Active control of a mixing layer by upstream influence from an oscillating edge

Abstract

A mixing layer having finite length impinges on a downstream oscillating leading edge. Several classes of modulation of the upstream vortex development and its interaction with the edge are produced. At excitation frequencies, ƒe, relative to the inherent instability frequency (IIF), ƒ0*, of the mixing layer-edge system, there are four distinct regimes of response: (i) at low perturbation frequency, ƒe/ƒ0* All of these classes of vortex interaction are phase-locked with respect to the edge motion. Particularly noticeable is the occurrence of the largest-scale, IIF frequency vortices for ƒe/ƒ0* slightly greater than ½ and their attenuation at ƒe/ƒ0* = 1. In other words, the maximum response of this externally-excited system occurs not at the IIF of the self-excited system, but well below it. Over this range of excitation frequency ƒe/ƒ0*, new vortex-edge interactions continuously occur. This is because the upstream shear layer has been perturbed by the motion of the oscillating edge. Although the vortex-oscillating-edge interaction pattern varies with excitation frequency, it appears that the arrival of the incident vortex at the edge relative to the phase of edge position is such that the process of secondary vortex generation is, enhanced. This suggests that there is a phase relation of these events to the upstream influence on the sensitive region of the shear layer at the upstream separation, such that the vortex patterns are phase-locked.