Supersonic jet noise is studied by large-eddy simulations and discussed in relation to the similarity spectra that represent the fine- and large-scale turbulence sources. The highest jet temperature is comparable to those observed in realistic jet engine exhaust at afterburner conditions. There is an excellent agreement in the sideline direction between the noise distributions and the fine-scale similarity spectra. In the peak directions and a far downstream direction at 150°, the agreement with the large-scale similarity spectra is good around the peak and lower frequencies. But there is a mismatch in the high-frequency range, and this mismatch increases as the temperature increases. In heated jets where Mach wave radiation is present, the source responsible for the high-frequency mismatch in the peak direction is different from that in the far downstream direction. Small-scale turbulence structures traveling at phase speeds greater than the ambient sound speed are believed to be the source responsible for the high-frequency mismatch in the peak directions. The noise generation mechanism of this small-scale source is Mach wave radiation, which is different from that of the fine-scale turbulence source. Thus, it cannot be completely accounted for by the fine- or large-scale similarity spectra. The source responsible for the high-frequency mismatch in the far downstream direction is observed near the nozzle lip and also in the jet plume. The latter is believed to be a fine turbulence source and its strength increases as the temperature increases. A combination of fine- and large-scale similarity spectra represents very well the noise distribution in the far downstream direction.
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