Passive Control of Transonic Cavity Aeroacoustics

Abstract

In recent years the continuing trend for the internalization of stores within an aircraft fuselage has led to a renewed interest in the field of cavity aeroacoustics. Open cavities exposed to transonic flow exhibit large pressure fluctuations which can result in damage to stores or components carried within the cavity. This study investigates the use of a passive resonant absorber based on Helmholtz resonators to attenuate the unsteady pressure fluctuations that arise in such cavity flows. The arrays are expected to remove energy from the high intensity cavity oscillations at the frequency to which they are tuned and therefore, to reduce the cavity noise. Six resonant arrays were designed to target individual Rossiter modes within a cavity. The arrays were tested in a small scale wind tunnel at both Mach 0.8 and Mach 0.9. The performance of the arrays were tested individually at both the front and rear wall of the cavity as well as in a combined arrangement. A peak attenuation of 14 dB was measured for an array at the front wall at Mach 0.9. A smaller attenuation of 8 dB was achieved when the same array was tested at Mach 0.8. Combined resonator installations at both the front and rear walls of the cavity further increased the peak sound pressure level (SPL) attenuation up to 18 dB at Mach 0.9. The investigation shows that passive resonant absorbers are a promising palliative method for the reduction of cavity acoustic modes at high subsonic speeds.