Understanding the Mechanism of Cavity Resonance Suppression Using a Cylindrical Rod in Crossflow

Abstract

A number of previous experiments have shown that high frequency excitation can lead to dramatic suppression of cavity ∞ow resonances typically associated with modern aircraft weapons bays. However, despite all the documented efiectiveness of this method, there is a lack of clear understanding of the physics driving the suppression mechanism. In this regard, the key issue addressed in this paper is whether resonant acoustics suppression due to high frequency excitation is a result of shear layer stabilization, and, if this is so, what is the mechanism that causes this stabilization. High frequency excitation was introduced into the shear layer of a cavity by means of spanwise coherent vortex shedding in the wake of a smooth, circular cylinder (the ‘shedding+wake’ conflguration). It was found that this conflguration dramatically suppressed the self-sustained resonant acoustic tones in a cavity having length-to-depth ratio, L=D = 2:0, over a wide range of subsonic freestream Mach numbers. On the other hand, a conflguration that neither introduced high frequency excitation, nor had a wake-type proflle (the ‘nonshedding+no-wake’ conflguration) showed inconsistent resonance suppression characteristics. Time-averaged velocity proflles at various streamwise locations for all the conflgurations revealed that the baseline and nonshedding+no-wake conflgurations developed similar to free shear layers whereas the shedding+wake conflguration exhibited a wake-type mean velocity proflle due to the velocity deflcit region downstream of the cylinder. Linear stability calculations on the mean proflles close to the upstream edge of the cylinder revealed that the wake type proflle of the shedding+wake conflguration had a much smaller growth rate compared to that of either the baseline or nonshedding+no-wake conflgurations. It is hoped that the results presented in this paper will ofier better insight into designing efiective cavity resonance suppression devices for future aircraft applications.