The combination of a shear layer with a significant wake component has the potential to produce a rich and complex flow pattern. Typically, wakes are classified as absolutely unstable and shear layers are in general convectively unstable, see Huerre & Monkewitz [1]. The linear stability calculations of Wallace & Redekopp [2] indicate that a bifurcation boundary exists between convectively unstable and absolutely unstable modes for a shear flow with a significant wake component. The boundary is a function of the velocity ratio in the shear layer and the size of the wake deficit.  In order to freely vary these two parameters experimentally a gravity driven vertical soap-film tunnel, with a splitter plate of the form described by Georgiev & Vorobieff [3], was constructed. The flow was illuminated with a low-pressure sodium lamp. The photograph in Figure 1 shows a visualization of a classical shear layer with a row of vortical structures, spinning in an anti-clockwise sense. Figure 2 shows the classic von Karman street formed behind a cylinder, in this case a 3mm diameter pin. In order to introduce a wake component to the shear layer the 3mm diameter cylinder was introduced at the trailing edge of the splitter plate. The photographs in Figure 3 show visualizations of the resultant wake-shear layer structures. The introduction of the cylinder has had the effect of creating a non-symmetric von Karman vortex street. As the flow develops the vortical structures of a clockwise sense produced on the low speed side of the wake appear to be merging with the stronger structures formed on the high speed side of the wake as the flow transitions to a conventional shear layer.

[1] P. Huerre and P.A. Monkewitz, Annu. Rev. Fluid Mech. 22, 473 (1990).

[2] D.A. Wallace and L.G.  Redekopp,  Phys. Fluids A 4, 189 (1992).

[3] D. Georgiev and P. Vorobieff, Rev. Sci. Inst. 73 (3), 1177 (2002).

               Figure 1:Classic shear layer                           Figure 2: Karman wake behind a cylinder

                                                  Figure 3: Shear layer-wake interactions