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 Sk = 0.13, which is well below this threshold. However, both the higher speed mixing layers have Sk that meet this nominal threshold: Sk = 0.46 for M = 2.5 and Sk = 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.

Original languageEnglish (US)
Title of host publication18th AIAA/CEAS Aeroacoustics Conference (33rd AIAA Aeroacoustics Conference)
PublisherAmerican Institute of Aeronautics and Astronautics Inc.
ISBN (Print)9781600869327
StatePublished - 2012
Event18th AIAA/CEAS Aeroacoustics Conference 2012 (33rd AIAA Aeroacoustics Conference) - , United States
Duration: Jun 4 2012Jun 6 2012

Publication series

Name18th AIAA/CEAS Aeroacoustics Conference (33rd AIAA Aeroacoustics Conference)


Conference18th AIAA/CEAS Aeroacoustics Conference 2012 (33rd AIAA Aeroacoustics Conference)
Country/TerritoryUnited States

ASJC Scopus subject areas

  • Aerospace Engineering
  • Mechanical Engineering
  • Acoustics and Ultrasonics


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