Large-Eddy Simulation and Mechanism of Secondary Instability of Flow around a Cylinder

摘要

Flow around a circular cylinder is simulated with large-eddy simulation at Re=200,and the mechanism of the origin of secondary vortex street is studied.The results show that the vortex street generated in the cylinder wake disappears as the flow moves downstream.Then,secondary instability occurs in the far wake of the cylinder after the primary vortex street dying away.The processes of the first instability and the secondary instability in the cylinder wake are recorded in the simulation.With these,instability of the entire flow field is studied with the energy gradient method It is found that it is the high mechanical energy gradient generated in the wake by the zero velocity gradient that leads to the instability.As the vortex developing at rear of the cylinder,the value of the energy gradient function becomes low,which leads to the vortex dying away.In the downstream,the value of the energy gradient function is enlarged again due to perturbation,which leads to the secondary instability,as the flow moving further in far wake of the cylinder.It can be concluded that the interaction of the variation of the value of the energy gradient function and the perturbation leads to the secondary instability.