Effect of aft wall offset and ramp on pressure oscillation from confined supersonic flow over cavity

A study is conducted on supersonic flow of Mach number 1.65 over rectangular cavity of length to depth ratio (L/D) 3 with aft wall ramp angles of 30°, 60° and 90° for with and without aft wall offset. Unsteady pressure measurement and schlieren flow visualization is employed to gain insight into the flow physics. Schlieren visualization revealed shock train like pattern for the cavities without offset. These shock structures are unsteady and oscillated longitudinally inside the test section over the cavity. The shock pattern disappeared as the blockage produced due to cavity pressure rise and shear layer oscillation is minimized by offsetting the cavity aft wall. Experimental investigations confirmed that the size of the cavity relative to the test section could have significant effect on the nature of flow field over the cavity and also on the acoustic oscillations within the cavity. For 90° ramp angle the dominant mode frequency in pressure oscillations shifted from second to third mode as the cavity aft wall is offset; however the shift in dominant mode frequency is not observed for the 60° cavity with aft wall offset. The forward traveling acoustic wave inside the cavity is confirmed to be present for 60° and 90° ramps while it is found to be absent for 30° ramp angled cavities. Analysis of the unsteady pressure measured inside the cavity revealed that the shock train has significant effect on the nature of acoustic oscillations from within the cavity. Aft wall offset is found to be effective in avoiding the occurrence of shock train and it is also found to increase the level of cavity pressure fluctuations. The highest pressure occurred at the aft wall and static pressure at all locations inside the cavity decreased as the aft wall ramp angle is reduced.