On the morphology of two-dimensional laminar vortex streets behind triangles

The two-dimensional laminar vortex streets behind a triangle have two morphologically distinct structures depending on the Reynolds number and the aspect ratio of the triangle. These two structures are the conventional structure and the separated rows structure, where the latter is characterized by a thin layer of irrotational fluid between two vortex rows. In this paper, by means of numerical simulation, we find that the separated rows structure occurs when the thickness of boundary layers is less than 25% of their separation distance. We also show from the linear stability analysis that the criterion is related to the coupling of two boundary layers in producing unstable modes.