Linear dynamics of over-expanded annular supersonic jets

This article delves into the dynamics of inviscid annular supersonic jets, akin to those exiting converging–diverging nozzles in over-expanded regimes. It focuses on the first azimuthal Fourier mode of flow fluctuations and examines their behaviour with varying mixing layer parameters and expansion regimes. The study reveals that two unstable Kelvin–Helmholtz waves exist in all cases, with the outer-layer wave being more unstable due to differences in the velocity gradient. The inner-layer wave is more sensitive to changes in base flow and extends beyond the jet, potentially contributing to nozzle resonances. The article also investigates upstream propagating guided-jet modes, which are found to be robust and not highly sensitive to changes in base flow, which makes them essential for understanding jet dynamics. A simplified model is used to obtain ideal base flows but with realistic shape in order to study the effects of varying nozzle pressure ratios on the dynamics of the waves supported by the jet.