The present study investigated the effects of active flow control by high-frequency flapping jets on a supersonic cavity flow. The discharging position was varied at the leading edge, the bottom wall upstream side, the bottom-wall downstream side, and the trailing edge. The flapping jets’ frequencies were varied at 0 (non-flapping case), 15, and 22 kHz, respectively. Discharging non-flapping jets at the leading edge and the bottom wall upstream side remarkably enhanced pressure fluctuations and density gradients. The reverse flow near the bottom wall and the amplitude of the recirculation inside the cavity were enhanced when non-flapping jets were discharged at the leading edge. Discharging non-flapping jets at the bottom wall downstream side reduced the pressure fluctuations of the frequency lower than 8 kHz. Discharging flapping jets changed flowfields, although the flow control effects of discharging positions were dominant. The flapping-jet discharging at the leading edge reduced the size of the primary recirculation. Discharging flapping jets at the bottom wall downstream side showed further suppression effects on fluctuations than the non-flapping-jet discharging case. There were no significant effects on the pressure fluctuations when the jets were discharged at the trailing edge.

Supersonic microjets issuing from a convergent rectangular nozzle whose height is 500 µm at its exit are visualized using the acetone-based PLIF (planar laser-induced fluorescence) and MTV (molecular tagging velocimetry) techniques. The jet Reynolds numbers are ranged between 200 and 7000 and the fully-expanded jet Mach-number is set to 1.5. The PLIF visualization results reveal that the jets spread abruptly at a certain location. Instantaneous MTV images are captured in the vicinity of the location where a jet spreads abruptly and the process of the jet collapse is successfully visualized. The distance between the nozzle exit and the jet-collapse location is estimated from each PLIF image. The distance is found to be constant for the Reynolds numbers higher than ~700 and to be inversely proportional to the Reynolds number for the numbers lower than ~700. Moreover, the distance changes abruptly at the Reynolds number of ~700.