Lattice Boltzmann Simulation Of Flow Field Geometry Mutation Site Mechanical Environment

Flow around cylinder and blood flow in vessel,two phenomena existing extensively in nature,have attracted many researchers'attention for a long time because of their importance of human being and were studied by theoretical and experimental methods in the past.After the inventation of computer,the numerical simulation method,an indispensable method in modern research work,has been used to deal with many complex problems which are difficult to solve by theoretical and experimental methods,this method is a new way for researchers to investigate some complex phenomena.In the thesis,the flow around the square cylinder and blood flow in a vessel with bifurcation are investigated by Lattice Boltzmann method,a numerical simulation method of studying flow phenomenon,the investigation includes two fields:(1) Flow around square cylinder.This field includes three parts:first the flow around a square cylinder placed in the center of a channel is simulated,it is found that the wake of the square cylinder presents two different patters at different Reynolds numbers:a pair of symmetrical vortices appear at small Reynolds numbers and a vortex street appear at big Reynolds numbers,the transition from symmetrical vortices to vortex street appears at Re≈60.The Strouhal numbers are also calculated at different Reynolds numbers,and they are in good agreement with published data; In the second part,the flow around two square cylinders arranged side by side is investigated at invariable Reynolds number,the results show that,the flow pattern is decided by gap ratio:the asymmetrical flow pattern occurs at small gap ratio and the symmetrical flow pattern occurs at big gap ratio,the transition of flow from asymmetrical pattern to symmetrical pattern appears at s=1.5.When s＜1.5,the flow is biased,and the two square cylinders have different lift and drag coefficients,their Strouhal numbers are fluctuant distinctly;When s＞1.5,the flow is symmetrical and the two square cylinders have equal drag coefficients and Strouhal numbers,their lift coefficients are symmetrical with x=0;In the third part,the flow is simulated at different gap ratios and different Reynolds numbers,it shows that,the time averaged lift and drag coefficients of two square cylinders are fluctuant with the increase of Reynolds number at s=1.5 and 1.8,but the fluctuations almost disappear with the increase of Reynolds number at s=2.5,this difference is induced by the difference of flow pattern.At s=1.5 and 1.8,the flow is in the critical state from biased pattern to symmetrical pattern and it is unsteady,in this situation,the increase of Reynolds number would make the two square cylinders,they are in the symmetrical state and have similar averaged lift and drag coefficients at small Reynolds numbers,in the asymmetrical state and have fluctuant averaged lift and drag coefficients;At s=2.5, the flow is in symmetrical state and it is unchanged with the increase of Reynolds numbers,in this state,the two squarec cylinders are also in the symmetrical state and have similar variable averaged lift and drag coefficients,but they are not fluctuant.(2) Blood flow in a vessel with bifurcation.The blood flow in a vessel with one bifurcation,secondary-bifurcation and multi-bifurcation are investigated,respectively. The first investigation is about the flow in the vessel with one bifurcation,the flow characteristics near the bifurcation were observed,and the results reveal that the flow velocity decrease before entering the branches,the maximum velocity is not at the centerline of main tube but at the places near the walls.In the second investigation, the flow in the vessel with secondary-bifurcation is simulated with different bifurcation angles,it shows that the maximum velocity increase and shift towards the walls of branches with the increase of bifurcation angles as the force from the walls changing the flow velocity and its direction.In the third investigation,the flow in the self-similar veseel with multi-bifurcation is simulated,it is found that the flow running in the inner branches and almost motionless in the external branches,from the main tube to branches,the flow is slower and slower.This characteristic is similar with the flow characteristics of most biologic branch systems.