Sound Scattering by Shear Layers of High-Speed Isothermal and Hot Turbulent Jets

The scattering of sound waves by turbulent shear layers is investigated for jets with acoustic Mach numbers between 0.3 and 1.3 and temperatures equal to 1, 1.5, or 2.25 times the ambient temperature, computed by large-eddy simulations (LESs) in previous work. Sound scattering is studied by solving the two-dimensional linearized Euler equations using the LES unsteady axisymmetric flowfields as the base flow. An acoustic source radiating at Strouhal numbers between 3 and 27 is placed on the jet axis two diameters downstream of the nozzle exit. For all jets, sound scattering is mainly related to the large-scale shear-layer turbulent structures, and spectral broadening occurs, yielding components on both sides of the source frequency. These components have amplitudes increasing strongly with Mach number and slightly with jet temperature and usually form two lobes in the sound spectra. However, for a jet Mach number of 0.9, no lobes clearly emerge upstream of the source because of the leakage of upstream-propagating guided jet waves. An additional lobe associated with the initial shear-layer instability waves is also seen at low Mach numbers. Finally, for hot jets, sound scattering is caused by both the jet velocity fluctuations and the temperature fluctuations.