Large and small scale instabilities for shear layer along a perforated surface

The shear flow of a fully turbulent boundary layer along a perforated plate,which is bounded by a closed cavity on its backside, can give rise to highly coherent,self-sustained oscillations. These oscillations are characterized in terms of unsteadypressure fluctuations and quantitative images of the instantaneous and averaged flowstructure using a technique of high-image-density particle image velocimetry (PIV).Variations of the effective length L of the perforated plate show nearlyinvariant values of dimensionless frequency fL/U; in which f is the predominantfrequency of oscillation and U is the freestream velocity. In fact, this relationshipholds even when the diameter of the hole pattern is altered. Variation of the holediameter D does, however, strongly influence the amplitude and degree oforganization of the self-sustained oscillation. It is demonstrated that, as the holediameter becomes larger relative to the inflow boundary layer thickness, theamplitude of the predominant spectral peak is substantially attenuated and, in alimitingcase, undetectable.The present investigation also demonstrates that self-sustained oscillationscan be effectively attenuated by three-dimensional surface elements in the form ofvortex generators. In absence of the three-dimensional surface elements, a well-defined front of the unstable oscillation propagates along the surface of the plate. Inpresence of an appropriate surface element, the front is no longer detectable, andinstantaneous and averaged defects of the streamwise velocity and surface-normalvorticity exist along the span of the plate.Keywords: Self-sustained oscillations, Perforated plate, Vortex generators, PIV