Formation of the secondary vortex street in the wake of a circular cylinder

Formation of the secondary vortex street in the wake of a circular cylinder was studied at Reynolds numbers lower than 160 by means of smoke wire technique of flow visualization and of hot-wire technique. It was seen by flow patterns and equi-vorticity lines that the secondary vortex street is formed by merging of vortices in the primary vortex street. When the wake is forced with a sound of a given frequency, the number of vortices to merge into a larger one is two or three, corresponding to the forcing frequency, which is one half or one third of the frequency of velocity fluctuations in the primary vortex street.