On the inversion of the von Karman street in the wake of a confined square cylinder

This paper considers the incompressible two-dimensional laminar flow around a square cylinder symmetrically positioned in a channel. In this type of flow, even if vortices of opposite sign are alternately shed from the body into the wake as in the unconfined case, an inversion of their position with respect to the flow symmetry line takes place further downstream. A numerical analysis is carried out to investigate the physical origin of this phenomenon and to characterize the position in the wake at which the vortices cross the symmetry line. It is shown that, for low to moderate blockage ratios, the fundamental cause of the inversion of the vortices is the amount of vorticity present in the incoming flow, and a dynamic interpretation in terms of vorticity interference in the wake is given. Further insight is gained through a linear stability analysis of the vortex shedding instability.