Study On A Flow Around Rigid Cylinder And Its Stability

For a long time, the flow past cylinder is one of the most classical problems in fluid dynamcis. When the flow past a cylinder, the area of cross section is contracted, its velocity increases and its pressure is reduced. Becouse of the viscous force, boundary layer near the cylinder separates and forms the Karman vortex street phenomenon. In computational fluid dynamics( CFD), the numercial study of the flow past a cylinder problem has always been a classic case to verify the reliablity of the CFD program.Firstly we fast generate appropriate grid by controlling parameters of the grid based on the open source program OpenFOAM. The drag coefficient Cd, lift cofficient Cl, Strouhal number St in laminar flow simulation is discussed and compared with previous experimental and computational results. To simulate subcritical region of flow past a cyliner, SST k-ω turbulence model is applicated and its result is verified by comparing with other’s result. Finally the relationship between mean value of drag coefficient Cd,rms 、the mean value of the maximum number of lift coefficient Clmax,rms and Reynolds number Re is established.Secondly by using LES( Large Eddy Simulation), the research of channel flow and critical region of flow past a cylinder is conducted.We generated turbulence flow in a short time according to the instability of the coherent structure under sinusoidal disturbances, then the relation ship of the streamwsie velocity near the wall and the distance to the wall is discusssed.We get the critical Re of the flow past a smooth cylinder at low turbulent kinetic energy.Finnaly, we simulate the subcritical region of the flow past a single degree of freedom cylinder by using 6DoF dynamic mesh model. The data of the results is extracted, processed,analysed using Romberg numerical integration methond and other numerical fit method by C++programming language. We get that for flow past a stationary cylinder, its Cl has a sinusoidal variation and is periodic in a certain time. So in a certain time, cylinder with a single freedom is stable. But becouse of the difference of intial flow fields and the mass of cylinders, the stability of cylinder differs espacially cylinder’s amplitude of deviation. And the coupling of fluid and solid will influence the frequency of cylinder’s viration which causes deviation of the cylinder.On the other hand, after a certain period of time, even for a flow past a stationary cylinder, Cl thedoes not show periodic variation, so the cylinder will eventually offset. According to the similarity of the flow around a sphere and a cylinder, we study the effect of Karman vortex street in ball games by using the result of cylinder and parameters of balls.