Faculty Profiles - CHO Yonghwan

Papers - CHO Yonghwan

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Mechanism of tsunami-induced erosion of bridge-abutment backfill and its countermeasures Reviewed International journal

Nakamura Tomoaki, Sugiyama Chisato, Cho Yong-Hwan, Mizutani Norimi

WATER Vol. 13 ( 24 ) 2021.12

Numerical Reproducibility of Wave Response for an Oscillating Wave Surge Converter Using Inverted Triangle Flap Reviewed International coauthorship International journal

Kim, T.-G, Kim, D.-S., Cho, Y.-H., Lee, K.-H.

Journal of Korean Society of Coastal and Ocean Engineers Vol. 33 ( 5 ) page： 203 - 216 2021.10

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

Analyzing various wave interactions with oscillating wave surge converters (OWSC) is essential because they must be operated efficiently under a wide range of wave conditions and designed to extract optimal wave energy. In the conceptual design and development stage of OWSC, numerical analysis can be a good alternative as a design tool. This study performed a numerical analysis on the behavioral characteristics of the inverted triangle flap against the incident waves using open source CFD to examine the essential behavioral attributes of OWSC. Specifically, the behavioral characteristics of the structure were studied by calculating the free water surface displacement and the flap rotation angle near the inverted triangular flap according to the change of the period under the regular wave conditions. By comparing and examining the numerical analysis results with the hydraulic model experiments, the validity of the analysis performed and the applicability in analyzing the wave-structure interactions related to OWSC was verified. The numerical analysis result confirmed that the hydrodynamic behavior characteristic due to the interactions of the wave and the inverted triangle flap was well reproduced.

DOI： 10.9765/KSCOE.2021.33.5.203

Motion of floating caisson with extended bottom slab under regular and irregular waves Reviewed International journal

Nakamura, T., Nagayama, N., Cho, Y.-H., Mizutani, N., Kurahara, Y., Takeda, M.

Journal of Marine Science and Engineering Vol. 9 ( 10 ) page： 1129 2021.10

Simple Breaker Index Formula Using Linear Model Reviewed International coauthorship International journal

Lee Kwang-Ho, Cho Yong-Hwan

JOURNAL OF MARINE SCIENCE AND ENGINEERING Vol. 9 ( 7 ) page： 731 2021.7

Language：English Publishing type：Research paper (scientific journal) Publisher：Journal of Marine Science and Engineering

Breaking waves generated by wave shoaling in coastal areas have a close relationship with various physical phenomena in coastal regions. Therefore, it is crucial to accurately predict breaker indexes such as breaking wave height and breaking depth when designing coastal structures. Many studies on wave breaking have been carried out, and many experimental data have been documented. Representative studies on wave breaking provide many empirical formulas for the prediction of breaking index, mainly through hydraulic model experiments. However, the existing empirical formulas for breaking index determine the coefficients of the assumed equation through statistical analysis of data under the assumption of a specific equation. This study presents an alternative method to estimate breaker index using representative linear-based supervised machine learning algorithms that show high predictive performance in various research fields related to regression or classification problems. Based on the used machine learning methods, a new simple linear equation for the prediction of breaker index is presented. The newly proposed breaker index formula showed similar predictive performance compared to the existing empirical formula, although it was a simple linear equation.

DOI： 10.3390/jmse9070731

Web of Science

Scopus

Other Link： https://www.mdpi.com/2077-1312/9/7/731

Development of Numerical Model for Mixed Soil Problems Using Dry Bulk Density and Investigation of Its Numerical Stability Reviewed International coauthorship International journal

Journal of Korean Society of Coastal and Ocean Engineers Vol. 33 ( 3 ) page： 110 - 121 2021.6

STUDY ON SEA LEVEL RISE EFFECTS ON SHORELINE RETREAT ON A GRAVEL BEACH

FLORENTINA Sinta, NAKAMURA Tomoaki, CHO Yonghwan, MIZUTANI Norimi, TAKEUCHI Masayuki

Journal of Japan Society of Civil Engineers, Ser. B3 (Ocean Engineering) Vol. 77 ( 2 ) page： I_301 - I_306 2021

Language：English Publishing type：Research paper (scientific journal) Publisher：Japan Society of Civil Engineers

Climate change effects have become global concern through decades due to its emerging hazards. Sea Level Rise (SLR) is one of the effects which may cause harm to the coastal communities due to the risk of damages, such as shoreline retreat and beach erosion. There are many previous studies regarding the effect of SLR on sandy beaches. However, the study regarding gravel beach response under SLR is still rare. Therefore, this study is conducted to investigate the effect of SLR on the past shoreline change by using Bruun rule on currently eroded gravel beach, Ida beach in Shichiri-mihama coast, Mie Prefecture. By comparing the calculation result with the past shoreline change data obtained from network camera system, the proportion of SLR contribution in shoreline change can be assessed. Results shows that SLR contribution is ranging between 5.1 – 8.7% of the actual shoreline retreat from November 2017 to November 2019. This indicates that there are other factors such as extreme waves playing more dominant role in the shoreline change compared to SLR in this gravel beach.

DOI： 10.2208/jscejoe.77.2_i_301

CiNii Research

CHARACTERISTICS OF TOPOGRAPHIC CHAGE FOCUSED ON THE VARIATION OF DRY BULK DENSITY BY SLIP-OUT

AKAHOSHI Rei, CHO Yong-hwan, WATANABE Mikiya, NAKAMURA Tomoaki, MIZUTANI Norimi

Journal of Japan Society of Civil Engineers, Ser. B3 (Ocean Engineering) Vol. 77 ( 2 ) page： I_949 - I_954 2021

Language：Japanese Publishing type：Research paper (scientific journal) Publisher：Japan Society of Civil Engineers

We conducted numerical analysis on changes in dry bulk density, hydraulic experiments on changes in fine particle fraction under uniform flow. The effect of the change in density on the change characteristics of the topography consisting of mixed sediment was investigated. As a result, when considering the total suspended amount, in sandy mixed earth and sand, it was suggested that it is necessary to consider the increase in the total suspended amount due to the increase in the proportion of fine particles and the decrease in the suspended amount due to the increase in the adhesive strength in the adhesive mixed sediment and sand.

DOI： 10.2208/jscejoe.77.2_i_949

CiNii Research

EVALUATION OF EFFECT OF WALL BEHIND BOTTOM-HINGED OWSC UNDER REGULAR WAVES

SATO Harutaka, CHO Yonghwan, NAKAMURA Tomoaki, MIZUTANI Norimi

Journal of Japan Society of Civil Engineers, Ser. B3 (Ocean Engineering) Vol. 77 ( 2 ) page： I_223 - I_228 2021

Language：Japanese Publishing type：Research paper (scientific journal) Publisher：Japan Society of Civil Engineers

Two-dimensional hydraulic experiments were conducted using a reversed triangular bottom-hinged flap with the reflection wall behind it. Electricity generation performance when the power take-off system is installed was evaluated from the experimental result based on linear theory. Efficiency is lowest near the antinodes of the standing waves. There are some cases in which D/L when efficiency is highest is slightly different from nodes depends on wave period. Efficiency is high when the wave period is short and the water depth is small. There is a possibility that there is a specific wave period in which efficiency is extremely high. The increasing ratio of efficiency by installing the wall reached 6 at most and 2 at least.

DOI： 10.2208/jscejoe.77.2_i_223

CiNii Research

EXPERIMENTAL STUDY ON EFFECTIVENESS AND MECHANISM OF ANTI-OSCILLATION TANKS AGAINST MOTION OF FLOATING CAISSON

NAKAMURA Tomoaki, SHIRAI Kaito, CHO Yonghwan, MIZUTANI Norimi, KURAHARA Yoshinosuke, TAKEDA Masahide

Journal of Japan Society of Civil Engineers, Ser. B3 (Ocean Engineering) Vol. 77 ( 2 ) page： I_619 - I_624 2021

Language：Japanese Publishing type：Research paper (scientific journal) Publisher：Japan Society of Civil Engineers

The effectiveness and mechanism of flume-type anti-oscillation tanks against the motion of a floating caisson under regular waves was investigated from hydraulic experiments. From experimental results, it was found that the amplitude of heave was little affected by free water filled in the anti-oscillation tanks. In contrast, the maximum value of the amplitude of pitch tended to decrease with an increase in the mass of free water. From anti-oscillation moment due to free water estimated from image analysis, it was revealed that gravitational force due to the free water accounted for a substantial fraction of the anti-oscillation moment, while horizontal inertia force had an effect of increasing the amplitude of the anti-oscillation moment. Furthermore, analytical solutions of the amplitude of pitch estimated from the anti-oscillation moment agreed reasonably well with experimental results. This suggested that the mechanism of anti-oscillation tanks can be explained in the same way as in the field of naval architecture.

DOI： 10.2208/jscejoe.77.2_i_619

CiNii Research

A Proposal of New Breaker Index Formula Using Supervised Machine Learning Reviewed International coauthorship

Byung-Jong Choi, Chang-Wook Park, Yong-Hwan Cho, Do-Sam Kim, Kwang-Ho Lee

Journal of Korean Society of Coastal and Ocean Engineers Vol. 32 ( 6 ) page： 384 - 395 2020.12

Modeling Tsunami-Induced Erosion of Bridge-Abutment Backfill Reviewed International journal

Tomoaki Nakamura, Yuto Nakai, Yong-Hwan Cho and Norimi Mizutani

Journal of Marine Science and Engineering Vol. 8 ( 11 ) page： 1 - 16 2020.11

An Experimental Study of a Bottom-Hinged Wave Energy Converter with a Reflection Wall in Regular Waves-Focusing on Behavioral Characteristics Reviewed International coauthorship International journal

Cho Yong-Hwan, Nakamura Tomoaki, Mizutani Norimi, Lee Kwang-Ho

Applied Sciences Vol. 10 ( 19 ) page： 1 - 23 2020.10

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

The hybrid system of wave energy converters (WECs) using coastal structures is an attractive issue in terms of a decrease in construction costs and an improvement of the ability to capture wave energy. Most studies on the utilization of reflected waves from structures, which is one of the hybrid systems, are limited to mathematical analysis based on linear theories. Therefore, this paper presents fundamental experimental results in the presence of a reflection wall simplified as a coastal structure behind a bottom-hinged flap-type WEC under unidirectional regular waves. The behavioral characteristics and the power generation efficiency ke of the flap were investigated, focusing on wave steepness, initial water depth, and distance from the reflection wall. The results show that the condition of the initial water depth being smaller than the flap height is more effective in terms of avoiding unstable rotating of the flap. The maximum ke appeared slightly far from the node position of the standing waves because the flap shape and the power take-off (PTO) damping induce the phase difference between the reciprocating behavior of the flap and the period of the standing wave. The results imply that the optimum position of a WEC is dependent on WEC shape, PTO damping, and installation water depth.

DOI： 10.3390/app10196734

Web of Science

Scopus

Other Link： https://www.mdpi.com/2076-3417/10/19/6734

Experimental study on spatiotemporal profile change in gravel beach Reviewed International journal

Kim, S., Nakamura, T., Cho, Y., and Mizutani, N.

International Journal of Offshore and Polar Engineering Vol. 30 ( 3 ) page： 349 - 356 2020.9

Influence of Tidal Current, Wind, and Wave in Hebei Spirit Oil Spill Modeling International coauthorship International journal

Lee Kwang-Ho, Kim Tag-Gyeom, Cho Yong-Hwan

JOURNAL OF MARINE SCIENCE AND ENGINEERING Vol. 8 ( 2 ) 2020.2

STUDY ON TSUNAMI FORCE ACTING ON INLAND STRUCTURE BEHIND COASTAL PROTECTION FACILITY WITH RESILIENT STRUCTURE COMBINED WITH COASTAL DIKE AND EMBANKMENT

NAKAMURA Tomoaki, SUZUKI Ryota, CHO Yonghwan, MIZUTANI Norimi

Journal of Japan Society of Civil Engineers, Ser. B3 (Ocean Engineering) Vol. 76 ( 2 ) page： I_156 - I_161 2020

Language：Japanese Publishing type：Research paper (scientific journal) Publisher：Japan Society of Civil Engineers

Tsunami-induced force acting on an inland building located at the landward side of a coastal protection facility with resilient structure combined with a coastal dike and an embankment was investigated numerically. Numerical results showed that the relationship between the tsunami force considering the erosion of the embankment and that assuming the constant profile of the embankment was affected by the scale of the erosion. This suggested that it is essential to take into account the erosion process of an embankment in assessing tsunami force acting on inland buildings.

DOI： 10.2208/jscejoe.76.2_i_156

CiNii Research

BEACH DEFORMATION ANALYSIS BY COMBINING NETWORK CAMERA AND UAV-SFM/MVS SURVEY

CHO Yonghwan, NAKAYAMA Ryoya, KIKU Masami, NAKAMURA Tomoaki, MIZUTANI Norimi

Journal of Japan Society of Civil Engineers, Ser. B2 (Coastal Engineering) Vol. 76 ( 2 ) page： I_655 - I_660 2020

Language：Japanese Publishing type：Research paper (scientific journal) Publisher：Japan Society of Civil Engineers

This study describes the improvement of the image analysis of network camera using the orthomosaic created from UAV-SfM/MVS survey and the morphological characteristics of Shichiri-Mihama Ida beach based on the estimated shoreline positions extracted by a network camera and DEM obtained by UAV-SfM/MVS survey. The estimated shoreline extracted from the network camera image denotes the position in the range of 1~2 m above sea level by comparison with the DEM so that it was located on the land side from the actual shoreline. The application of the network camera including UAV-SfM/MVS survey makes it possible to identify the waves that caused the beach deformation between observations.

DOI： 10.2208/kaigan.76.2_i_655

CiNii Research

EFFECT OF WAVE FIELD ON TIME SCALE OF EROSION OF SAND SEABED UNDER RUBBLE MOUND OF COMPOSITE BREAKWATER COVERED WITH WAVE DISSIPATING BLOCKS

MURAOKA Hiroki, NAKAMURA Tomoaki, CHO Yonghwan, MIZUTANI Norimi

Journal of Japan Society of Civil Engineers, Ser. B2 (Coastal Engineering) Vol. 76 ( 2 ) page： I_865 - I_870 2020

Language：Japanese Publishing type：Research paper (scientific journal) Publisher：Japan Society of Civil Engineers

Numerical analyses of 1/55.8-scale small experiments and 1/4-scale large experiments on sand erosion under the rubble mound of a composite breakwater were conducted to investigate the effect of wave field on the time scale of the erosion. Numerical results showed that the time for the averaged flow inside the mound to be offshore throughout the mound and the time for the wave field around the breakwater to be equilibrium were longer in the small-scale experiments than in the large-scale experiments, and consequently the erosion observed in the large-scale experiments was underestimated in the small-scale experiments. This suggested that the time for the erosion to reach the same depth as that of the large-scale experiments was longer than that estimated according to the Froude similarity law.

DOI： 10.2208/kaigan.76.2_i_865

CiNii Research

FUNDAMENTAL STUDY ON POWER GENERATION CHARACTERISTICS OF BOTTOM-HINGED WAVE ENERGY CONVERTER

CHO Yonghwan, HONG Hyeonseon, KIM Shinwoong, NAKAMURA Tomoaki, MIZUTANI Norimi

Journal of Japan Society of Civil Engineers, Ser. B3 (Ocean Engineering) Vol. 76 ( 2 ) page： I_899 - I_904 2020

Language：Japanese Publishing type：Research paper (scientific journal) Publisher：Japan Society of Civil Engineers

The paper presents power generation characteristics of the proposed wave energy converter based on the bottom-hinged pendulum. The pendulum behavior becomes unstable and the power generation efficiency decreases when the wave energy is small, and the pendulum is installed at the position of D/L ≈ 0.5, which is the antinode of the standing wave. When the initial water depth was deeper than the pendulum, it was found that larger wave energy was required to rotate the pendulum stably. The maximum power generation efficiency (ke = 0.77) was obtained at the position of D/L ≈ 0.32.

DOI： 10.2208/jscejoe.76.2_i_899

CiNii Research

NUMERICAL ANALYSIS OF WAVE DAMPING MECHANISM OF TEMPORARY FLOATING BREAKWATER

NAKAMURA Tomoaki, YOSHIMURA Kazuki, CHO Yonghwan, MIZUTANI Norimi, KOTAKE Yasuo, SHIBUTANI Yoko

Journal of Japan Society of Civil Engineers, Ser. B3 (Ocean Engineering) Vol. 76 ( 2 ) page： I_372 - I_377 2020

Language：Japanese Publishing type：Research paper (scientific journal) Publisher：Japan Society of Civil Engineers

A temporary floating breakwater has been proposed to keep calm wave conditions in an offshore construction site. In this study, its wave damping mechanism was investigated from numerical simulations. Numerical results showed that the trend of the transmission coefficient can be predicted reasonably well. Furthermore, an energy damping coefficient of the breakwater changed depending on the breakwater width relative to the wavelength, and the change was explained in terms of vortices formed inside and under the breakwater. From numerical experiments, it was found that the barrier height of the breakwater and the free water depth poured inside the breakwater for most effectively reducing transmitted waves were dependence on the breakwater width relative to the wavelength.

DOI： 10.2208/jscejoe.76.2_i_372

CiNii Research

STUDY ON INFLUENCE OF FOOTING WIDTH ON MOTION OF FLOATING CAISSON

NAKAMURA Tomoaki, NAGAYAMA Naoto, CHO Yonghwan, MIZUTANI Norimi, KURAHARA Yoshinosuke, TAKEDA Masahide

Journal of Japan Society of Civil Engineers, Ser. B3 (Ocean Engineering) Vol. 76 ( 2 ) page： I_366 - I_371 2020

Language：Japanese Publishing type：Research paper (scientific journal) Publisher：Japan Society of Civil Engineers

In order to reduce the motion of a floating caisson, the effectiveness of a footing was investigated in terms of its width. Free oscillation experiments showed that the natural period of the pitch and the nonlinear extinction coefficients of the heave and pitch increased with the relative footing width. Forced oscillation experiments showed that the amplitude of the heave became large when the wave period was the same as the natural period of the heave. Similarly, the amplitude of the pitch increased when the wave period was one or two times larger than the natural period of the pitch. These suggested that it is essential to determine footing width considering the findings of this study.

DOI： 10.2208/jscejoe.76.2_i_366

CiNii Research

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Papers - CHO Yonghwan