Experimental investigation of the throttling process and oscillation mechanism in a supersonic cascade

The structure and oscillation characteristics of the shock train during steady and oscillatory flows in a supersonic cascade was experimentally studied. The study was based on the SAV21 supersonic cascade designed by the German Aerospace Centre operating under incoming Mach number equal to 2.27 and flow turning angle of 45 degrees. Before the onset of stall, the expansion waves generated on the curved configuration of the suction surface (stage III) and the separation bubble (stage IV) suppresses the movement of the shock train. A striking feature of stage 1 of the oscillatory flows is that as the throttling ratio increases, the dominant frequency of the pressure fluctuation keeps decreasing and the high-energy oscillations take up more time and a wider frequency range. Then stage 2 (which is composed of large-amplitude oscillations and secondary oscillations) appears. With the enhancement of the throttling ratio, the occurrence probability of large-amplitude oscillations rises in stage 3. The shock train propagates into the upstream passage in stage 4, the final stage. Through cross-wavelet transform, it was discovered that the immediate change of the entrance local conditions leads to the in-phase fluctuation of pressure in the entire flowfield during the oscillatory flows, which is consistent with the observation of Dailey. Further analysis indicates that all the oscillation stages except stage 4, are triggered by the suction-surface flow separation, which belongs to the Dailey criterion. Furthermore, the feedback loop established by the communicating ways (acoustic wave propagation, shock movement, spillage, and separation bubble deformation) is brought forward.