研究者詳細 - 趙　容桓

Small ship collisions in coastal areas during tsunamis: Impact on seawalls behind sand dunes

Cho, Y; Nakamura, T; Mizutani, N; Sugimoto, W

COASTAL ENGINEERING 189 巻 2024年4月

出版者・発行元：Coastal Engineering

Coastal regions often feature seawalls situated behind sand dunes, introducing unique topographical factors that significantly influence ship drift and collision behavior. This study investigates the collision dynamics of a small ship, particularly fishing boats, in coastal areas during tsunamis and the subsequent impact forces on a seawall behind dunes. Through hydraulic model experiments, the characteristics of tsunami-driven drift, collision probabilities, and instantaneous impact forces were examined. The likelihood of ship collisions with seawalls increased when the dimensionless initial position of a ship (the ratio of its distance from the shore to the tsunami breaking point) was less than 0.6. The impact force experienced by a ship was influenced by the behavior of the water mass rising in front of the seawall post-tsunami. Collisions before the descending water mass resulted in increased impact forces, while collisions post-descent showed forces decreased owing to reduced drift velocity. In cases where ships encountered falling water masses before a collision, impact forces decreased significantly compared to other scenarios. An equation to estimate ship impact velocity, accounting for dune effects, based on the dimensionless time difference between the tsunami and ship collision and the tsunami propagation velocity calculated from near-shore tsunami height was introduced. This equation considers the damping effect of impact velocity, improving accuracy within the experimental range of small impact forces.

DOI： 10.1016/j.coastaleng.2024.104486

Numerical investigation of solitary wave attenuation by a vertical plate-type flexible breakwater using an FVM-FEM coupled model. Part 1: Linear elastic isotropic material 査読有り国際誌

Sun, WY; Nakamura, T; Cho, YH; Mizutani, N

OCEAN ENGINEERING 285 巻 2023年10月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Ocean Engineering

The use of flexible breakwaters to protect offshore areas from wave impacts has increased in recent years, owing to their cost effectiveness, environmental friendliness, and simple installation. This study proposes a finite-volume-method–finite-element-method coupled model based on waves2Foam, preCICE, and CalculiX to investigate wave attenuation by a vertical plate-type flexible breakwater under the impact of a solitary wave. The numerical model was validated by comparing the current results with previous numerical and experimental results. The proposed coupled model appropriately predicted the intricate flow phenomena of solitary waves impacting the flexible breakwater. The study systematically discusses the wave attenuation performance and structural response of the flexible breakwater under the impact of a solitary wave by considering several prominent factors: the mass coefficient, stiffness coefficient, and Poisson's ratio. Implementing a flexible breakwater effectively reduced wave energy intensity through various mechanisms, such as wave overtopping, wave breaking, and the dynamic response of the flexible breakwater. The material density had a negligible impact on the efficiency of the flexible breakwater in terms of wave dissipation and structural response; by contrast, the flexibility and compressibility of the breakwater significantly influenced the efficiency of the flexible breakwater. Employing a flexible breakwater fabricated using a compressible material is recommended for protecting coastal regions from wave-induced damage.

DOI： 10.1016/j.oceaneng.2023.115368

Development of Coupled Numerical Model between Floating Caisson and Anti-Oscillation Tanks

Shirai, K; Nakamura, T; Cho, YH; Mizutani, N; Kurahara, Y; Takeda, M

JOURNAL OF MARINE SCIENCE AND ENGINEERING 11 巻 ( 9 ) 2023年9月

出版者・発行元：Journal of Marine Science and Engineering

Floating caissons can oscillate owing to ocean waves when towed to an installation site. To reduce these oscillations, free-surface anti-oscillation tanks mounted on floating caissons have been proposed. However, no coupled numerical model exists between the motion of the floating caisson and fluid flow in the tanks based on computational fluid dynamics (CFD). In this study, a coupled model is developed and compared to existing physical experiments for validation. In the coupled model, the vertical and rotational motion of the floating caisson are computed as a rigid body, and the motion of the free water in the tank is computed using a CFD model. Numerical results show the predictive capability of the coupled model in terms of the rotational motion (pitch) of the floating caisson within ±20% of experimental data, regardless of the absence or presence of water in the tank. The numerical results also show that the fluid flow with complex air–water interface motion in the tank can be analyzed in detail using the coupled model. This suggests that the coupled model developed in this study is a useful tool for quantitatively assessing the effectiveness of an anti-oscillation tank for reducing the pitch of a floating caisson.

DOI： 10.3390/jmse11091669

Development of coupled numerical model between floating caisson and anti-oscillation tanks 査読有り国際誌

11 巻 ( 9 ) 2023年8月

Simplified BEM-based mathematical model for bottom-hinged oscillating wave surge converters under large-amplitude regular waves 査読有り国際誌

Liu, Y; Cho, YH; Mizutani, N; Nakamura, T

OCEAN ENGINEERING 272 巻 2023年3月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Ocean Engineering

The linear relationship between wave amplitude and wave torque based on the boundary element method (BEM) is widely employed for small-amplitude waves. However, linear relationships are unsuitable for bottom-hinged oscillating wave surge converters (OWSCs) under high waves owing to nonlinearities, such as wave overtopping. This paper presents an improved BEM introducing an incident wave amplitude correcting factor to simplify the complex nonlinearities under high regular waves. The correcting factors were calibrated with computational fluid dynamics (CFD) using OpenFOAM. An Oyster 800-like OWSC was adopted as the study model. A quadratic fitting curve of correcting factor vs. wave height is applicable because the calibrated correcting factors significantly correlate with wave heights but are almost irrelevant to wave periods. Though the time–history responses differ in BEM and CFD under high waves, BEM-based time-averaged results show good accuracy and high computational efficiency, such as equivalent pitch amplitude and capture width ratio (CWR). At mid-low periods, simultaneously varying power take-off (PTO) stiffness and damping than solely varying PTO damping, significantly increases CWR, especially under low waves. The capturing power under high waves substantially increases because of pitch motion amplification. At long periods, adjusting PTO inertia produces negligible efficiency improvement even with amplified oscillation.

DOI： 10.1016/j.oceaneng.2023.113893

Investigation of the Spiral Wave Generation and Propagation on a Numerical Circular Wave Tank Model 査読有り国際誌

Islam M.S., Nakamura T., Cho Y.H., Mizutani N.

Journal of Marine Science and Engineering 11 巻 ( 2 ) 2023年2月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Journal of Marine Science and Engineering

A two-phase incompressible flow model in three-dimensional cylindrical coordinates is applied for oblique wave generation in a numerical circular wave tank. The governing equations are discretized by a finite volume method, and a mass source function is added to reproduce oblique waves through a spiral wave generator positioned at the center of the tank. The volume of fluid method is implemented to track the free surfaces between the air and fluid, and the zonal embedded grid system is adopted to obtain a grid-independent solution in the cylindrical coordinates. A permeable, circumferentially sloping topography (1:7), similar to a natural beach profile, is set up for investigating wave propagation and other characteristics. The simulation and physical experimental results are compared, which show a good agreement in randomly selected water surface elevation profiles and wave heights under the same wave and sloping conditions. The spiral waves are reproduced and propagated in a phenomenon similar to that observed in the physical experiment. The results also show that the wave-breaking positions differ in different wave conditions and suggest a relationship with cross-shore and longshore velocity distribution in terms of incident wave heights and wave-breaking positions in different wave periods on the same sloping topography. Furthermore, this model can be used to investigate the mechanism of longshore current generation and the influences of beach slope on the generated wave propagation in a sloping topography.

DOI： 10.3390/jmse11020388

反射壁の幅が振り子型波力発電装置の運動特性に与える影響に関する実験的研究

趙容桓, 山本健太, 中村友昭, 水谷法美

土木学会論文集 79 巻 ( 18 ) 頁： n/a 2023年

記述言語：日本語出版者・発行元：公益社団法人土木学会

本研究では，逆三角形型振り子の背後遊水域に設置した反射壁の幅による振り子の運動特性と発電効率の上昇に有効な幅を検討する三次元実験を実施した．その結果，三次元条件における振り子の固有周期は二次元条件の値より小さくなることが確認された．反射壁を設置した場合，二次元と同様に，定常波の節の位置で発電効率が最大となり，比較的短い入射波周期において発電効率が高くなることが判明した．反射壁の幅は小さくなるにつれて反射壁前面の定常波の形成が弱まり，節と腹の位置における効果は少なく，振り子の設置場所によって反射壁なしの条件より発電効率が低下する傾向があった．本実験において反射壁の幅が振り子の幅より1.5倍以上になると反射壁なしの条件より発電効率を向上させる可能性が示された．

DOI： 10.2208/jscejj.23-18082

規則波下における曳航時の浮遊ケーソンの動揺に関する3次元数値解析

中村友昭, 森和大, 趙容桓, 水谷法美, 山野貴司

土木学会論文集 79 巻 ( 18 ) 頁： n/a 2023年

記述言語：日本語出版者・発行元：公益社団法人土木学会

3次元流体・構造連成解析による浮遊ケーソンの曳航・回頭時の作業限界の評価に向けた取り組みとして，本研究では，浮遊ケーソンを規則波下において直線的に曳航したときに生じるケーソンの動揺特性を流体・構造連成解析が可能な3次元数値計算モデルFS3Mにより考究した．その結果，HeaveとPitchの変動はケーソンから見た見かけの入射波周期（出会い波周期）で生じる傾向があること，Heaveの振幅は見かけの入射波周期がHeaveの固有周期と一致すると同調が生じて大きくなること，Pitchの固有周期は曳航索の影響を受け，見かけの入射波周期が曳航索も含めたPitchの固有周期と一致すると同調が生じてPitchの振幅が大きくなることを確認した．また，曳航索に作用する索張力の時間平均値は，波高の増加とともに大きくなること，HeaveやPitchの同調が生じるときに大きくなることを確認した．

DOI： 10.2208/jscejj.23-18011

礫浜海岸への地形変化モデルXBeachの適用に関する一検討

中村友昭, 牧亮太, 趙容桓, 水谷法美

土木学会論文集 79 巻 ( 18 ) 頁： n/a 2023年

記述言語：日本語出版者・発行元：公益社団法人土木学会

地形変化モデルXBeachを礫浜の地形変化に関する既往の断面実験に適用し，漂砂に関わるパラメータが与える影響を調べるとともに，断面実験結果との比較により地形変化の再現性を評価した．その結果，礫浜表面を出入りする浸透滲出流の影響を考慮するため，岸向き掃流砂量の倍率uprushfacを1より大きくし，沖向き掃流砂量の倍率backwashfacを1より小さくすることで，断面実験で確認されていた堆積型の地形が得られることを示した．また，XBeachを礫浜に適用した既往の研究と同様に浸透滲出流の機能を使用するだけでは，堆積型の地形の再現は困難であることが判明した．上述した岸向き掃流砂量と沖向き掃流砂量の倍率に加えて底質の静止摩擦角reposeangleをキャリブレーションすることで，波浪条件や人工リーフの有無によらず地形の時空間的な変化の再現性が向上することを確認した．

DOI： 10.2208/jscejj.23-18058

深層学習による気象情報と海岸画像を用いた波浪推定精度向上に関する検討

宮下侑莉華, 中村友昭, 菊雅美, 趙容桓, 水谷法美

土木学会論文集 79 巻 ( 18 ) 頁： n/a - n/a 2023年

記述言語：日本語掲載種別：研究論文（学術雑誌）出版者・発行元：公益社団法人土木学会

海岸管理において，波浪情報を把握することは重要である．本研究では，七里御浜井田海岸において，現地海岸画像から作成したオルソモザイク画像と気象情報に深層学習を適用することで高精度な波浪推定モデルの構築を試みた．第三世代波浪推算モデルSWANにより，NOWPHAS三重尾鷲沖の波浪が推定可能であることを示すとともに，SWANの推算結果と海岸画像の二つに畳み込みニューラルネットワーク(CNN)を適用することで有義波高を良好に推定可能であることを示した．また，風速データにLSTMを適用して推定した波浪と海岸画像の二つにCNNを適用することで，海岸画像のみにCNNを適用した波浪推定結果よりも有義波高と波向について推定精度が向上することが示され，有義波周期においても，良好に推定可能であることが示された．

DOI： 10.2208/jscejj.23-18094

津波漂流小型船舶による砂丘背後の防潮壁への衝突力の評価に関する実験的研究

趙容桓, 金子凌太朗, 中村友昭, 水谷法美, 杉本渉

土木学会論文集 79 巻 ( 18 ) 頁： n/a 2023年

記述言語：日本語出版者・発行元：公益社団法人土木学会

本研究では，沿岸部の砂丘背後に設置された防潮壁を対象に津波が小型船舶を漂流させる挙動と，その漂流船舶による防潮壁への衝突力を検討する1/100縮尺の水理模型実験を実施した．その結果，船舶の初期位置（汀線からの距離）と砕波位置の比で表される無次元初期位置が0.6より小さくなると，船舶が防潮壁に衝突する可能性が大きくなることを確認した．船舶の防潮壁への衝突力は防潮壁前面に発生する波のせり上がりによって低減される傾向があり，防潮壁前面に砂丘等の構造物が存在する場合，船舶の漂流速度と津波および船舶がそれぞれ防潮壁に到達する時間の差を考慮すれば，跳ね返す水塊による衝突力の減衰効果を概ね評価できることが判明した．

DOI： 10.2208/jscejj.23-18080

底質の混合が津波の遡上・打ち上げと波圧・波力に与える影響に関する実験的研究

中村友昭, 八木亮多, 趙容桓, 水谷法美

土木学会論文集 79 巻 ( 18 ) 頁： n/a 2023年

記述言語：日本語出版者・発行元：公益社団法人土木学会

黒い津波のように底質を巻き込むことが津波の遡上・打ち上げ・波圧・波力に与える影響を検討するため，底質の有無以外の点で同一条件となるように，底質を入れていない清水状態の水と底質としてカオリンを予め混合させて濁水状態とした水をゲート急開により開放して段波状の津波を発生させる水理実験を実施した．その結果，カオリンを混合させた場合には津波の遡上が局所的に早くなる可能性があることが判明した．鉛直壁に作用するサージフロント波圧による極大波圧は平均的には清水時よりもカオリン混合時の方が小さくなったが，底面近傍ではばらつきが大きくなり，カオリンの濃度が高い場合には大きな波圧が生じる可能性があることが判明した．持続波圧による最大波圧は，カオリンによる流体の見かけの密度の増加と比例して大きくなることが判明した．

DOI： 10.2208/jscejj.23-18092

Performance enhancement of a bottom-hinged oscillating wave surge converter via resonant adjustment 査読有り国際誌

Liu, Y; Mizutani, N; Cho, YH; Nakamura, T

RENEWABLE ENERGY 201 巻頁： 624 - 635 2022年12月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Renewable Energy

Based on the boundary element method, the frequency-domain dynamic equations of bottom-hinged oscillating wave surge converters under regular and irregular waves in shallow waters were derived. The nonlinear hydrostatic restoring moment and drag moment were linearized as time-independent stiffness and constant damping, respectively. A Python code was developed to efficiently solve the response without significantly sacrificing accuracy. The feasibility of performance enhancement via adjusting Power Take-Off (PTO) parameters was discussed. Under regular waves, the adjusting approaches are classified as no artificial resonance, ideal resonance, and near resonance. The adjustment towards resonance can boost the capturing power, although the flap-type absorber was recognized as a wave torque dominating device. A near-resonance situation, found by scanning the PTO parameters, is more effective to improve the hydrodynamic performance than the ideal resonance, where the amplification of the damping item is disadvantageous. An increasing hysteretic phase angle of velocity relative to wave torque with the increase of wave period represents the best status of wave energy harvesting. Under irregular waves, two adjusting approaches are presented: no artificial resonance and adjusting towards resonance. The performance at short peak periods can be improved by adjusting PTO stiffness, while, adjusting PTO inertia torque is almost ineffective.

DOI： 10.1016/j.renene.2022.10.130

Influence of Tsunami-Driven Shipping Containers' Layout on Their Motion 査読有り国際誌

Hou, YD; Nakamura, T; Cho, YH; Mizutani, N; Tomita, T

JOURNAL OF MARINE SCIENCE AND ENGINEERING 10 巻 ( 12 ) 2022年12月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Journal of Marine Science and Engineering

This study investigated the interaction between containers under extreme hydrodynamic conditions modeled on tsunamis to assess whether the number and layout of containers affect their motion and to guide future studies on numerical simulations describing tsunami-debris motion. The three-dimensional Fluid–Structure–Sediment–Seabed Interaction Model (FS3M), which can compute tsunami–container interaction, was used as a numerical test model, and numerical results for the specific target of the simulation were compared with experimental data to check the validity and computational accuracy of the FS3M. The study showed that the number of rows (Nx), columns (Ny), and stacks (Nz) in the initial arrangement of containers constitute the main factor affecting the area where the containers spread and their drift motion velocity. An increase in Nx and Nz can effectively reduce the container drift velocity. Conversely, as Ny increases, the drift motion velocity of the center of gravity of the entire group increases. The results of this study can facilitate the development of more realistic building structure scenarios in future research that consider the proposed characteristic damage estimation and comprehensive assessment methods laid out herein.

DOI： 10.3390/jmse10121911

深層学習による海岸画像を用いた波浪推定に関する検討査読有り

宮下侑莉華，中村友昭，菊　雅美，趙　容桓，水谷法美

土木学会論文集Ｂ２（海岸工学） 78 巻 ( 2 ) 頁： 127 - 132 2022年11月

橋台背面盛土と基礎地盤の侵食が橋桁への作用津波力とその評価式に与える影響査読有り

中村友昭，熊澤諒大，趙　容桓，水谷法美

土木学会論文集Ｂ２（海岸工学） 78 巻 ( 2 ) 頁： 223 - 228 2022年11月

減揺タンク付浮遊ケーソンの動揺に与える波浪諸元の影響に関する実験的研究査読有り

中村友昭，竹山俊介，白井開斗，趙　容桓，水谷法美，倉原義之介，武田将英

土木学会論文集B3（海洋開発） 78 巻 ( 2 ) 頁： 253 - 258 2022年10月

Experimental Investigation on the Mechanism of Longshore Sediment Transport Using a Circular Wave Basin 査読有り

Islam, MS; Akita, N; Nakamura, T; Cho, YH; Mizutani, N

JOURNAL OF MARINE SCIENCE AND ENGINEERING 10 巻 ( 9 ) 2022年9月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Journal of Marine Science and Engineering

In this paper, a hydraulic model experiment was conducted to investigate the mechanism of longshore sediment transport on a sloping beach of 1:7 using a circular wave tank equipped with a spiral wave maker. The experimental results revealed that cross-shore sediment transport was predominant up to the formation of equilibrium cross-section. Then the longshore sediment transport was actively dominant near the bar after the development of equilibrium topography. The wave-breaking positions and the characteristics of sand displacement in the vicinity of the wave-breaking zone were observed throughout the installation of fluorescent sands in different cross-sections under several wave conditions on the same terrain to presume the coastal erosion. From the outcomes, it was confirmed that the gradient of the average water level due to the obliquely incident waves breaking and the differences in breaking positions along the shoreline accelerated the generation of longshore currents and longshore sediment transport in the sloping beach profile. Moreover, the actively longshore sediment transport tendency throughout the breaking waves and the longshore current had been confirmed in this study where the gradually increasing wave heights influenced the stronger degree of wave breaking on the sloping beach, which enhanced the sediment transport procedure to deform the beach profile.

DOI： 10.3390/jmse10091189

Nonlinear hydrodynamic analysis and optimization of oscillating wave surge converters under irregular waves 査読有り国際誌

Liu, Y; Mizutani, N; Cho, YH; Nakamura, T

OCEAN ENGINEERING 250 巻 ( 15 ) 2022年4月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Ocean Engineering

To evaluate the mean annual capture width ratio (CWR) of the oscillating wave surge converters (OWSCs) under irregular waves, the time-domain dynamic equation was derived by considering the nonlinear hydrostatic stiffness, drag, and nonlinear power take-off (PTO) system. A Python code was developed to solve the response and capture performance with 4th-order Runge–Kutta integration. The wave spectrum was corrected according to the water depth in shallow water. Unlike the inflexible drag coefficients in steady flow, the drag coefficients in irregular waves (calibrated with OpenFOAM) are strongly affected by OWSC thickness. Multi-objective genetic algorithm (MOGA) optimization of OWSC geometric sizes, internal water filling, and PTO parameters was conducted for two objective functions: maximizing the mean annual CWR and minimizing the structural mass per unit width. The optimized result presents a slender OWSC that neutrally balances these two objectives. The effects and local sensitivities of the width, thickness, axis depth, water filling, PTO stiffness, damping, and friction were comprehensively discussed. The results show that the axis depth has the greatest positive influence on the CWR, and the increase of thickness creates significant economical disadvantage due to a heavier structure. However, inertia adjustment by filling water does not benefit the mean annual CWR.

DOI： 10.1016/j.oceaneng.2022.110888