An experimental study of mass injection pattern effects on cavity oscillations

An experimental study was performed to investigate the effects of mass injection on the suppression of aeroacoustic interaction in an open, rectangular cavity in supersonic flow. The oscillatory behavior of flow over a cavity can induce substantial pressure fluctuations and result in a potentially detrimental environment. Through the use of upstream mass injection, boundary layer and shear layer growth were enhanced, and the aeroacoustic effects were attenuated and, in some cases, virtually eliminated. The injectant was introduced perpendicular to a supersonic freestream through a series of small holes. Freestream Mach number was held constant at a nominal M=1.8. Additionally, the pattern of the injectant holes was varied to study three-dimensional and various vortical interactions induced by the hole patterns. Acoustic data and boundary-layer surveys were obtained for each injectant model and for various injectant flow rates. Mass injection upstream of an open cavity was shown to be an effective method of attenuating cavity aeroacoustics. Variation of the injectant hole patterns demonstrated that three-dimensional interactions can also be an effective means of aeroacoustic suppression; however, the hole pattern which most closely approximates a two-dimensional injectant/freestream interaction produced the maximum attenuation of the patterns considered.