The ‘whistler-nozzle’ phenomenon

Abstract

The ‘whistler nozzle’ is a simple device which can induce jet self-excitations of controllable amplitudes and frequencies and appears highly promising for many applications involving turbulent transport, combustion and aerodynamic noise. This paper documents the characteristics of this curious phenomenon for different values of the controlling parameters and explains the phenomenon. It is shown that the whistler excitation results from the coupling of two independent resonance mechanisms: shear-layer tone resulting from the impingement of the pipe-exit shear layer on the collar lip, and organ-pipe resonance of the pipe nozzle. The crucial role of the shear-layer tone in driving the organ-pipe resonance is proven by reproducing the event in pipe–ring and pipe–hole configurations in the absence of the collar. It is also shown that this phenomenon is strongest when the self-excitation frequency matches the ‘preferred mode’ of the jet.The ‘whistler-nozzle’ phenomenon occurs for both laminar and turbulent initial boundary layers; the excitation can be induced without the pipe nozzle (say, by ring or hole tone) when the exit flow is laminar but not when it is turbulent. Unlike the shear-layer tone and jet tone phenomena, where successive stages overlap, adjacent stages of the whistler-nozzle excitation are separated by ‘dead zones’ where the conditions for both resonance mechanisms cannot be simultaneously met. Also, unlike the shear-layer and jet tones, the whistler frequency cannot be varied continuously by changing the speed. Since the phenomenon is the coupling of two resonance mechanisms, the frequency data appear to defy a simple nondimensional representation for the entire range of its operation. Reasonable collapse of data is achieved, however, when the exit momentum thickness is used as a lengthscale, thus emphasizing the role of the shear-layer tone in the phenomenon.