The Formation of the Secondary Vortex Street Instability

It is widely known that the near wake of bluff bodies is subject to the Karman vortex street instability. It is much less known that a qualitatively similar instability also exists in the far wake, in which large-scale vortices form, whether a near-wake Karman exists more upstream, or not. The cause of this downstream “secondary” vortex street instability is a source of controversy in the literature.

In this work, we investigate the secondary street in the far wake of solid plates (i.e. with Karman vortex shedding) and highly porous plates (i.e. without Karman vortex shedding). We perform a combination of LES and high-speed planar PIV at turbulent Reynolds numbers. The results are analyzed using the modal decomposition techniques. Our results suggest that the secondary vortex street formation is a sequential process, in which smaller vortices give their place to larger ones, and so forth. The effects of plate homogeneity and porosity on the resulting flow field are also discussed.