Interaction of a slot-tone with a pipe

The self-sustained tone generated by a low Mach number free flow impinging on a slotted plate, usually referred to slot-tone, is experimentaly studied. In this work we focus on its coupling with the pipe from which the jet flows out. The jet nozzle and the beveled slot are aligned and have the same large aspect ratio. The tones generated have a single well defined frequency (around 1 kHz), a high amplitude (typically, 105 to 115 dB) and exhibit typical characteristics of self-sustained tones. Their frequencies are about the same order of magnitude than the natural frequency of the shear layer calculated from the linear stability theory. For some Reynolds numbers and some plate distances, a standing wave pattern is observed in the pipe. The outer and inner acoustic fields show high coherence and the slot-tone locks on a pipe eigenmode, which indicates an acoustic feedback. However, especially for lower Reynolds numbers, this coherence vanishes but the self-sustained tones are still present. It suggests that the hydrodynamic feedback becomes preponderant.