A supersonic flow over a rectangular cavity is known to oscillate at certain predominant frequencies. The present study focuses on the effect of the cavity length-to-depth ( / ) ratio on the frequency for a free-stream Mach number of 1.7. The pressure oscillations are measured by changing the / ratio from 0.5 to 3.0, and the power spectral density is calculated from the temporal pressure signals for each / ratio. The results demonstrate that the spectral peaks for an / ratio of less than ∼1 and greater than ∼2 are accounted for by the feedback mechanisms of the transverse and longitudinal oscillations, respectively. The results also demonstrate that the spectral peaks in the transition (1 <∼ / <∼ 2) are accounted for by either of the two feedback mechanisms of transverse and longitudinal oscillations; that is, the flows under the transition regime oscillate both transversely and longitudinally.
A mechanism of cavity-induced pressure oscillation in supersonic flows is not well understood in spite of a lot of former investigations. Especially, the process by which the pressure wave is generated and the path of the pressure wave propagating inside the cavity remain unclear. In order to clarify these, the oscillatory behaviors in the supersonic flow over a rectangular cavity are visualized by the schlieren method with a high-speed camera in the present study. The inlet Mach number of the flow is 1.68. The length and depth of the cavity are 14.0mm and 11.7mm respectively; i.e., the length-to-depth ratio of the cavity is 1.20. The pressure oscillation near the trailing edge of the cavity is also measured by use of the semiconductor-type pressure transducer simultaneously with the visualization. As a result, the pressure waves propagating inside as well as outside the cavity are successfully visualized. In addition, the relationship between the shear layer displacement, pressure wave generation and pressure oscillation at the trailing edge of the cavity are clarified experimentally.
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