Phenomenology of Kármán vortex streets in oscillatory flow

Vortex wakes of circular cylinders at low Reynolds numbers have been investigated. Sound waves are superimposed on the flow in mean flow direction. In this configuration the Kármán vortices are shed at the sound frequency or at subharmonics of the sound frequency. The Karman vortex street is treated as a nonlinear self-excited flow oscillator with forced oscillations. Using a flow visualization technique a variety of wake structures has been identified as a function of sound frequency and sound amplitude, but independent of the Reynolds number. The superimposed sound influences the distribution of circulation and accordingly the shedding mechanism. Primary vortex and secondary vortex are shed simultaneously from one side of the cylinder. The alternate vortex shedding is arranged spatially and temporally. Structures along the vortex axes are revealed.Parts of this paper have been presented at IUTAM Congress 1984, Lyngby, Denmark, and at ICNM Conference 1985, Shanghai, P. R. China