Phenomenology of Kármán vortex streets in oscillatory flow

The interaction of vortex formation and shedding from a circular cylinder with sound waves is investigated in order to understand the coupling mechanism. The sound waves are superimposed on the low Reynolds number flow in the mean flow direction. In this configuration the Karman vortices are shed at the sound frequency or at its subharmonics. The Karman vortex street is treated as a nonlinear self-excited flow oscillator with forced oscillations. Using flow visualization, various wake structures are identified as a function of sound frequency and sound amplitude, but independent of the Reynolds number. It is found that the superimposed sound influences the circulation distribution and, accordingly, the shedding mechanism. The primary and secondary vortices are shed simultaneously from one side of the cylinder, and the alternate vortex shedding has a spatio-temporal distribution. (S.A.V.)