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Experimental measurements indicate that the noise radiated from a jet depends not just on the jet-exit velocity alone, but is significantly affected by the jet temperature. Now, there is evidence to support the proposition that jet mixing noise consists of two principal components. These are the noise from the large turbulence structures of the jet flow and the fine-scale turbulence. The prediction of fine-scale turbulence noise from hot jets is considered. Earlier Tam and Auriault (Tam, C. K. W., and Auriault, L., "Jet Mixing Noise from Fine-Scale Turbulence," AIAA Journal, Vol. 37, No. 2, 1999, pp. 145-153) developed a semi-empirical theory capable of predicting the fine-scale turbulence noise from cold to moderate temperature jets. In this work, their semi-empirical theory is extended to high-temperature jets, up to a temperature ratio above that of present day commercial engines. The density gradient present in hot jets promotes the growth of Kelvin-Helmholtz instability in the jet mixing layer. This causes a higher level of turbulent mixing and stronger turbulence fluctuations. In addition, recent experiments reveal that the two-point space-time correlation function of turbulent mixing for hot jets is substantially different from that for cold jets. The eddy decay time is shorter, and the eddy size is slightly reduced. These changes have an appreciable impact on the noise radiated. In the present extended fine-scale turbulence theory, both effects are taken into account. Extensive comparisons between computed noise spectra and measurements for hot jets over the Mach-number range of 0.5-2.0 are reported here. Good agreements are found over inlet angle from 50 to 110 deg. This is the directivity for which fine-scale turbulence noise is dominant.

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Noise from auxiliary power units (APU) is an important contributor to the overall level of ramp noise. Currently, ramp noise is regulated by international governing bodies as well as by individual airport. A significant component of APU noise is combustion noise. In this study, the unique spectral shape of APU combustion noise is identified. It is found that the spectral shape is the same regardless of engine size, power setting and directivity. Also, it is practically the same as that of open flame combustion noise. The frequency at the peak of the combustion noise spectrum is found to lie in the narrow range between 250 to 350 Hz. The peak sound pressure level of a given APU varies as the square of the fuel consumption rate. In the literature, suggestions have been made concerning a second combustion noise mechanism arising from the passage of hot entropy spots through the exhaust nozzle or constriction. In this investigation, no evidence has been found to indicate the existence of a second APU combustion noise component.

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