Attenuation of Self-Excited Oscillations of Shear Flow Past a Perforated or Slotted Plate: Effects of Leading-Edge Ramp

Grazing flow past a perforated or slotted plate, bounded on its backside by a closed cavity, can give rise to highly coherent, self-sustained oscillations that have a wavelength much longer than the length scale of the slot or perforate. This investigation aims to suppress these purely hydrodynamic oscillations via small amplitude deflection of the inflow, induced by a ramp located at the leading-edge of the plate. Both unsteady pressure measurements and quantitative imaging of the flow structure reveal the essential features of the most effective attenuation. A central aspect is formation of a large-scale bubble involving steady entrained flow through the slots or perforates. Remarkably, attenuation can be achieved when the height of the ramp $h$ is approximately two orders of magnitude smaller than the effective length $L$ of the plate, i.e., $h/L \quad \ge $ 0.025.