Effects of shear layer growth on the indirect noise in compound nozzles

A one-dimensional analytical model is proposed to account for mixing and compute the indirect noise in compound nozzle flows. The model is based on the work of Morton et al. (Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences 234 (1196), 1956, 1-23) and serves as an extension to Younes & Hickey (Journal of Sound and Vibration 442, 2019, 609-623). The model is valid for subsonic flows in the limit of small velocity differences. By studying various inlet Mach numbers and area ratios, a parametric study is conducted to quantify the shear layer growth at the outlet of the nozzle. It is shown that shear layer growth is maximized at the highest convective Mach number and lowest primary area ratio. The effect of the shear layer on the indirect noise is explored by computing the entropic response with and without the presence of shear. It is shown that the effect becomes pronounced at shear layer thicknesses above 50%. In general, the shear layer acts to shift the peak noise to the left of the frequency spectrum, while attenuating the noise at high frequencies. This finding provides new insight into the design of subsonic noise suppressors for multi-stream nozzle devices. (C) 2019 Elsevier Ltd. All rights reserved.