Abstract
The excitation mechanism of acoustic resonances in an in-line tube array with intermediate tube spacings is investigated experimentally. The array is tested in air and water flows. The acoustical mode of the wind tunnel is simulated in the water channel by a standing free-surface wave in the transverse direction. Resonances are found to be excited due to the coupling between the resonant sound field and the unstable shear layers which separate from the tubes. This resonance phenomenon is quite distinct from, and therefore not related to, the jet instability which generates the vorticity-shedding excitation in the absence of resonance. The flow structures under resonant and non-resonant conditions are found to be entirely different. Since the jet instability dominates and the shear layer instability is suppressed at non-resonant conditions, the occurrence of resonance necessitates the switching of the flow instability from the jet mode to the shear-layer mode. The transition mechanism as well as the conditions under which this mode switching can be achieved are described in this paper.
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