Secondary Vortex Street in the Wake of a Thin Rectangular Cylinder

The transition from the primary (Karman) and two-layered vortex streets to the secondary vortex street in the far wake of a thin rectangular cylinder is examined through direct numerical simulations. It is found that at Reynolds numbers of 110 and 120 the secondary vortices are induced by the hydrodynamic instability of the two parallel shear layers (without vortex merging), whereas at Reynolds numbers above 130 the secondary vortices are formed from direct merging of the two-layered vortices. In summary, depending on the Reynolds number either hydrodynamic instability or vortex merging may be the physical cause for the secondary vortex street.