Aft wall offset effects on open cavities in confined supersonic flow

The effect of aft wall offset in open cavities in a Mach 1.71 flow is investigated experimentally. Two different rectangular cavities with length to depth ratios of 2 and 3 are investigated to validate the effects over the range of the open cavities. Three different offsets of 0%, 5% and 10% are selected for exploring the effects of aft wall offset in cavity flow fields. For investigating the flow physics, unsteady pressure measurements and schlieren flow visualisation techniques are employed. All the cases tested, exhibited the characteristic cavity flow features. Schlieren flow visualisation clearly indicated the presence of shear layer and other shock features that are associated with the cavity flow field. Statistical analysis techniques viz. Fast Fourier Transforms, correlation, coherence and spectrogram are utilised for analysing the unsteady pressure data. Experimental results confirmed the reduction in tonal amplitude in offset cavities for both the length to depth ratios. The 10% offset case provided the maximum reduction in both the length to depth ratio, 2 as well as length to depth ratio, 3 cavity. Spectrogram studies indicated the presence of temporal mode switching in the higher offset value cases. Higher number of modes are also noted to be excited for the offset cases as evident from the spectrogram. From the correlation plot, the existence of the acoustic wave inside the cavity for all the cases is deduced. The maximum OASPL and static pressure values are observed at the aft wall.