Bimodal vortex shedding in a perturbed cylinder wake

Cylinder wakes display distinct modes of vortex shedding when perturbed by appropriate means. By investigating experimentally the wake of a circular cylinder perturbed by a periodic fluctuation imposed on the inflow velocity, it is shown that bimodal behavior is possible. During a given experiment, the wake switches back and forth between two different vortex shedding modes, more specifically, a 2S↔2P transition is observed. No discernible change in the timing of vortex formation is found to accompany the transition. Modal decomposition of the velocity field is employed to exemplify the interaction of the imposed symmetrical perturbation and the intrinsic antisymmetrical instability of the near wake.