavior of vessels Reviewed

Mizutani, N., Koike, T., Nakamura, T., and Koyasu, Y.

Journal of Coastal Engineering, JSCE Vol. 56 page： 841-845 2009.11

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

This paper investigates the reflection of a tsunami due to a quay wall and the behavior of a drifted vessel due to the tsunami by using hydraulic experiments and a numerical model based on MARS with the body force type of IB method. A reflection coefficient is evaluated by considering the difference between water surface elevations in the presence of the wall and those in the absence of the wall. It is found that the height of the wall has a great influence on the reflection, and hence the tsunami run-up. When the vessel is run up over the wall, it is drifted at the velocity faster than the fluid velocity. On an apron behind the wall, it is drifted at the almost same velocity as the fluid velocity. It is also revealed that the vessel can be run up on the apron when the water depth at the wall is larger than the draft of the vessel.

A three-dimensional coupled fluid-sediment interaction model with bed-load/suspended-load transport for scour analysis around a fixed structure Reviewed

Nakamura, T., Mizutani, N., and Yim, S. C.

Journal of Offshore Mechanics and Arctic Engineering, ASME Vol. 131 ( 3 ) page： 9 2009.8

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

The predictive capability of a three-dimensional (3D) numerical model for sediment transport and resulting scour around a structure is investigated in this study. Starting with the bed-load and suspended-load sediment transport (reference) model developed by Takahashi et al. (2000, "Modeling Sediment Transport Due to Tsunamis With Exchange Rate Between Bed Load Layer and Suspended Load Layer," Proceedings of the 27th International Conference on Coastal Engineering, ASCE, Sydney, Australia, pp. 1508-1519), we first introduce an extension to incorporate Nielsen's modified Shields parameter to account for the effects of infiltration/exfiltration flow velocity across the fluid-sand interface on the sediment transport (the modified Shields-parameter model). We then propose a new model to include the influence of the effective stress to account for the stress fluctuations inside the surface layer of the sand bed (the effective-stress model). The three analytical models are incorporated into a 3D numerical solver developed by Nakamura et al. (2008, "Tsunami Scour Around a Square Structure," Coast. Eng. Japan, 50(2), pp. 209-246) to analyze the dynamics of fluid-sediment interaction and scour. Their solver is composed of two modules, namely, a finite-difference numerical wave tank and a finite-element coupled sand-skeleton pore-water module. The predictive capability of the three alternative coupled models is calibrated against hydraulic experiments on sediment transport and resulting scour around a fixed rigid structure due to the run-up of a single large wave in terms of the sediment transport process and the final scour profile after the wave run-up. It is found that, among the three models considered, the proposed effective-stress model most accurately predicts the scouring process around the seaward corner of the structure. The results also reveal that the deposition and erosion patterns predicted using the effective-stress model are in good agreement with measu

Development of a three-dimensional coupled fluid-sediment interaction model and its application to erosion and scour phenomena Reviewed

Nakamura, T. and Yim, S. C.

Annual Journal of Civil Engineering in the Ocean, JSCE Vol. 25 page： 1227-1232 2009.7

Authorship：Lead author Language：Japanese Publishing type：Research paper (scientific journal)

A three-dimensional two-way coupled fluid-sediment interaction model (FSM) is developed in this study. FSM consists of a generalized Navier-Stokes solver (GNS) with the volume of fluid (VOF) module for water-air interface tracking and a bed-load sediment transport module (STM) for fluid-sediment interface tracking. To connect STM with GNS, a two-way coupling procedure is implemented at every time step. FSM is applied to predict cross-shore beach profile change due to solitary waves, and its predictive capability is validated against experimental data. The sensitivity of model parameters involving the sediment transport on the profile change is analyzed. FSM is applied to predict local scour in front of a quay wall due to a jet flow to demonstrate its applicability to three-dimensional phenomena.

Local scour in front of quay wall caused by bow thruster and its countermeasure using filter units Reviewed

Mizutani, N., Nakamura, T., Shinoda, Y., and Koyama, H.

Proc., 19th International Offshore and Polar Engineering Conference, ISOPE page： 1280-1287 2009.6

Language：English

This study is intended to investigate local scour in front of a quay wall and its generation mechanism. Hydraulic model experiments have been conducted to investigate the local scour in front of quay wall. Also, countermeasures against local scour using armor stones and filter units are investigated based on the experiment. To consider the mechanism of local scour, numerical simulation has been conducted. The numerical computation method employed in this study includes a flow simulation inside seabed and dynamic response of seabed like sediment transport. Time variation of pore pressure obtained in the experiments indicates the generation of local liquefaction due to strong flow and its response of seabed, and this may encourage the local scour. Experimental results clearly show that armoring by filter units is effective to reduce the local scour.

Mechanism of backfilling sand discharge from a gap under a vertical revetment Reviewed

Nakamura, T., Hur, D.-S., and Mizutani, N.

Journal of Waterway, Port, Coastal, and Ocean Engineering, ASCE Vol. 134 ( 3 ) page： 178-186 2008.11

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

Coastal disasters such as sliding, settlement, and overturning of coastal structures as well as subsidence of reclaimed areas occur due to toe scouring and backfilling sand discharge. In this study, the sand discharge mechanism from a gap under a vertical revetment without wave absorption and foot protection is investigated using hydraulic model experiments and newly developed numerical simulations. In the hydraulic model experiments, sand discharge and sinkholes similar to those observed in the field surveys are reproduced, and it is found that sand discharge is very sensitive to wave steepness and relative gap height. In numerical simulations, a comparison with the experimental data demonstrates the validity of the numerical methods. It is revealed that sand discharge tends to occur with an increase of the maximum seaward velocity just outside the gap under the revetment as well as the maximum volumetric strain inside the most offshore side of the sand inside the gap.

Study on estimation of collision force of a drifted container using fluid-structure interaction analysis Reviewed

Yeom, G.-S., Nakamura, T., Usami, A., and Mizutani, N.

Annual Journal of Coastal Engineering, JSCE Vol. 55 page： 281-285 2008.11

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

This study proposes a calculation scheme which enables to estimate the collision time and force of drifted container by run-up tsunami. Simulated behaviors of the drifted container by a run-up wave are found to be in good agreement with the laboratory experimental results. A set of this calculated values are used as the initial values of the fluid-structure interaction analysis which simulates the collision of the drifted container. It is confirmed from these simulations that the added mass (water mass at the rear of the container) plays an important role on the collision and the collision force and time become constant when the adequate volume of the water at the rear of the drifting container is considered. Moreover, the effect of the added mass for the collision force and time is verified in full scale collision simulation.

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Papers - NAKAMURA Tomoaki