Turbulence suppression in free turbulent shear flows under controlled excitation. Part 2. Jet-noise reduction

It is shown that reduction of broadband (even total) far-field jet noise can be achieved via controlled excitation of a jet at a frequency in the range 0. 01 less than St// theta less than 0. 02, where St// theta is the Strouhal number based on the exit momentum thickness of the shear layer. Hot-wire measurements in the noise-producing region of the jet reveal that the noise suppression is a direct consequence of turbulence suppression, produced by early saturation and breakdown of maximally growing instability modes.