Effects of Forward Flight on Jet Mixing Noise from Fine-Scale Turbulence

Abstract

It is known experimentally that a jet in forward flight radiates less noise than the samejetin a static environment. At a forward flight Mach number of 0.2, the noise reduction, depending on the jet operating conditions, could be as large as 4-5 dB in the sideline directions. In the past, a way to predict flight effects was to use the method of relative velocity exponent. Another method was to extrapolate measured static jet noise to the flight condition by means of scaling formulas. Both methods are semi-empirical. The fine scale turbulence jet mixing noise theory of 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) is extended for application to jets in simulated forward flight. It will be shown that the calculated noise spectra at different simulated forward flight Mach numbers for both supersonic and subsonic jets compare well with experiments. The effects of forward flight on the sources of fine-scale turbulent jet mixing noise is also investigated. It is found that in the presence of forward flight the dominant noise sources move downstream. The turbulence intensity and the size of turbulent eddies responsible for noise emission are reduced.