Experimental Study of Subcavity in Supersonic Cavity Flow

An experimental study is conducted at Mach number 1.71 on rectangular open cavity of length-to-depth ratio (L/D) 2 with subcavity at the front wall. Subcavity at the front wall has been established as a passive control device. In the current study it is found that subcavity at the front wall can also act as a passive resonator.The dominance of fluid-dynamic or fluid-resonant oscillations accordingly causes the subcavity at the front wall to act as a passive control device or as a passive resonator in supersonic cavity flow. In the current study the presence of the fluid-dynamic and fluid-resonant cavity oscillations is clearly brought out and the inherent differences between the two types of oscillations are also highlighted. Fluid-resonant cavity oscillations are not often observed experimentally in most of the experimental works related to supersonic cavity flow. A total of six cavity models with different subcavity lengths are investigated and are also designated by the ratio of subcavity length to main cavity length (l/L = 0.00,0.10,0.20, 0.25, 0.30, and 0.40). High-speed schlieren flow visualization and unsteady pressure measurements are employed to gain further insight into the flow physics. High-speed schlieren flow visualization clearly indicates the presence of six different types of waves and five different flow features that are associated with the cavity flow field. Statistical analysis techniques, namely, fast Fourier transform, spectrogram, correlation, and coherence, are employed for analyzing the unsteady pressure data. Fluid-dynamic-type cavity oscillations are dominant in l/L = 0.00, 0.10 and 0.20 cavities. Pressure oscillations inside the cavity are suppressed as l/L is increased from 0.00 to 0.20. As l/L is increased from 0.20 to 0.25, the type of cavity oscillations changes from fluid-dynamic to fluid-resonant. This change in the type of cavity oscillations results in increased pressure oscillations inside the cavity for l/L = 0.25, 0.30, and 0.40. Distinct cavity tones at certain frequencies in power spectrum plots are classified into fluid-dynamic mode and fluid-resonant mode. Modal frequencies of fluid-resonant modes differ widely from those predicted by modified Rossiter formula. Correlation analysis suggests the presence of fluid-resonant behavior in l/L = 0.25,0.30, and 0.40 cavities. With reference to l/L = 0.00 cavity, there is maximum reduction of 30.2 dB in sound pressure level for the second fluid-dynamic mode, and there is also 4.4 dB reduction in overall sound pressure level for the l/L = 0.20 cavity.