Noise and flow characteristics of a howling internally mixed jet

The noise emitted by axisymmetric, dual-stream, internally mixed jets was studied. At a jet Mach number of 0.90, the jet howled loudly (i.e., acoustic tones dominated over the broadband jet noise) for multiple mixing-duct lengths and for both unheated and heated core flows. Particle-image velocimetry data revealed that the jet’s potential core was substantially shortened when the howling occurred and there were intense velocity fluctuations in the jet’s shear layer. Using schlieren images, it was shown the jet’s instability waves were excited at the frequency of the fundamental acoustic tone, suggesting a flow/acoustic interaction was responsible for these observations.