A Numerical Study On Three-dimensional Fluid Flow With Free Surface Based On Gas-liquid Two-phase Flow Model

The gas-liquid two-phase flow numerical simulation has become a hot and advanced research on Computational Fluid Dynamics (CFD) in recent years. To accurately describe the moving interface between the air and water phases is the key technique in developing the gas-liquid two-phase flow model, which directly affects the convergence of iterative computation and the stability of calculation results. Therefore, the establishment of a three-dimensional two-phase flow numerical model with free surface tracking method of high precision is the main research approach to numerical study on fluid flow of free surface.This thesis first comprehensively reviewes various numerical methods dealing with fluid flow of free surface with the advantage and disadvantage of these methods analyzed respectively and then introduces the VOF method in detail. Considering the necessity of simulating the large-scale flow with complex interface, a 3D numerical model of two phase flow is presented based on the 3D unsteady Navier-Stokes equations, in which the governing equations are numerically discreted on grids with divided structure by means of finite volume method (FVM). In this model, two High-resolution schemes of VOF method(CICSAM&STACS) are applied to capture the interface between the air and water phases. These methods are characterized by simple programming and good description of the combination, breaking and splitting of the free surface. Meanwhile, with algebraic multi-grid method (AMG), linear system of equations is solved, and the flow solver is parallelized by domain decomposition using OpenMP. In order to verify the accuracy and the effectiveness of simulation of complex fluid flow with free surface using this model, we verified these two methods in a shifting, rotating, shearing and 3D deformation current field. By comparing and analysing the difference between the results,it can be concluded that the STACS-VOF method can achieve much higher precision in tracking interface.The numerical model of two phase flow adopted in this thesis takes the role of surface tension into account. The numerical simulation of typical 3D problems on free surface are used to test the model by using the STACS-VOF model and the CSF model, for example, the free-fall water droplet impacting onto a liquid pool, 3D gas bubble rising in a quiescent liquid and the sloshing liquid. The simulation results are similar to the actual physical phenomena, which verify the reliability and effectivenessof the model.Finally, by using Large Eddy Simulation (LES) based on STACS-VOF model, this thesis probes into the movement of dam-break waves on a dry bottom and wet bottom in lower reaches respectively. Moreover, the numerical model simulates the breaking of dam break wave and its propagation over a uniform slope. It is found that the gas-liquid two-phase flow model put forward in this thesis performs very well in simulating the movement of air-entrained flow and such phenomena as the breaking, overturning, splash-up and so on in the propagation process.