Visualization of wave propagation within a supersonic two-dimensional cavity by digital streak schlieren

Optical measurements were carried out in planar two-dimensional open shallow cavities in order to determine how the flow field inside the cavity changes if the length-to-depth ratio of the cavity and the conditions of the boundary layer at the cavity leading edge are varied. The main challenge in this configuration, namely the fact that there are often no clearly identifiable wave fronts in the flow within the cavity, was overcome by applying a digital streak schlieren technique in combination with time-resolved high-speed flow visualizations. Using this approach, one can identify the propagation of waves within the cavity which allows one to determine the frequencies of flow oscillations inside the cavity entirely by optical means. The results from these measurements showed excellent agreement with independently conducted pressure measurements, simulations and analytical predictions. The applied technique also provides a measurement for the convective flow velocity within the cavity, for which different values can be found in the literature. The paper presents the results obtained for a Mach 2 supersonic flow over shallow rectangular open cavities with length-to-depth ratios of 3, 5, 6 and 8.