Jet engine exhaust noise due to rough combustion and nonsteady aerodynamic sources

Internal sources are accounted for in terms of fluctuations of mass and momentum at the nozzle exit plane. At low Mach numbers, mass flow fluctuations generated at the exit plane by acoustic resonant type fluctuations inside the engine are found to be dominant. In the subsonic Mach number range between 0.3 and 0.5, exit plane mass flow fluctuations at frequencies characteristic of turbulence become most dominant. Above Mach 0.5, the turbulent momentum fluctuations at the exit plane become dominant, and the jet contribution is not found significant at subsonic speeds.