Reducing the generation of discrete-tones by micro-perforating the back of an open shallow cavity under a subsonic flow

Abstract

An experimental and numerical study has been carried out to investigate the effect of micro-perforating the back of an open shallow cavity under a low-speed air flow in order to reduce the amplitudes of the discrete tones due to the shear layer-cavity interaction. Open shallow cavities with length-to-depth ratio of 10 and 17 have been considered flush-mounted on the test section of a low-speed wind-tunnel with mean flow velocity 30 m/s. The bottom plate of the cavity was either a plain or an unbacked micro-perforated panel (MPP). A set of wall-pressure measurements over the bottom wall showed that the MPP was able to reduce by up to 8 dB the amplitude of the discrete tones, mostly noticeable beneath the upstream edge of the cavity within the recirculation bubble where the acoustic components dominate over the broadband components. However, the MPP may enhance the broadband components over and beyond the reattachment zone, thereby suggesting to micro-perforate the back wall only in the attenuation zone. These results were assessed against aeroacoustic numerical simulations performed in time domain using Lattice Boltzmann Method. The effect of the cavity length-to-depth ratio on the damping induced by the micro-perforated treatment was also examined.