Vortex streets behind triangular objects

When a triangular object, instead of a circular object, is used to produce a vortex street, an exotic vortex arrangement is observed in addition to the commonly observed mushroom-like structure. The exotic structure is morphologically distinctive from the classical von Karman vortex street and is characterized by a thin layer of irrotational fluid that separates two rows of vortices. This structure appears when a dimensionless ratio between the thickness of the boundary layer and the size of the object is less than a certain value, which is measured to be 0.4 in our experiment. The experiment is modeled by a double shear layer profile, whose linear stability is solved numerically. The theoretical consideration shows a rough agreement with the measurement.