Mitigation of pressure oscillations induced by supersonic flow over slender cavities

Abstract

Supersonic flow over a slender cavity excited high-frequency and high-g vibrational forces inside the cavity. The excitation of the whistle is caused by vortex shedding at the upstream end of the cavity and vortex interaction with the downstream end of the cavity associated with sound generation. The sound is fed back upstream and drives the vortex-shedding process. To suppress the acoustic whistle, an experimental test program was undertaken to characterize the acoustic cavity oscillations in a deep and narrow cavity, using microphones and flow visualization, and reduce the coherence of the vortices by geometric changes of the upstream end of the cavity. The changes that included multisteps and pins extending into the supersonic approach flow were selected to break up the orderly development of coherent vortices under acoustic excitation. With these passive shearflow control devices, acoustic amplitude reductions by a maximum factor of 5 were obtained, essentially eliminating acoustic pressure oscillation in the cavity over the entire range of Mach numbers tested. In this article, the detailed geometric and flow parameter variation and its effect on the cavity pressure oscillations are described. I. Introduction A COUSTIC oscillations inside cavities, induced by flow over the cavity, were studied extensively in the last two decades.16 This phenomenon is important in subsonic and supersonic flight of airframes with cavities such as wheel wells and weapons bays, due to the excitation of structural vibrations. The oscillations are usually characterized by broadband and pure-tone components. The broadband components are primarily associated with shallow cavities, whereas deep cavities usually excite pure periodic oscillations similar to those excited by the edge-tone phenomenon. Earlier investigations1 were aimed to quantify the sound emissions caused by cavity resonance. Later investigations attempted to develop analytical predictions for the phenomenon.2'3 Plumbee et al. 2 developed a mathematical model that was able to predict the resonant modes by relating the cavity oscillation to resonance induced by turbulent noise from the shear-layer flowing over the open end. This model was particularly successful in predicting the noise produced by deep cavities. East3 showed that some of the acoustic modes can be excited even at low subsonic Mach numbers. Over the years, different investigations proposed various theories for the excitation of the cavity pressure oscillations. Basically, most works relate these oscillations to interaction between the separated shear layer that spans across the open end of the cavities and the acoustic response of the cavity. Rossiter5 studied the problem experimentally in the Mach number range of 0.4-1.2 and cavity length to depth (L/D) ratios of 1-10 and concluded that an acoustic feedback is driving the oscillations. Rossiter proposed that the vortices in the shear layer that is separated from the upstream lip of the cavity, interact with the downstream lip generating acoustic pulses. These pulses propagate upstream, inside the cavity, exciting the shear layer at the upstream lip and thus enhancing the shedding of new vortices in the shear-layer. Using these arguments he devised an empirical equation for the tone frequencies /