Formation of type II vortex streets

In experiments in 2D soap film, we observe two kinds of vortex streets. The ``type I'' vortex street is stable and its K\'{a}rm\'{a}n ratio (the ratio of the transverse spacing to the longitudinal spacing of constituent vortices) is between 0.3 and 0.5. In contrast, the ``type II'' vortex street is characterized by its meta-stability and a much higher K\'{a}rm\'{a}n ratio, measured between 0.5 and 0.7. We studied the condition of formation of the type II vortex street by independently controlling two length scales of the system - the head-on width of the obstacle $W$ and the thickness of the boundary layer $\delta$ before detachment. Our experiment suggests that the vortex street is type II when $\delta/W<0.4$. The type II vortex street eventually evolves into the type I at downstream. The lifetime of this meta-stable configuration is strongly affected by the thickness $\delta$ of the boundary layer.