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Screech characteristics of under-expanded high aspect ratio elliptic jet

Screeching behavior and the closure mechanism of a feedback loop for the flapping mode in under-expanded supersonic jet are investigated by schlieren imaging and near-field acoustic measurements for a high aspect ratio elliptic nozzle. Near-field measurements revealed a single screech frequency for the measured Mach number range. The cross spectrum of pressure signals shows that the upstream propagating wave is out of phase, which was identified as the asymmetric flapping mode. The proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD) methods were applied to the time-resolved schlieren data to extract the coherent information associated with screech and its harmonics. The first three spatial POD modes reveal the periodic flapping of the jet and the asymmetric upstream propagation of acoustic waves. POD modes 4 and 5 identify the flow structures and acoustic wave patterns associated with the harmonics of the fundamental screech tone. The spatial DMD mode corresponds to the frequency of ∼10 kHz and exhibits the radial distortion of the jet shear layer and the acoustic radiation pattern. The interaction of the acoustic waves with the jet shear layer indicates that the upstream-traveling guided jet mode is responsible for closing the feedback loop of the flapping mode.