Nonlinear Propagation of Noise Radiated from Supersonic Jets

This paper describes how the role of nonlinearity in the propagation of noise from supersonic jets is not currently well understood, but it is of interest because the noise radiation from military jet aircraft can approach or exceed levels of 150 dB or more at close range. These high noise amplitudes suggest that the inclusion of nonlinear propagation effects in long-range noise estimation models may be of importance in order to correctly assess the environmental impact the aircraft have. A finite-amplitude noise spectrum undergoes an evolution as it propagates, in that energy is transferred from mid to higher frequencies, and in some cases, from mid to lower frequencies, resulting in a spectral broadening. Waveform steepening accounts for the energy transfer to high frequencies as portions of the waveform become more shock-like. If these steepened portions begin to grow, the number of zero crossings decreases, consequently resulting in an energy increase at lower frequencies. The nonlinear propagation of noise has been studied over the past few decades by many others. While many of these studies employed a variety of theoretical and numerical techniques in an attempt to understand and predict the nonlinear spectral evolution, few have included accompanying experimental evidence. Crighton, in particular, has stressed the need for additional aeroacoustic experiments in order to properly assess the relevance and accuracy of the abundance of analytical and numerical work already carried out. Recent laboratory experiments performed on both cold and heat-simulated (HS) model-scale supersonic jets demonstrate evidence of nonlinear propagation effects. This paper represents continued analysis of those data. The experimental setup is first briefly described and is followed by the presentation of power spectral results.