NUMERICAL SIMULATION OF EXTREME WAVE GENERATION USING VOF METHOD

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.     

THREE-DIMENSIONAL NUMERICAL SIMULATION OF WAVE SLAMMING ON AN OPEN STRUCTURE

The three-dimensional numerical simulation of wave slamming on an open structure in the splash zone is carried out based upon the Volume Of Fluid (VOF) method.A wave basin is established by solving the...     

NUMERICAL SIMULATION OF IRREGULAR WAVE-SIMULATING IRREGULAR WAVE TRAIN

In real sea states, damage incidents on offshore floating structures are not due to the whole time series of wave elevation characterized as statistical one but due to few extreme waves or wave groups in irregular wave train. So, using CFD tools to precisely simulate predetermined irregular wave train will lay sound basis for understanding the local characteristic of the flow field and impact loads on offshore floating structures when damage incidents occur. In this article, the generation of single extreme wave is investigated in a numerical wave tank. First, experimental irregular wave train is decomposed into certain number of small-amplitude waves. The Fourier series expansion is performed to determine the amplitude and initial phase angle of each wave component. A hydrodynamic transfer function is used to calculate the amplitude of wave-maker motion associated with each wave component. Then superposition is carried out on all of the wave-maker motion components to get the final wave-maker motion. With the wave-maker motion as input, simulation of the single extreme irregular wave train is modeled successfully. Then the method is applied to simulating a much more complicated irregular wave train. Once again main features of the complicated irregular wave train are reproduced compared with experiment carried out in the new deepwater experimental basin at Shanghai Jiao Tong University. In the simulation, dynamic mesh method is enabled to model the piston-type wave-maker, the Volume Of Fluid (VOF) method is employed to capture the free surface and a dissipation zone is introduced to deal with wave reflection.     

REFLECTION OF OBLIQUE INCIDENT WAVES BY PERFORATED CAISSONS WITH TRAVERSE WALL

The interaction of oblique incident waves with infinite number of perforated caissons is investigated. The fluid domain is divided into infinite sub-domains by the caissons, and eigen-function expansion is applied to expand velocity potentials in each domain. A phase relation is introduced for wave oscillation in each caisson, and the structure geometry is considered in constructing the models of reflection waves. The reflected waves with the present analysis include all of the waves traveling in different directions when incident wave period is short. Numerical examinations show that velocities at the inner and outer sides of the front walls of caissons are close to each other, and reflection coefficients satisfy the energy conservation relation very well when porous effect parameter is infinite. Numerical results show that the reflection coefficients of oblique incident waves are smaller for shorter caissons at low frequency, and decrease with the increase of wave incident angle.     

VISCOSITY EFFECTS ON THE BEHAVIOR OF A RISING BUBBLE

In the incompressible fluid flow regime,without taking consideration of surface tension effects,the viscosity effects on the behavior of an initially spherical buoyancy-driven bubble rising in an infinite and initially stationary liquid are investigated numerically by the Volume Of Fluid(VOF) method.The ratio of the gas density to the liquid density is taken as 0.001,and the gas viscosity to the liquid viscosity is 0.01,which is close to the case of an air bubble rising in water.It is found by numerical exp...     

NUMERICAL SIMULATIONS OF HIGHLY NONLINEAR STEADY AND UNSTEADY FREE SURFACE FLOWS

A numerical simulation model based on an open source Computational Fluid Dynamics (CFD) package-Open Field Operation and Manipulation (OpenFOAM) has been developed to study highly nonlinear steady and unsteady free surface flows. A two-fluid formulation is used in this model and the free surface is captured using the classical Volume Of Fluid (VOF) method. The incompressible Euler/Navier-Stokes equations are solved using a finite volume method on unstructured polyhedral cells. Both steady and unsteady free surface flows are simulated, which include: (1) a submerged NACA0012 2-D hydrofoil moving at a constant speed, (2) the Wigley hull moving at a constant speed, (3) numerical wave tank, (4) green water overtopping a fixed 2-D deck, (5) green water impact on a fixed 3-D body without or with a vertical wall on the deck. The numerical results obtained have been compared with the experimental measurements and other CFD results, and the agreements are satisfactory. The present numerical model can thus be used to simulate highly nonlinear steady and unsteady free surface flows.     

Three-dimensional numerical simulation of wave interaction with perforated quasi-ellipse caisson

The finite difference method and the volume of fluid (VOF) method were used to develop a three-dimensional numerical model to study wave interaction with a perforated caisson.The partial cell method was adopted to solve this type of problem for the first time. The validity of the present model, with and without the presence of caisson structures, was examined by comparing the model results with experimental data. Then, the numerical model was used to investigate the effects of various wave and structure parameters on the wave force and wave runup of the perforated quasi-ellipse caisson. Compared with the solid quasi-ellipse caisson, the wave force on the perforated quasi-ellipse caisson is significantly reduced with increasing porosity of the perforated quasi-ellipse caisson. Furthermore, the perforated quasi-ellipse caisson can also reduce the wave runup, and it tends to decrease with the increase of the porosity of the perforated quasi-ellipse caisson and the relative wave height.     

AN UNSPLIT LAGRANGIAN ADVECTION SCHEME FOR VOLUME OF FLUID METHOD

An Unsplit Lagrangian Advection(ULA) scheme for Volume Of Fluid(VOF) method is presented in this article.The ULA scheme is developed based on an algorithm of Piecewise Linear Interface Construction(PLIC).The volume fluxes between cells are calculated through solving the new equation of the linear interfaces in cells in the ULA scheme.The fluxes flowing out from one cell is the inflow fluxes for another cell.In this way the whole fluid volume is conserved strictly without using any redistribution algorithms....     

LARGE EDDY SIMLIATION WAVE BREAKING OVER MUDDY SEABED

The Large Eddy Simulation (LES) of the wave breaking over a muddy seabed is carried out with a Coupled Level Set and Volume Of Fluid (CLSVOF) method to capture the interfaces.The effects of the mud on the wave breaking are studied.The existence of a mud layer beneath an otherwise rigid bottom is found to have a similar effect as an increase of the water depth.As compared with the case of a simple rigid bottom,the inception of the wave breaking is evidently delayed and the breaking intensity is much reduced.The dissipation of the wave energy is shown to have very different rates before,during and after the breaking.Before and after the breaking,the mud plays an important role.During the breaking,however,the turbulence as well as the entrainment of the air also dissipate a large amount of energy.     

Experimental and Numerical Investigations on Horizontal Oil-Gas Flow

Experiments were carried out to investigate the characteristics of oil-gas flow in a horizontal pipe on a large scale (with the inner diameter D = 125 mm). With the experimental data, the flow patterns were presented. Through the analyses for the flow regime transition, it was found that there was a critical superficial velocity of liquid phase for the flow regime transiting from stratified flow to slug flow. The slug flow could not occur until the superficial velocity of liquid phase was higher than the critical velocity. For the flow pattern transiting from stratified to slug flow, the transmitting velocity of gas phase decreases with the augmentation of superficial velocity of liquid phase. On the basis of the experiments, numerical simulations of different flow patterns and their transitions were performed with the use of the Volume Of Fluid (VOF) technique. The results of the computations are shown to match well with the measured data in the experiments.     

A NEW NUMERICAL WAVE FLUME COMBINING THE 0-1 TYPE BEM AND THE VOF METHOD

A new coupling numerical wave model, based on both the Boundary Element Method (BEM) and the Volume Of Fluid (VOF) method, is established by taking advantages of the both methods to solve the wave-structure interaction problems. In this model, the wave transformation in front of structures is calculated by the 0-1 type BEM, and the intense wave motions near the structures are calculated by the VOF method. In this paper, the characteristics of the BEM and the VOF method are discussed first, and then the coupling treatments are described in detail. In the end, the accuracy and the validity of the coupling model are examined by comparing the numerical results with experiment results and other numerical results available for the interactions between regular waves with a monolayer horizontal plate.     

APPLICATION OF VOF TECHNIQUE TO CHANNEL IMPROVEMENT FOR PANJIATAI SHOAL IN THE YELLOW RIVER

In this paper, Volume Of Fluid (VOF) technique is used to simulate the free surface and the flow field affected by the groynes in the experimental flume and the natural river. A series of flume experiments are carried out. According to the orientation of groynes, angles from the flow direction to the axis of the groyne used in the experiments are assorted into obtuse angle, right angle, and acute angle as well. For each arrangement option, flow conditions are classified as the submerged and the non submerged one. Velocities and the water level affected by single groyne or double-groyne are measured. The calculated results are agreement well with the experimental data. Because of the high demands in the mesh quality and the computer capability, the VOF technique is seldom used to simulate the natural river. In this paper, the VOF technique is also used to simulate the Panjiatai Shoal of 2km long, one of the important improvement reaches in the Yellow River. Two improvement schemes are compared and analyzed, and the better one is recommended.     

NUMERICAL SIMULATION OF VENTILATED CAVITATING FLOW AROUND A 2D FOIL

By using a pressure-based method and the finite volume method in the framework of the time dependent Reynolds-averaged Navier-Stokes equations, the authors studied the unsteady process of ventilated cavities generated forcing air through an orifice in a 2D hydrofoil without natural cavitation physically. The computation was carried out with the Volume Of Fluid （VOF） technique to track the gas-liquid two-phase interface. The results of simulation indicate that the ventilation rate is an important parameter in determining the morphology of cavity. There exists a critical value to convert sheet cavity into supereavity. A high ventilation rate can induce a two phase interface fluctuation and enable the ventilated eavitating flow to present a characteristic of periodicity.     

NUMERICAL SIMULATION OF PROPAGATION AND BREAKING PROCESSES OF A FOCUSED WAVES GROUP

A two-dimensional numerical wave flume is developed to study the focused waves group propagation and the consequent breaking processes. The numerical model is based on the Reynolds-Averaged Navier-Stokes (RANS) equations, with the standard k-ε turbulence model to simulate the turbulence effects. To track the complicated and broken free-surface, the Volume Of Fluid (VOF) method is employed. The numerical model combines the "Partial Cell Treatment (PCT)" method with the "Locally Relative Stationary (LRS)" concept to treat the moving wave paddle so that various waves can be generated directly in a fixed Cartesian grid system. The theoretical results of the linear and nonlinear waves are used to validate the numerical wave flume firstly, and then a plunging breaking wave created by a focused waves group is simulated. The numerical results are compared to the experimental data and other simulation results, with very good agreements. The turbulence intensity, the flow field and the energy dissipation in the breaking processes are analyzed based on the numerical results. It is shown that the present numerical model is efficient and accurate for studying the waves group generation, the waves packet propagation, and the wave breaking processes.     

NUMERICAL SIMULATION OF SKIMMING FLOW OVER MILD STEPPED CHANNEL

1. INTRODUCTION The stepped channel/spillway finds its extensiveapplications in hydraulic and environmentalengineering, which is used to dissipate the energy ofreleased flood and also to be a landscape structure.Flow over the stepped channel/spillway is a…     

NUMERICAL SIMULATION OF HYDRODYNAMIC CHARACTERISTICS ON AN ARC CROWN WALL USING VOLUME OF FLUID METHOD BASED ON BFC

In the present study, a new algorithm based on the Volume Of Fluid (VOF) method is developed to simulate the hydrodynamic characteristics on an arc crown wall. Structured grids are generated by the coordinate transform method in an arbitrary complex region. The Navier-Stokes equations for two-dimensional incompressible viscous flows are discretized in the Body Fitted Coordinate (BFC) system. The transformed SIMPLE algorithm is proposed to modify the pressure-velocity field and a transformed VOF method is used to trace the free surface. Hydrodynamic characteristics on an arc crown wall are obtained by the improved numerical model based on the BFC system (BFC model). The velocity field, the pressure field and the time profiles of the water surface near the arc crown wall obtained by using the BFC model and the Cartesian model are compared. The BFC model is verified by experimental results.