Triggered flash schlieren photography has been used to study the jet structure and sound generation in subsonic air jets (Mach 0.6-0.8) in which the flow upstream of the nozzle exit is subjected to various types of acoustic perturbation. The experiments form part of a research study into the contribution from internal noise sources in aeroengines to the radiated jet noise. Photographs show the formation, convection and coalescence of large vortices in the jet and the diffraction and refraction of internally-generated sound waves. Strong acoustic emission is observed at 900 to the jet axis.
S. N. Heavens, S. N. Heavens,
"Visualization Of Unsteady Jets And Aerodynamic Noise Fields", Proc. SPIE 0189, 13th Intl Congress on High Speed Photography and Photonics, (27 July 1979); doi: 10.1117/12.957670; https://doi.org/10.1117/12.957670