Inuence of shear flow on liner impedance computed by multimodal method

A numerical method is proposed to compute the liner impedance under shear grazing flow. The flow-acoustic coupling that mainly occurs at the perforations is simulated using multimodal method. The Floquet-Bloch theorem is utilized to educe the liner impedance after the multimodal computation. Such simple calculation gives results that are in agreement with the empirical model in the order of magnitude of the changes in impedance with flow speed. Furthermore, the proposed method allows to take into account the thickness of the boundary layer. Results indicate that the shear flow eduction gives better predictions than the uniform eduction with Ingard-Myers condition, particularly for a relatively thick boundary layer. The liner resistance is shown to maintain the no-flow value until flow speed reaches a threshold value. After the threshold, the resistance increases almost linearly with flow Mach number, and with a larger slope in the case of a thinner boundary layer. It is also found that as flow conditions vary, the change of resistance is consistent with the variation in the level of flow-acoustic coupling, which indicates that the flow inuence on liner impedance results from such coupling.