Xizeng Zhao | Zhejiang University (original) (raw)

Papers by Xizeng Zhao

Research paper thumbnail of Hydrodynamic performance of a new-type vertical wall breakwater with inclined culvert on a permeable rubble mound foundation

Research paper thumbnail of Easily generating and absorbing waves using machine learning

High-order wave-making theories are becoming available but are limited to certain ranges of waves... more High-order wave-making theories are becoming available but are limited to certain ranges of waves and wavemaker types in their applicability. Alternatively, machine learning can be considered to find nonlinear functional relationships. Therefore, this paper proposes a simple and universal framework for generating and absorbing waves based on machine learning. This framework trains neural networks to establish the transfer function between the free-surface elevation on the wavemaker and the wavemaker velocity. Significantly, penalty term and data augmentation techniques based on wave-making mechanisms are introduced to increase the generalization ability of neural networks, rather than pure data-driven. Therefore, once the target wave profiles in front of the wavemaker are given, it can realize generating waves and absorbing reflected waves at the same time. Taking piston and plunger wavemakers as examples, an in-house numerical solver is applied to simulate both wave generation and ...

Research paper thumbnail of An active-controlled heaving plate breakwater trained by an intelligent framework based on deep reinforcement learning

Research paper thumbnail of Numerical simulation of focused wave generation using CIP method

Freak wave (extreme wave, rogue wave, huge wave, water wall, et al) is extremely large water wave... more Freak wave (extreme wave, rogue wave, huge wave, water wall, et al) is extremely large water wave in ocean and may occur all over the sea area. Despite its low possibility of occurrence, such a wave may lead to damage of ships and offshore platforms. Until recently, there was no means to generate a realistic extreme wave appearing in a random wave train. A relatively easy way to generate freak waves is using wave focusing. Along with physical experiments, numerical simulations are useful tools for designing coastal structures as well as for understanding natural hydrodynamic processes in the field of ocean engineering. This work is motivated by the requirement of hydrodynamics laboratories to generate ! focused waves and to investigate the survivability of offshore wave energy converters due to focused waves. The purpose of this paper is to demonstrate a CFD approach applied to simulations of focused wave. In this paper, a constrained interpolation profile (CIP)-based Cartesian grid...

Research paper thumbnail of Some improvements on Moment-of-Fluid method in 3D rectangular hexahedrons

The moment-of-fluid method (MOF) is an extension of the volume-of-fluid method with piecewise lin... more The moment-of-fluid method (MOF) is an extension of the volume-of-fluid method with piecewise linear interface construction (VOF-PLIC). In MOF reconstruction, the optimized normal vector is determined from the reference centroid and the volume fraction by iteration. The state-of-art work by \citet{milcent_moment--fluid_2020} proposed an analytic gradient of the objective function, which greatly reduces the computational cost. In this study, we further accelerate the MOF reconstruction algorithm by using Gauss-Newton iteration instead of Broyden-Fletcher-Goldfarb-Shanno (BFGS) iteration. We also propose an improved initial guess for MOF reconstruction, which improves the efficiency and the robustness of the MOF reconstruction algorithm. Our implementation of the code and test cases are available on our Github repository.

Research paper thumbnail of A TensorFlow-based new high-performance computational framework for CFD

Journal of Hydrodynamics, 2020

In this study, a computational framework in the field of artificial intelligence was applied in c... more In this study, a computational framework in the field of artificial intelligence was applied in computational fluid dynamics (CFD) field. This Framework, which was initially proposed by Google AI department, is called “TensorFlow”. An improved CFD model based on this framework was developed with a high-order difference method, which is a constrained interpolation profile (CIP) scheme for the base flow solver of the advection term in the Navier-Stokes equations, and preconditioned conjugate gradient (PCG) method was implemented in the model to solve the Poisson equation. Some new features including the convolution, vectorization, and graphics processing unit (GPU) acceleration were implemented to raise the computational efficiency. The model was tested with several benchmark cases and shows good performance. Compared with our former CIP-based model, the present TensorFlow-based model also shows significantly higher computational efficiency in large-scale computation. The results indicate TensorFlow could be a promising framework for CFD models due to its ability in the computational acceleration and convenience for programming.

Research paper thumbnail of Wave power extraction by a nearshore oscillating water column converter with a surging lip-wall

Research paper thumbnail of A numerical study of tsunami wave run-up and impact on coastal cliffs using a CIP-based model

Natural Hazards and Earth System Sciences Discussions, 2017

There is a general lack of the understanding of tsunami wave interacting with complex geographies... more There is a general lack of the understanding of tsunami wave interacting with complex geographies, especially the process of inundation. Numerical simulations are performed to understand the effects of several factors on tsunami wave impact and run-up in the presence of submarine gentle slopes and coastal cliffs, using an in-house code, named a Constrained Interpolation Profile (CIP)-based model in Zhejiang University (CIP-ZJU). The model employs a high-order finite difference method, the CIP method as the flow solver, utilizes a VOF-type method, the Tangent of hyperbola for interface capturing/Slope weighting (THINC/SW) scheme to capture the free surface, and treats the solid boundary by an immersed boundary method. A series of incident waves are arranged to interact with varying coastal geographies. Numerical results are compared with experimental data and good agreement is obtained. The influences of submarine gentle slope, coastal cliff and incident wave height are discussed. It...

Research paper thumbnail of A numerical study of tsunami wave impact and run-up on coastal cliffs using a CIP-based model

Natural Hazards and Earth System Sciences, 2017

There is a general lack of understanding of tsunami wave interaction with complex geographies, es... more There is a general lack of understanding of tsunami wave interaction with complex geographies, especially the process of inundation. Numerical simulations are performed to understand the effects of several factors on tsunami wave impact and run-up in the presence of gentle submarine slopes and coastal cliffs, using an in-house code, a constrained interpolation profile (CIP)-based model. The model employs a high-order finite difference method, the CIP method, as the flow solver; utilizes a VOF-type method, the tangent of hyperbola for interface capturing/slope weighting (THINC/SW) scheme, to capture the free surface; and treats the solid boundary by an immersed boundary method. A series of incident waves are arranged to interact with varying coastal geographies. Numerical results are compared with experimental data and good agreement is obtained. The influences of gentle submarine slope, coastal cliff and incident wave height are discussed. It is found that the tsunami amplification ...

Research paper thumbnail of Numerical investigation of the gate motion effect on a dam break flow

Journal of Marine Science and Technology, 2016

The effects of a moving gate on dam break flow experiments are numerically considered in this pap... more The effects of a moving gate on dam break flow experiments are numerically considered in this paper. The dam break flow process is modelled by a sudden gate release. A Constrained Interpolation Profile (CIP)-based model is employed to solve the Navier-Stokes equations with the free surface boundary condition to deal with the water-air-gate interactions. In addition, the VOF-type THINC/SW method is used to capture the free surface. The movable gate is simulated by an immersed boundary, and a gate motion formula is proposed from existing experimental data and applied in the dam break flow computations. The difference between the water profile and pressure domain with/without gate motion is examined. The dam breaks with different gate motions are discussed as well. The numerical results show that the proposed gate motion formula correlates well with the numerical modelling, and the gate motion has a significant influence in the water collapse process and cannot be neglected in the study of dam break.

Research paper thumbnail of Numerical Study on Flow around Four Square-Arranged Cylinders at Low Reynolds Numbers

Mathematical Problems in Engineering, 2017

In this paper, numerical simulations of flow past four square-arranged cylinders are carried out ... more In this paper, numerical simulations of flow past four square-arranged cylinders are carried out at different spacing ratios (1.5≤L/D≤5.0; L is the center to center distance; D is the cylinder diameter) and Reynolds numbers (100≤Re≤1000). The effects of spacing ratio and Reynolds number on the wake flow characteristics are investigated, such as the instantaneous vorticity contours, force coefficients, and vortex shedding frequencies. The results show that the flow characteristics behind the four-cylinder cases are significantly affected by the spacing ratios and Reynolds numbers. At the same spacing ratio, the transformation of flow pattern is advanced quickly with increasing of Reynolds numbers, the values of force coefficients are correspondingly fluctuated with large amplitude, and the vortex shedding frequency is increased significantly with Re.

Research paper thumbnail of Computation of Flow Past an In-Line Oscillating Circular Cylinder and a Stationary Cylinder in Tandem Using a CIP-Based Model

Mathematical Problems in Engineering, 2015

Viscous flow past an upstream in-line forced oscillating circular cylinder with a stationary cyli... more Viscous flow past an upstream in-line forced oscillating circular cylinder with a stationary cylinder downstream at Reynolds number of 100 is investigated using a CIP model. The model is established in a Cartesian coordinate system using a high-order difference method to discretise the Navier-Stokes equations. The fluid-structure interaction is treated as a multiphase flow with fluid and solid phases solved simultaneously. An immersed boundary method is used to deal with the fluid-body coupling. The CFD model is firstly applied to the computation of flow past a fixed circular cylinder for its validation; then flow over two stationary tandem cylinders is investigated and good agreements are obtained comparing with existing ones. Computations are then performed with flow past two tandem cylinders with an upstream in-line oscillating cylinder with a small spacingL=2D. Considerable attention is paid to the spectrum characteristics and vortex modes.

Research paper thumbnail of Efficient Focusing Models for Generation of Freak Waves

Research paper thumbnail of Effects of Wave Breaking on Freak Wave Generation in Random Wave Train

中国海洋工程 英文版, 2010

The experimental studies of the wave breaking effects on freak wave generation are presented with... more The experimental studies of the wave breaking effects on freak wave generation are presented within a finite-depth random wave train in a laboratory wave tank. The main attention is paid to the abnormal index,AI=Hmax/Hs,being used to characterize the freak waves,and the changes of the coefficient due to wave breaking. The results show that the occurrence probability of freak wave events in non-breaking waves is much larger than that in breaking waves and such occurrence in deep water is larger than that in shallow water.

Research paper thumbnail of A CIP-based numerical simulation of freak wave impact on a floating body

Ocean Engineering, 2014

Freak (Extreme, rogue) waves are extremely large water waves in ocean and may occur all over the ... more Freak (Extreme, rogue) waves are extremely large water waves in ocean and may occur all over the world sea area. Such a wave may lead to damage of coastal and offshore structures. Accurate prediction of extreme waveinduced forces and motions is of importance and necessaries for researchers and engineers for the purpose of structure design and disaster prevention. Due to the complexity of nonlinear wave-structure interactions related with distorted free surface and relatively large amplitude of structure response, a great deal of effort is required to investigate the physics. Here, a Computational Fluid Dynamics (CFD) model has been developed to study focused wave impact on a floating structure and validated by a newly designed experiment. Focused waves are generated based on the mechanism of wave focusing in a two-dimensional wave tank. In the experiment, a model of a box-shaped floating body with a small freeboard is adopted in order to easily obtain green water phenomena. The computations are performed by a Constrained Interpolation Profile (CIP)-based Cartesian grid method. The CIP algorithm is adopted as the base scheme to obtain a robust flow solver of the Navier-stokes equation with free surface boundary. An improved THINC scheme (THINC, tangent of hyperbola for interface capturing), the more accurate THINC/SW scheme (THINC with Slope Weighting), is applied as the free surface/ interface capturing method. Main attentions are paid to the three degrees of freedom (3-DOF) body motions, pressure domain around the structure and nonlinear phenomena, such as water on deck. The highly nonlinear wave-structure interactions, including significant body motion and water on deck, are modeled successfully in comparison with experimental measurements. It is concluded that the present model with the aid of the CIP technique can provide with acceptably accurate numerical results on the route to practical purposes.

Research paper thumbnail of Simulation of wave impact on a horizontal deck based on SPH method

Journal of Marine Science and Application, 2010

A numerical model was established for simulating wave impact on a horizontal deck by an improved ... more A numerical model was established for simulating wave impact on a horizontal deck by an improved incompressible smoothed particle hydrodynamics (ISPH). As a grid-less particle method, the ISPH method has been widely used in the free-surface hydrodynamic flows with good accuracy. The improvement includes the employment of a corrective function for enhancement of angular momentum conservation in a particle-based calculation and a new estimation method to predict the pressure on the horizontal deck. The simulation results show a good agreement with the experiment. The present numerical model can be used to study wave impact load on the horizontal deck.

Research paper thumbnail of A CIP-Based Numerical Simulation of Green Water on a 2-D Floating Body

Volume 9: Odd M. Faltinsen Honoring Symposium on Marine Hydrodynamics, 2013

Numerical simulation of green water violent impact on a 2-D floating body is carried out using a ... more Numerical simulation of green water violent impact on a 2-D floating body is carried out using a Constrained Interpolation Profile (CIP)-based model and validated by a newly designed experiment, which is carried out in a two-dimensional wave flume in the Research Institute for Applied Mechanics, Kyushu University, Japan. In this work, focused wave are used for green water phenomena. Numerical simulations are performed by a CIP-based Cartesian grid method, which is described in the paper. Fluid-structure interaction is treated as a multi-phase flow problem. The CIP algorithm is adopted as the base scheme to obtain a robust flow solver for the multi-phase flow problem. The free surface/interface boundary is captured by THINC/SW (THINC: tangent of hyperbola for interface capturing; SW: Slope Weighting). A model of a box-shaped floating body with a small freeboard is adopted in order to easily obtain green water phenomena in the experiment. Main attentions are paid to the body responses...

Research paper thumbnail of Numerical Simulation of Extreme Wave Generation Using VOF Method

Journal of Hydrodynamics, 2010

Numerical simulations of extreme wave generation are carried out by using the Volume Of Fluid (VO... more Numerical simulations of extreme wave generation are carried out by using the Volume Of Fluid (VOF) method. Extreme waves are generated based on wave focusing in a 2-D numerical model. To validate the capability of the VOF-based model described in this article, the propagation of regular waves is computed and compared with the theoretical results. By adjusting the phases of wave components, extreme waves are formed at given time and given position in the computation. The numerical results are compared with theoretical solutions and experimental data. It is concluded that the present model based on the VOF technique can provide acceptably accurate numerical results to serve practical purposes.

Research paper thumbnail of A numerical study of the transformation of water waves generated in a wave flume

Fluid Dynamics Research, 2009

This paper is concerned with the transformation of water waves generated in a numerical wave flum... more This paper is concerned with the transformation of water waves generated in a numerical wave flume. To set up the numerical wave tank, the non-periodic spectral method is used to solve potential flow equations incorporated with fully nonlinear free-surface boundary conditions and the wavemaker boundary condition. The flume is employed to simulate the transformation of water waves with different steepness and wavelength. The calculation results of the free-surface elevation and the corresponding amplitude wave spectrum are compared with the linear wave solutions and the analytical solutions. The result of the investigations is that short waves agree well with the linear solutions, whereas the long waves with limited Ursell numbers are consistent with the analytical solutions. Physical experiments are conducted in a wave flume to verify the numerical scheme. Good agreement between the experimental and the numerical results is obtained. The time-frequency information of waves is also investigated based on the wavelet transform. The numerical results show that short waves with smaller Benjamin-Feir index (BFI) are more stable than long waves with larger BFI in finite wave depth.

Research paper thumbnail of Validation of the initialization of a numerical wave flume using a time ramp

Fluid Dynamics Research, 2010

ABSTRACT This study investigates the initialization of nonlinear free-surface simulations in a nu... more ABSTRACT This study investigates the initialization of nonlinear free-surface simulations in a numerical wave flume. The numerical wave flume has been detailed in a previous paper (Zhao et al 2009 Fluid Dyn. Res. 41 035510). Due to mismatch between linear input wavemaker motion and the kinematics of fully nonlinear waves, direct numerical simulations of progressive waves, generated by a sinusoidally moving wavemaker, are prone to high-frequency wave instability unless the flow is given sufficient time to adjust. A time ramp is superimposed on wavemaker motion at the start, which allows nonlinear free-surface simulations to be initialized with linear input. Three time ramp functions are introduced to check the difference of the time ramp. The time–frequency information of waves is also investigated by means of wavelet analysis. Numerical results show that the difference of the time ramp function is minor and the time ramp scheme is effective in stabilizing wave instability at the start of the simulation in a wave flume.

Research paper thumbnail of Hydrodynamic performance of a new-type vertical wall breakwater with inclined culvert on a permeable rubble mound foundation

Research paper thumbnail of Easily generating and absorbing waves using machine learning

High-order wave-making theories are becoming available but are limited to certain ranges of waves... more High-order wave-making theories are becoming available but are limited to certain ranges of waves and wavemaker types in their applicability. Alternatively, machine learning can be considered to find nonlinear functional relationships. Therefore, this paper proposes a simple and universal framework for generating and absorbing waves based on machine learning. This framework trains neural networks to establish the transfer function between the free-surface elevation on the wavemaker and the wavemaker velocity. Significantly, penalty term and data augmentation techniques based on wave-making mechanisms are introduced to increase the generalization ability of neural networks, rather than pure data-driven. Therefore, once the target wave profiles in front of the wavemaker are given, it can realize generating waves and absorbing reflected waves at the same time. Taking piston and plunger wavemakers as examples, an in-house numerical solver is applied to simulate both wave generation and ...

Research paper thumbnail of An active-controlled heaving plate breakwater trained by an intelligent framework based on deep reinforcement learning

Research paper thumbnail of Numerical simulation of focused wave generation using CIP method

Freak wave (extreme wave, rogue wave, huge wave, water wall, et al) is extremely large water wave... more Freak wave (extreme wave, rogue wave, huge wave, water wall, et al) is extremely large water wave in ocean and may occur all over the sea area. Despite its low possibility of occurrence, such a wave may lead to damage of ships and offshore platforms. Until recently, there was no means to generate a realistic extreme wave appearing in a random wave train. A relatively easy way to generate freak waves is using wave focusing. Along with physical experiments, numerical simulations are useful tools for designing coastal structures as well as for understanding natural hydrodynamic processes in the field of ocean engineering. This work is motivated by the requirement of hydrodynamics laboratories to generate ! focused waves and to investigate the survivability of offshore wave energy converters due to focused waves. The purpose of this paper is to demonstrate a CFD approach applied to simulations of focused wave. In this paper, a constrained interpolation profile (CIP)-based Cartesian grid...

Research paper thumbnail of Some improvements on Moment-of-Fluid method in 3D rectangular hexahedrons

The moment-of-fluid method (MOF) is an extension of the volume-of-fluid method with piecewise lin... more The moment-of-fluid method (MOF) is an extension of the volume-of-fluid method with piecewise linear interface construction (VOF-PLIC). In MOF reconstruction, the optimized normal vector is determined from the reference centroid and the volume fraction by iteration. The state-of-art work by \citet{milcent_moment--fluid_2020} proposed an analytic gradient of the objective function, which greatly reduces the computational cost. In this study, we further accelerate the MOF reconstruction algorithm by using Gauss-Newton iteration instead of Broyden-Fletcher-Goldfarb-Shanno (BFGS) iteration. We also propose an improved initial guess for MOF reconstruction, which improves the efficiency and the robustness of the MOF reconstruction algorithm. Our implementation of the code and test cases are available on our Github repository.

Research paper thumbnail of A TensorFlow-based new high-performance computational framework for CFD

Journal of Hydrodynamics, 2020

In this study, a computational framework in the field of artificial intelligence was applied in c... more In this study, a computational framework in the field of artificial intelligence was applied in computational fluid dynamics (CFD) field. This Framework, which was initially proposed by Google AI department, is called “TensorFlow”. An improved CFD model based on this framework was developed with a high-order difference method, which is a constrained interpolation profile (CIP) scheme for the base flow solver of the advection term in the Navier-Stokes equations, and preconditioned conjugate gradient (PCG) method was implemented in the model to solve the Poisson equation. Some new features including the convolution, vectorization, and graphics processing unit (GPU) acceleration were implemented to raise the computational efficiency. The model was tested with several benchmark cases and shows good performance. Compared with our former CIP-based model, the present TensorFlow-based model also shows significantly higher computational efficiency in large-scale computation. The results indicate TensorFlow could be a promising framework for CFD models due to its ability in the computational acceleration and convenience for programming.

Research paper thumbnail of Wave power extraction by a nearshore oscillating water column converter with a surging lip-wall

Research paper thumbnail of A numerical study of tsunami wave run-up and impact on coastal cliffs using a CIP-based model

Natural Hazards and Earth System Sciences Discussions, 2017

There is a general lack of the understanding of tsunami wave interacting with complex geographies... more There is a general lack of the understanding of tsunami wave interacting with complex geographies, especially the process of inundation. Numerical simulations are performed to understand the effects of several factors on tsunami wave impact and run-up in the presence of submarine gentle slopes and coastal cliffs, using an in-house code, named a Constrained Interpolation Profile (CIP)-based model in Zhejiang University (CIP-ZJU). The model employs a high-order finite difference method, the CIP method as the flow solver, utilizes a VOF-type method, the Tangent of hyperbola for interface capturing/Slope weighting (THINC/SW) scheme to capture the free surface, and treats the solid boundary by an immersed boundary method. A series of incident waves are arranged to interact with varying coastal geographies. Numerical results are compared with experimental data and good agreement is obtained. The influences of submarine gentle slope, coastal cliff and incident wave height are discussed. It...

Research paper thumbnail of A numerical study of tsunami wave impact and run-up on coastal cliffs using a CIP-based model

Natural Hazards and Earth System Sciences, 2017

There is a general lack of understanding of tsunami wave interaction with complex geographies, es... more There is a general lack of understanding of tsunami wave interaction with complex geographies, especially the process of inundation. Numerical simulations are performed to understand the effects of several factors on tsunami wave impact and run-up in the presence of gentle submarine slopes and coastal cliffs, using an in-house code, a constrained interpolation profile (CIP)-based model. The model employs a high-order finite difference method, the CIP method, as the flow solver; utilizes a VOF-type method, the tangent of hyperbola for interface capturing/slope weighting (THINC/SW) scheme, to capture the free surface; and treats the solid boundary by an immersed boundary method. A series of incident waves are arranged to interact with varying coastal geographies. Numerical results are compared with experimental data and good agreement is obtained. The influences of gentle submarine slope, coastal cliff and incident wave height are discussed. It is found that the tsunami amplification ...

Research paper thumbnail of Numerical investigation of the gate motion effect on a dam break flow

Journal of Marine Science and Technology, 2016

The effects of a moving gate on dam break flow experiments are numerically considered in this pap... more The effects of a moving gate on dam break flow experiments are numerically considered in this paper. The dam break flow process is modelled by a sudden gate release. A Constrained Interpolation Profile (CIP)-based model is employed to solve the Navier-Stokes equations with the free surface boundary condition to deal with the water-air-gate interactions. In addition, the VOF-type THINC/SW method is used to capture the free surface. The movable gate is simulated by an immersed boundary, and a gate motion formula is proposed from existing experimental data and applied in the dam break flow computations. The difference between the water profile and pressure domain with/without gate motion is examined. The dam breaks with different gate motions are discussed as well. The numerical results show that the proposed gate motion formula correlates well with the numerical modelling, and the gate motion has a significant influence in the water collapse process and cannot be neglected in the study of dam break.

Research paper thumbnail of Numerical Study on Flow around Four Square-Arranged Cylinders at Low Reynolds Numbers

Mathematical Problems in Engineering, 2017

In this paper, numerical simulations of flow past four square-arranged cylinders are carried out ... more In this paper, numerical simulations of flow past four square-arranged cylinders are carried out at different spacing ratios (1.5≤L/D≤5.0; L is the center to center distance; D is the cylinder diameter) and Reynolds numbers (100≤Re≤1000). The effects of spacing ratio and Reynolds number on the wake flow characteristics are investigated, such as the instantaneous vorticity contours, force coefficients, and vortex shedding frequencies. The results show that the flow characteristics behind the four-cylinder cases are significantly affected by the spacing ratios and Reynolds numbers. At the same spacing ratio, the transformation of flow pattern is advanced quickly with increasing of Reynolds numbers, the values of force coefficients are correspondingly fluctuated with large amplitude, and the vortex shedding frequency is increased significantly with Re.

Research paper thumbnail of Computation of Flow Past an In-Line Oscillating Circular Cylinder and a Stationary Cylinder in Tandem Using a CIP-Based Model

Mathematical Problems in Engineering, 2015

Viscous flow past an upstream in-line forced oscillating circular cylinder with a stationary cyli... more Viscous flow past an upstream in-line forced oscillating circular cylinder with a stationary cylinder downstream at Reynolds number of 100 is investigated using a CIP model. The model is established in a Cartesian coordinate system using a high-order difference method to discretise the Navier-Stokes equations. The fluid-structure interaction is treated as a multiphase flow with fluid and solid phases solved simultaneously. An immersed boundary method is used to deal with the fluid-body coupling. The CFD model is firstly applied to the computation of flow past a fixed circular cylinder for its validation; then flow over two stationary tandem cylinders is investigated and good agreements are obtained comparing with existing ones. Computations are then performed with flow past two tandem cylinders with an upstream in-line oscillating cylinder with a small spacingL=2D. Considerable attention is paid to the spectrum characteristics and vortex modes.

Research paper thumbnail of Efficient Focusing Models for Generation of Freak Waves

Research paper thumbnail of Effects of Wave Breaking on Freak Wave Generation in Random Wave Train

中国海洋工程 英文版, 2010

The experimental studies of the wave breaking effects on freak wave generation are presented with... more The experimental studies of the wave breaking effects on freak wave generation are presented within a finite-depth random wave train in a laboratory wave tank. The main attention is paid to the abnormal index,AI=Hmax/Hs,being used to characterize the freak waves,and the changes of the coefficient due to wave breaking. The results show that the occurrence probability of freak wave events in non-breaking waves is much larger than that in breaking waves and such occurrence in deep water is larger than that in shallow water.

Research paper thumbnail of A CIP-based numerical simulation of freak wave impact on a floating body

Ocean Engineering, 2014

Freak (Extreme, rogue) waves are extremely large water waves in ocean and may occur all over the ... more Freak (Extreme, rogue) waves are extremely large water waves in ocean and may occur all over the world sea area. Such a wave may lead to damage of coastal and offshore structures. Accurate prediction of extreme waveinduced forces and motions is of importance and necessaries for researchers and engineers for the purpose of structure design and disaster prevention. Due to the complexity of nonlinear wave-structure interactions related with distorted free surface and relatively large amplitude of structure response, a great deal of effort is required to investigate the physics. Here, a Computational Fluid Dynamics (CFD) model has been developed to study focused wave impact on a floating structure and validated by a newly designed experiment. Focused waves are generated based on the mechanism of wave focusing in a two-dimensional wave tank. In the experiment, a model of a box-shaped floating body with a small freeboard is adopted in order to easily obtain green water phenomena. The computations are performed by a Constrained Interpolation Profile (CIP)-based Cartesian grid method. The CIP algorithm is adopted as the base scheme to obtain a robust flow solver of the Navier-stokes equation with free surface boundary. An improved THINC scheme (THINC, tangent of hyperbola for interface capturing), the more accurate THINC/SW scheme (THINC with Slope Weighting), is applied as the free surface/ interface capturing method. Main attentions are paid to the three degrees of freedom (3-DOF) body motions, pressure domain around the structure and nonlinear phenomena, such as water on deck. The highly nonlinear wave-structure interactions, including significant body motion and water on deck, are modeled successfully in comparison with experimental measurements. It is concluded that the present model with the aid of the CIP technique can provide with acceptably accurate numerical results on the route to practical purposes.

Research paper thumbnail of Simulation of wave impact on a horizontal deck based on SPH method

Journal of Marine Science and Application, 2010

A numerical model was established for simulating wave impact on a horizontal deck by an improved ... more A numerical model was established for simulating wave impact on a horizontal deck by an improved incompressible smoothed particle hydrodynamics (ISPH). As a grid-less particle method, the ISPH method has been widely used in the free-surface hydrodynamic flows with good accuracy. The improvement includes the employment of a corrective function for enhancement of angular momentum conservation in a particle-based calculation and a new estimation method to predict the pressure on the horizontal deck. The simulation results show a good agreement with the experiment. The present numerical model can be used to study wave impact load on the horizontal deck.

Research paper thumbnail of A CIP-Based Numerical Simulation of Green Water on a 2-D Floating Body

Volume 9: Odd M. Faltinsen Honoring Symposium on Marine Hydrodynamics, 2013

Numerical simulation of green water violent impact on a 2-D floating body is carried out using a ... more Numerical simulation of green water violent impact on a 2-D floating body is carried out using a Constrained Interpolation Profile (CIP)-based model and validated by a newly designed experiment, which is carried out in a two-dimensional wave flume in the Research Institute for Applied Mechanics, Kyushu University, Japan. In this work, focused wave are used for green water phenomena. Numerical simulations are performed by a CIP-based Cartesian grid method, which is described in the paper. Fluid-structure interaction is treated as a multi-phase flow problem. The CIP algorithm is adopted as the base scheme to obtain a robust flow solver for the multi-phase flow problem. The free surface/interface boundary is captured by THINC/SW (THINC: tangent of hyperbola for interface capturing; SW: Slope Weighting). A model of a box-shaped floating body with a small freeboard is adopted in order to easily obtain green water phenomena in the experiment. Main attentions are paid to the body responses...

Research paper thumbnail of Numerical Simulation of Extreme Wave Generation Using VOF Method

Journal of Hydrodynamics, 2010

Numerical simulations of extreme wave generation are carried out by using the Volume Of Fluid (VO... more Numerical simulations of extreme wave generation are carried out by using the Volume Of Fluid (VOF) method. Extreme waves are generated based on wave focusing in a 2-D numerical model. To validate the capability of the VOF-based model described in this article, the propagation of regular waves is computed and compared with the theoretical results. By adjusting the phases of wave components, extreme waves are formed at given time and given position in the computation. The numerical results are compared with theoretical solutions and experimental data. It is concluded that the present model based on the VOF technique can provide acceptably accurate numerical results to serve practical purposes.

Research paper thumbnail of A numerical study of the transformation of water waves generated in a wave flume

Fluid Dynamics Research, 2009

This paper is concerned with the transformation of water waves generated in a numerical wave flum... more This paper is concerned with the transformation of water waves generated in a numerical wave flume. To set up the numerical wave tank, the non-periodic spectral method is used to solve potential flow equations incorporated with fully nonlinear free-surface boundary conditions and the wavemaker boundary condition. The flume is employed to simulate the transformation of water waves with different steepness and wavelength. The calculation results of the free-surface elevation and the corresponding amplitude wave spectrum are compared with the linear wave solutions and the analytical solutions. The result of the investigations is that short waves agree well with the linear solutions, whereas the long waves with limited Ursell numbers are consistent with the analytical solutions. Physical experiments are conducted in a wave flume to verify the numerical scheme. Good agreement between the experimental and the numerical results is obtained. The time-frequency information of waves is also investigated based on the wavelet transform. The numerical results show that short waves with smaller Benjamin-Feir index (BFI) are more stable than long waves with larger BFI in finite wave depth.

Research paper thumbnail of Validation of the initialization of a numerical wave flume using a time ramp

Fluid Dynamics Research, 2010

ABSTRACT This study investigates the initialization of nonlinear free-surface simulations in a nu... more ABSTRACT This study investigates the initialization of nonlinear free-surface simulations in a numerical wave flume. The numerical wave flume has been detailed in a previous paper (Zhao et al 2009 Fluid Dyn. Res. 41 035510). Due to mismatch between linear input wavemaker motion and the kinematics of fully nonlinear waves, direct numerical simulations of progressive waves, generated by a sinusoidally moving wavemaker, are prone to high-frequency wave instability unless the flow is given sufficient time to adjust. A time ramp is superimposed on wavemaker motion at the start, which allows nonlinear free-surface simulations to be initialized with linear input. Three time ramp functions are introduced to check the difference of the time ramp. The time–frequency information of waves is also investigated by means of wavelet analysis. Numerical results show that the difference of the time ramp function is minor and the time ramp scheme is effective in stabilizing wave instability at the start of the simulation in a wave flume.