Abstract:

Flow-excited cavity resonance is a common, generally undesirable, sound-generating phenomenon that occurs in many engineering applications. In this study, low-speed cavity resonance suppression techniques were investigated. The focus was on two methods: leading edge spoilers and leading edge air mass injection. Experiments were conducted in a closed section wind tunnel, which included a secondary blowing apparatus for controlling boundary layer thickness. The cavity, attached to the wind tunnel test section, acted as a side-branch resonator. Smoke flow visualization was performed for various operating conditions. For example, a nondimensional spoiler length of 0.156 (based on cavity length) inclined at a 50-deg angle of attack typically provided a 12 dB (re: 20 (mu)Pa) reduction of the 140-dB, 124-Hz tonal cavity pulsation. Air mass injection through a perforated array at a rate of 2.8 liter/s yielded a 27-dB reduction. In general, the results suggested that such leading edge suppression techniques yield greater orifice shear layer thicknesses, similar to that of increasing the boundary layer thickness. The thickened shear layer may cause greater vortex diffusion, a key parameter establishing the level of the cavity pressure fluctuations. Diffuse vortices lead to weaker cavity pressure oscillations and lower vortex convection velocities.