Jet noise modification by the “whistler nozzle”

Spurred by the prospect that the “whistler nozzle” may find interesting technological applications, the farfield noise characteristics of a subsonic whistler nozzle jet have been measured as a function of the Mach number, emission angle, and mode of excitation. The whistler nozzle jet becomes a progressively less effective noise amplifier with increasing Mach numbers. The self-sustained whistler excitation produces boradband noise amplification with essentially unchanged spectral shape. The directivity pattern of the overall sound pressure level, and the "broadband amplification factor," computed after subtracting the contributions from the spectral peaks which are due to the whistler tones and the harmonics, are found to be functions of the Mach number, emission angle, and mode of whistler excitation. The total noise amplification and the broadband amplification factor monotonically increase with the emission angle, reaching their maxima normal to the jet axis. The directional effects of whistler excitation are different from the artificially excited axisymmetric jet. Causality arguments are presented to suggest that the far-field broadband noise amplification is a direct consequence of near-field broadband turbulence amplification and not of interaction of shear layer and jet mode coherent structures as proposed previously.