Source mechanisms of jet crackle

Fighter jets and other aircraft with high specific thrust engines can make a particularly intense type of aerodynamic jet noise that has become known as crackle. It is distinguished from low-speed jet noise by its frequency content, extreme intensity, and sporadic character, all of which make it particularly annoying to people working and living near these aircraft. There is evidence that source mechanisms and nonlinear propagation mechanisms both lead to crackle. We use detailed simulations to study the turbulence source and near acoustic field of a crackling temporally developing planar free shear flows, with Mach numbers M = 1.5, M = 2.5, and M = 3.5. The skewness of the acoustic pressure being Sk > 0.4 has been correlated with the perception of crackle. The sound radiated from the M = 1.5 mixing layer had S_k = 0.13, which is well below this threshold. However, both the higher speed mixing layers have S_k that meet this nominal threshold: S_k = 0.46 for M = 2.5 and S_k = 0.61 for M = 3.5. Space-time correlations inside the mixing layer show convective Mach numbers corresponding to that anticipated based upon the Mach angle of the near-field acoustic waves. However, turbulence correlation statistics suggest that the Mach waves observed are longer than would be produced by typical correlated structure. The clumping together of shorter waves in the very near acoustic field leads to the obvious Mach waves of greater extent further from the turbulent mixing layer.