Abstract
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.
Highlights
Modern aero-engines are faced with the challenges of high stability, high thrust-to-weight ratio, and high stall margin
On it the profiles of the supersonic cascade and the separation bubble are outlined with red lines, the transducers are drawn with Cambridge blue lines, the compression waves are filled with dark blue lines, and the expansion waves are painted with green dot lines
When throttling ratio (TR) increases to 65.8%, the internal shock detaches from the pressure surface tip, and an overflow occurs in the flowfield, which is defined as the beginning of oscillatory flows
Summary
Modern aero-engines are faced with the challenges of high stability, high thrust-to-weight ratio, and high stall margin. It is significant to study the shock train oscillation characteristics in a supersonic cascade in a curved passage. Violent shock oscillation occurs in the oscillatory flows of stall mode, resulting in drastic fluctuation of pressure throughout the passage. During the buzz flow of a supersonic/hypersonic inlet, the rapid change of the flowfield structure leads to the large amplitude fluctuation of load and a decrease in performance. Research on oscillatory flows was conducted to clarify the shock oscillation behavior and pressure fluctuation characteristics of different oscillation stages, the trigger and dominant mechanism of the stall, the lead-lag effect relationship of flowfield evolution at different locations, etc. The structure and oscillation characteristics of the shock train of steady and oscillatory flows are shown and discussed in sections IV and V, respectively. The underlying physical oscillation mechanism (including trigger mechanism, dominant mechanism, and feedback loop) of the supersonic cascade are discussed
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