Flow and acoustic properties of low Reynolds number underexpanded supersonic jets

Abstract

An experimental program to investigate the flow and acoustic properties of model underexpanded supersonic jets was conducted. In particular, the role played by large-scale organized fluctuations in the flow evolution and acoustic production processes was examined in detail. The experimental conditions were chosen as low Reynolds number ( Re = 8000) Mach number 1·4 and 2·1 underexpanded jets exhausting from convergent nozzles. A consequence of performing the experiments at low Reynolds number is that the broadband shock-associated noise is suppressed. The focus of the present study is on the generation of noise by large-scale instabilities in the presence of strong shock cell structures. The aeroacoustic properties of these underexpanded jets are shown to have some departures from those of perfectly expanded jets at identical Mach and Reynolds numbers. Also, the noise radiated by these model jets is shown to be very similar to the shock screech noise of conventional high Reynolds number underexpanded jets. It is demonstrated that the production of screech is related to the modulation and decay of large-scale turbulence structures. This is consistent with high Reynolds number jet flows of comparable Mach number and nozzle geometry.