Investigation of acoustic propagation and source localization in a hot jet flow

A jet flow typically occurs in open-jet wind tunnels and turbofan engines, where there are velocity and temperature gradients between the source region and the outside environment. These gradients form a thermal shear layer that determines acoustic propagation and source localization. Moreover, they induce Kelvin-Helmholtz instabilities in numerical simulations. To stabilize simulations, a gradient-term suppression and filtering method is proposed and validated using time-domain linearized Euler equations. For sound propagation, the influences of the steep gradients of the velocity and temperature together with the spreading angle and source frequency are investigated through a self-similar shear layer with superposition of compressibility and heat. Those refraction influences strongly change the beamforming-related phase rather than the directivity-related amplitude. Next, the computed database is processed by delay-and-sum beamforming to localize the source position. In the beamforming part, refraction and amplitude corrections based on Amiet's theory with an extension for temperature differences are numerically evaluated and later applied when analyzing the localization error. This localization error is basically proportional to the jet Mach number, temperature ratio, and source frequency and is inversely proportional to the spreading rate. Finally, the localization error is fitted as a linear function of the product of the jet Mach number and the Strouhal number. (C) 2020 Elsevier Ltd. All rights reserved.