On the mechanism of symmetric vortex shedding

In this paper, we study the vortex-shedding mechanism of a square cylinder subjected to a mean flow with a superimposed pulsatile flow, under symmetric vortex shedding conditions. The near-body vortical events are interpreted in accordance with the two-dimensional vorticity transport equation, and the interactions between different vortical regions are quantified by circulation balance for a control volume in the near wake. Strong wake counter flow that splits into two branches, with one branch considerably disrupting the boundary layer while other branch cuts off supply of vorticity to the shear layer, is found to be an essential feature of symmetric vortex shedding. To further clarify the roles of correlated terms contributing to the vorticity generation and transport in the wake, the Reynolds number was varied while keeping other excitation parameters constant. This results in the intensity of symmetric vortex shedding increasing with Reynolds number. The increase in wake counter-flow strength with increased Reynolds number cannot be explained either by a relative velocity between the mean and pulsatile flow components or relative acceleration between bluff body and fluid. Based on the results it is inferred that the brief rolling up of the shear layer to form a wake vortex during a part of the pulsation cycle, together with the associated unsteady tangential pressure gradient around the body, contribute to the stronger wake counter flow at higher Reynolds numbers. At lower pulsation amplitudes, the tendency of the fluid in the wake to form strong counter flow can interact significantly with the transverse entrainment flow to alter the natural vortex-shedding mechanism, to give different vortex-shedding modes.