Experiments Concerning Tones Produced By An Axisymmetric Choked Jet Impinging On Flat Plates

An experimental investigation of the sound produced by an axisymmetric supersonic jet impinging on plates with variable size and nozzle-to-plate spacings was performed. Spectral analysis and schlieren photography were used to determine the sound characteristics and the nature of the flow disturbances associated with the production of discrete tones. Two classes of tones, associated with small and large plates, existed for pressure ratios above p0 /pa = 2·70 (where p0 was the reservoir pressure and pa the ambient pressure). For lower pressure ratios, however, only large plate tones occurred for all plate sizes, with chocked jet screech appearing at the largest nozzle-to-plate spacings. Over the majority of spacings investigated, the large plate data fell along three parallel lines on a log-log plot. The occurrence of multiple tones appeared to be the rule rather than the exception, so the phenomenon is more complex than originally thought. Reflector tests indicated that the feedback loop for both classes of tones involved the nozzle. While the jet flow instabilities are excited at the nozzle in both cases, it now appears that the large oscillations of the shock waves in the small plate case may be an essential element in the production of the tones, while in the case of the large plate tones, these oscillations may be incidental. Schlieren photography revealed both symmetric and asymmetric disturbances in the large plate jet flows, which corresponded to symmetrical and helical modes of oscillation in the acoustic cross-spectrum studies. In addition to this, both symmetric and asymmetric oscillations of the shock waves occurred in the large plate case, while only symmetric oscillations of the stand-off shock waves occurred in the small plate case.