An experimental investigation of cavity aeroacoustics and control in subsonic flows

Mass injection upstream of a cavity was used to actively control the unsteady pressure oscillations inherent in a cavity at subsonic speeds. High amplitude pressure oscillations result when resonance (matching of frequencies) occurs between the shear layer's unsteady up and down movement and pressure disturbances that feed back, upstream, within the cavity. The measurements were performed in a blow down wind tunnel at nominal Mach numbers of 0.5 up to 0.8 and for unit Reynolds numbers between 3 and 7 million per foot. It was shown that mass injection significantly alters the upstream boundary layer and the shear layer over the cavity so that the feed back disturbances do not match with the unsteadiness of the shear layer. Once the two frequencies are shifted, the possibility of resonance interaction can not exist. Upstream mass injection has effectively eliminated the large amplitude oscillations, with acoustic levels which were measured as high as 167.5 dB, that normally exist when mass injection is not present. Sound reductions were as much as 16 dB and 17.3 dB at Mach numbers of 0.5 and 0.8 respectively. Cone probe measurements, over the cavity, have verified the existence of streamwise vortices that are possible contributing mechanisms for reducing pressure oscillations. Water table experiments have also been conducted, and show visually the effects of upstream mass injection on single and double cavity arrangements. This study is a continuation of a systematic study of the attenuation of aeroacoustic oscillations in flows over weapon bay cavities reported by Vakili et al.