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 ARTIFICIAL VENTILATED CAVITY

1. INTRODUCTION The cavitation phenomenon[1,2] has already been paid high attention as a primary characteristic in the process of underwater body’s manufacture and exploitation. For a long time, the purpose of cavitation research is to prevent the occurr…     

NUMERICAL INVESTIGATION OF THE VENTILATED CAVITATING FLOW AROUND AN UNDER-WATER VEHICLE BASED ON A THREE-COMPONENT CAVITATION MODEL

Based on the Reynolds-Averaged Navier-Stokes equations and mass transfer model,an approach,where a three-component cavitation model is proposed,is presented to simulate ventilated cavitating flow as well as natural cavitation.In the proposed cavitation model,the initial content of nucleus in the local flow field is updated instantaneously,and is coupled with the Rayleigh-Plesset equation to capture the cavity development.The proposed model is applied to simulate the cavitating flow around an under-water veh...     

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 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.     

NUMERICAL SIMULATION OF SLOSHING IN RECTANGULAR TANK WITH VOF BASED ON UNSTRUCTURED GRIDS

A new method for sloshing simulation in a sway tank is present, in which the two phase interface is treated as a physical discontinuity, which can be captured by a well-designed high order scheme. Based on Normalized Variable Diagram (NVD), a high order discretization scheme with unstructured grids is realized, together with a numerical method for free surface flow with a fixed grid. This method is implemented in an in-house code General Transport Equation Analyzer (GTEA) which is an unstructured grids finite volume solver. The present method is first validated by available analytical solutions. A simulation for a 2-D rectangular tank at different excitation frequencies of the sway is carried out. A comparison with experimental data in literature and results obtained by commercial software CFX shows that the sloshing load on the monitor points agrees well with the experimental data, with the same grids, and the present method gives better results on the secondary peak. It is shown that the present method can simulate the free surface overturning and breakup phenomena.     

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 STUDY ON AIR FLOW AROUND AN OPENING WITH LARGE EDDY SIMULATION

Jet characteristics of air supply opening in a ventilating or an air-conditioning system is primarily decided by the folw state in the duct connected to the opening. It is valuable to study the opening jet characteristics and the flow state in a duct. In this study, the Large Eddy Simulation (LES) technique combined with the Tarlor-Galerkin Finite Element Method ( FEM) in Computational Fluid Dynamics (CFD) was applied to the problem. The 3-D flow fields in ducts around air supply opening under typical conditions were investigated by numerical simulation as well as experimental measurements. Numerical results agree well with the available experimental data. It indicates that the LES method is available under the conditions with complicated boundaries and inner accompanied by anisotropic large-scale eddies, and it is credible to predict the jet deflection characteristics around an opening.     

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.     

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.     

NUMERICAL SIMULATION OF TURBULENT FREE SURFACE FLOW OVER OBSTRUCTION

A two-dimensional hybrid numerical model, FEM-LES-VOF, for free surface flows is proposed in this study, which is a combination of three-step Taylor-Galerkin finite element method, large eddy simulation with the Smagorinsky sub-grid model and Computational Lagrangian-Eulerian Advection Remap Volume of Fluid （CLEAR-VOF） method. The present FEM-LES-VOF model allows the fluid flows involving violent free surface and turbulence subject to complex boundary configuration to be simulated in a straightforward manner with unstructured grids in the context of finite element method. Numerical simulation of a benchmark problem of dam breaking is conducted to verify the present model. Comparisons with experimental data show that the proposed model works well and is capable of producing reliable predictions for free surface flows. Using the FEM-LES-VOF model, the free surface flow over a semi-circular obstruction is investigated. The simulation results are compared with available experimental and numerical results. Good performance of the FEM-LES-VOF model is demonstrated again. Moreover, the numerical studies show that the turbulence plays an important role in the evolution of free surface when the reflected wave propagates upstream during the fluid flow passing the submerged obstacle.     

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.     

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...     

CALCULATION OF FLOW FIELD AND ANALYSIS OF SPAWNING SITES FOR CHINESE STURGEON IN THE DOWNSTREAM OF GEZHOUBA DAM

The Chinese sturgeon (Acipenser Sinensis) is one of the unique and important fishery resources in China. Since the construction of the Gezhouba Dam, the traditional migration route of the sturgeon has been blocked; consequently, the length of natural spawning sites is reduced from 800 km in the past to less than 5 km at present. As an endangered species, the Chinese sturgeon has become one of the most conserved aquatic species. In this article, the flow field of its spawning states in the downstream of Gezhouba Dam was simulated and analyzed using N-S equations and k ? ε turbulence model. Volume Of Fluid (VOF) method with the Finite Volume Method (FVM) was used to simulate the water-air two-phase flow to examine the computed area. On the basis of the ecological-hydraulic characteristics of Chinese sturgeon, the features of the flow field were investigated to provide theoretical support for the proper management of the Three Gorges Reservoir.     

THREE-DIMENSIONAL NUMERICAL SIMULATION OF AERATED FLOWS DOWNSTREAM SUDDEN FALL AERATOR EXPANSION-IN A TUNNEL

Air entrainment is known to be one of efficient and inexpensive methods to prevent cavitation damages in hydropower projects.The shape of sudden expansion-fall is used as a common device for mitigating cavitation erosions.The complex flow patterns with cavitation are numerically simulated by using the realizable k-εturbulence model and the air-water mixture model.The calculated results are compared well with the experimental results as well as those obtained with the k-εturbulence model with the Volume Of Fluid(VOF)Model.The calculated results agree well with the experimental data for the aeration cavity and wall pressure.Moreover,the air concentration near sidewall is simulated by a mixture model.It is found that the mixture turbulence model is superior to the VOF turbulence model.     

NUMERICAL SIMULATIONS OF 2D PERIODIC UNSTEADY CAVITATING FLOWS

1. INTRODUCTION Cavitation is a natural phenomenon especially existing in liquids. A cavitating flow generally involves a large number of vapor structures such as bubbles or vortices which are convecting downstream. When they reach high pressure zones, th…     

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....     

NUMERICAL SIMULATION OF FLOW AND HEAT TRANSFER CHARA-CTERISTICS OUTSIDE A PERIODICALLY VIBRATING TUBE

Fluid flow and heat transfer characteristics outside a vibrating tube were numerically simulated by the dynamic mesh method. The mechanism of heat transfer enhancement via periodic vibration of the tube was explored by using the field synergy principle. It is found that the field synergy angle between fluid velocity vector and temperature gradient vector for a periodically vibrating tube is significantly smaller than that for a stationary tube, and it changes approximately according to the sinusoidal law in a vibration period. The effect of time phase of the vibration on the field synergy angle and convective heat transfer coefficient were also discussed. Results indicate that the vibration can enhance heat transfer and this effect is more remarkable when time phase angle ranges between 50° and 1400 in a half period. Especially when the time phase angle is 90°, the average field synergy angle outside the tube reaches the minimum, which leads to the best heat transfer performance.     

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.     

A MATRIX-FREE IMPLICIT METHOD FOR INCOMPRESSIBLE FLOW AROUND 3D COMPLEX UNDERWATER BODIES

1.INTRODUCTION In recent years, the solutions of incompressible Navier-Stokes equations become more important as an integral component in the understanding, simulation, and design analysis of highly complex three-dimensional flow phenomena about complex configurations. An efficient and accurate flow solver is the key to developing a useful engineering tool for flow analysis. Over last several decades, many methods have been developed to solve the incompressible RANS equations. Among tho…