Influence of nozzle geometry on screech instability closure

The effect of nozzle exit geometry on the triadic interactions between convective Kelvin-Helmholtz instabilities and the quasi-steady shock structure of supersonic jets is discussed in this study. A circular and rectangular nozzle with an aspect ratio (AR) of 2 were evaluated using results from near field acoustics and high-speed Schlieren images. The study investigated different types of screech instabilities in both nozzle configurations, including the multimode instability in rectangular jets. The near field acoustic measurements showed comparable trends in screech staging in both nozzle configurations. The Schlieren results revealed that the standing waves in the jet near field were influenced by the oscillation mode of the jets. The study also explored a new technique of noise visualization from Schlieren images based on near field acoustics and temporal Fourier decomposition. Spatial Fourier decomposition demonstrated that the resonance closure mechanism across both nozzle configurations was dependent on the relative influence of the first two shock cells. Comparisons between the AR 2 and AR 9 jet showed that jet aspect ratio can significantly affect the energy distribution from the triadic interactions.