Suppression of cavity oscillations using upstream mass injection

An investigation was conducted to determine the effectiveness of upstream secondary mass injection in suppressing oscillations that cause acoustic waves in rectangular cavities. The tests were conducted at a Mach number of approximately 1.8, and at a test section Reynolds number of approximately 1.7 x 10⁷/ft. The boundary layer became fully turbulent before reaching the cavity. The mass was injected through a series of holes just upstream of the cavity at several different mass flow rates. Two different hole density patterns (porosities), a high density and a low density, were used in the experiment to judge the relative effectiveness. Turbulent boundary layer characteristics were examined, and frequency spectrums of the cavity oscillations were measured. The results showed significant suppression of sound pressure levels; in the best case, the peak of the frequency spectrum was reduced from 172 dB (no suppression) to 145 dB. In addition, the low density hole pattern was judged to be much more effective in suppression at lower mass injection rates. It was shown that the low density configuration has a larger shear layer momentum thickness, and that the greater suppression is probably caused by this. An attempt was made to develop a nondimensional blowing parameter that could describe suppression effectiveness for any given configuration. It was established that the momentum thickness should be incorporated into the blowing parameter.