The appearance of subharmonic modes in bluff body wakes

This paper presents a series of observations of the onset of subharmonic modes in vortex wakes. Two main scenarios are investigated: the transition from two-dimensional to three-dimensional flow; and the appearance of subharmonic modes in flow-induced vibrations. Observations across flows past static bodies, bodies moving with a prescribed trajectory, and bodies freely responding to the flow forces are presented. In all cases, it is shown that the spatiotemporal symmetry of the classic Kármán vortex street must first be broken to allow a subharmonic mode to occur. The symmetry breaking can be introduced by modifying the geometry or some external forcing, or it can come from a prior spontaneous symmetry breaking bifurcation. However, the broken symmetry is not sufficient for the appearance of a subharmonic mode, and the strength of the asymmetry needs to reach a finite threshold before such modes arise. It is hypothesized that this finite threshold is related to the point at which interaction between vortices on the same side of the wake are stronger than interactions between vortices on opposite sides.