Three-dimensional vorticity modes in the wake of a flat plate

Abstract

The three-dimensional development of the wake behind a flat plate separating two laminar streams of equal velocity, subjected to perturbations in its initial conditions, is studied experimentally and numerically at moderate Reynolds numbers. The effect of single waves as well as subharmonic streamwise forcing is discussed. For the case of a single wave, streamwise forcing combined with a sinusoidal spanwise perturbation, the two distinct three-dimensional vorticity modes found in an early study [J. Fluid Mech. 190, 1 (1988)], are further studied and their symmetry properties analyzed. Depending on the orientation of the spanwise perturbation, the counter-rotating pairs of streamwise vortex tubes forming in the braids are shown to induce undulations in the cores of the Karman-like vortices, resulting in either an in-phase or a varicose configuration. Furthermore, when the wake is subjected to subharmonic streamwise forcing, additional modes of the topology of the vorticity field are shown to exist. The evolution of these subharmonic three-dimensional modes is analyzed as a function of the initial conditions.