Non-linear sound propagation is investigated computationally by simulating compressible time-developing mixing layers using the Large Eddy Simulation (LES) approach and solving the viscous Burgers Equation. The mixing layers are of convective Mach numbers of 0.4, 0.8 and 1.2. The LES results agree qualitatively with known flow behavior. Mach waves are observed in the near sound field of the supersonic mixing layer computed by the LES. These waves show steepening typical to non-linear propagation. Further calculations using the Burgers equation support this finding, where the initial wave slope has a role in kicking them. No visible non-linear propagation effects were found for the subsonic mixing layers. The effects of geometrical spreading and viscosity are also considered.

Supersonic crackle, an irritable component of aircraft jet noise, was investigated using model scale measurements. Near-field results showed Gaussian distribution but far-field had high skewness and even higher in its derivative. Skewness, a measure of asymmetry in the waveform, was compared to screech arising from shock associated noise which was also high but in contrast to crackle its skewness derivative had dropped to a much smaller value than its waveform. Both crackle and screech are nonlinear but their nonlinear properties are entirely different. Crackle is quantified when its derivative skewness becomes larger than its waveform skewness which should exceed 0.3.

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Screech and crackle are investigated by far-field acoustic measurements and visualizations of a cold jet from a Mach 2 convergent-divergent nozzle operated over-expanded, fully expanded, and under-expanded. The skewness of the sampled far-field pressure frequency distribution is evaluated at polar angles below 90°. It is found that skewness alone is insufficient for singling out crackling noise, however, a better characterization is obtained by jointly considering the skewness of the acoustic pressure time derivative. New evidence is provided of crackle and screech being competitive mechanisms over the nozzle pressure ratio range 4.3 to 10. These new physical insights advance the current understanding of high-speed jet aeroacoustics, towards enabling sustainable airport operations of supersonic civil aircraft.