Computational study on sound scattering by jet shear layers
Résumé
The scattering and spectral broadening of sound waves by velocity and density fluctuations in the shear layers of jets with acoustic Mach numbers Ma between 0.3 and 1.3 and temperatures between 1 and 2.25 times the ambient temperature are investigated numerically. For that, the propagation of acoustic waves generated by a monopolar source radiating at Helmholtz number He=2.7, 8.1 and 24.3 placed on the jet axis is simulated by solving the Linearized Euler Equations (LEE) using the Discontinuous Galerkin Method (DGM), with a base flow obtained from well-resolved Large Eddy Simulations. Two base flows consisting of the jet mean flow with or without axisymmetric flow fluctuations are considered. The objective is to analyze the effects of the flow fluctuations on the properties of the acoustic waves passing through the jet shear layers in space and in the frequency domain. The scattered field shows beams whose properties depend on the Mach number and the source frequency. The spatial scattering is evenly distributed throughout the domain, except upstream of the source for Mach numbers Mj equal to 0.9, given the directivities of the acoustic fields. In the frequency domain, the spectra of the acoustic field show a tone and two lateral lobes. They are symmetric when the acoustic Mach number is at least 0.6 or at positions downstream of the source. For the heated jets, the lateral lobes in the spectra increase and approach the central peak as the temperature of the jets rises.
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