Grazing Flow Effect on Surface Impedance with Circular Perforations of Small Diameters

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It is well known that introduction of perforations in the walls of flow surface causes not only significant sound attenuation but also reduction of aerodynamic sound radiation from a rotating wing. In order to enjoy the largest advantage of perforated surface, the optimal distribution of finite impedance may be realized, which means the impedance characteristics of the surface must be predicted under the grazing flow condition. In this paper grazing flow effect on the surface impedance was investigated by conducting a numerical simulation, where the diameter is 1mm and the Reynolds number based on the diameter ranges from 44 to 93. At the same time, the acoustic impedance without grazing effect was investigated by using the linear one-dimensional flow model, which is utilized as the limit impedance value resulted from zero flow speed. The complex surface impedance ratio marks far larger magnitude than the conventional impedance for larger diameters and higher Reynolds number, which reflects the effect of high viscosity of the flow.

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