An experimental investigation of the resonance appearance in an impinging jet

This work presents an experimental investigation of the resonance generated by a subsonic and supersonic jet issuing from a convergent nozzle and impinging on a flat plate normal to the nozzle axis. The tonal behaviour of jet impingement is explored in a systematic fashion as a function of the jet flow conditions and the nozzle-to-plate distance via acoustic measurements in the far field of the jet. Specifically, the fully expanded jet Mach number is varied in the range 0.4-1.5 and the nozzle-to-plate distance in the range 2-10 nozzle diameters. The resonance appearance and the staging dynamics between the different tones are found to be strongly dependent on the jet-plate distance as well as the jet Mach number, particularly in the supersonic flow regime. Specifically, competition between two resonant dynamics associated with jet impingement and screech is observed for small and intermediate nozzle-plate spacings. For larger distances, resonances disappear in the subsonic regime and the jet dynamics reproduces the behaviour of a free screeching jet in the supersonic regime. Non-linear spectral analysis is finally exploited to characterise the non-linearities associated with the resonance generation.