On the Feedback Phenomenon of an Impinging Jet

摘要

The resonance phenomenon which occurs when a high speed turbulent jet impinges on a large flat plate is experimentally studied. The far field noise is also investigated for a wide range of impinging jet speeds. The study involves far field, near field, and surface pressure measurements. A feedback of pressure waves between the nozzle and the plate is found to be the mechanism responsible for the resonance. The feedback loop is formed by downstream traveling large coherent structures, and upstream acoustic waves propagating in the nearby stagnant region around the jet. The upstream acoustic waves are generated by the impingement of large scale structures on the plate. Near the nozzle exit these waves are locked in-phase with the oscillations of the thin shear layer, which is forced to roll into large eddies at the resonance frequency f(r). Furthermore, the experimental data revealed the following interesting results: (1) The total period of the feedback loop is an integer N times the resonance period, N being constant over a resonance frequency stage. A jump from one frequency stage to another (occurring at certain nozzle-to-plate separation distances), takes place through a quantum unit change in N, so as to preserve the phase lock between the two oscillations at the nozzle. (2) As the wavy shear layer evolves into large vortices, a strong interaction develops among the initial instability vortices. This phenomenon is referred to as 'collective interaction', and is believed to be the mechanism responsible for the sharp drop in the passage frequency of the vortices near the nozzle exit. The frequency drops from the high instability frequency f(in) to that of the resonance frequencey f(r).