The possibility to predict flow acoustic coupling at a sharp edge of an area expansion in a flow duct is explored, starting from the analytical model proposed by Nilsson and Boij. In the model, the vortex shedding is treated as infinitely thin, causing an instability for all frequencies. For small and large values of the Strouhal number, the acoustic field is well defined and can be treated separately from the remaining flow field. For intermediate Strouhal numbers, this classification into acoustic and non-acoustic waves is more complex. In particular, a hydrodynamic wave and higher order, non-propagating acoustic waves are changing properties. The possibility to use the amount of deviation from the asymptotic behaviour as a measure of the strength of the interaction at the edge is explored. It is shown that the interaction is strongly dependent on the area ratio at the duct expansion. Further studies would aim to a more elaborate investigation of the implications of the wave number parameter dependence. The objective would be to better predict the circumstances for strong interaction, in order to design less noisy ventilation and exhaust systems and to enhance dissipation effects.
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