Discrete tones generated by the impingement of a high-speed jet on a circular cylinder

Discrete tones generated from the interaction of high-speed jets with solid boundaries are investigated experimentally. Measurement of far-field pressure waves and Schlieren flow visualization were conducted. Emphasis is placed upon the generation mechanism of a discrete tone that is generated when a circular cylinder is placed in a jet at a high Reynolds number. This discrete tone was observed to be generated for jet exit Mach numbers greater than 0.6 and distances from the nozzle lip to the cylinder less than about eight times the nozzle diameter. By the Schlieren flow visualization, it is confirmed that this discrete tone is radiated from the jet by a strong feedback resonance. When an eddy (a ring vortex) interacts with the cylinder, an acoustic pulse is emitted with a strong directional peak in the upstream direction. The acoustic pulse propagates outside the jet and, upon reaching the nozzle lip, forces the generation of a new eddy. As the new eddy approaches the cylinder it regains the strength of the original eddy by extracting energy from the potential core of the jet and so maintains the cycle.