The acoustic impedance of orifices in the wall of a flow duct with a laminar or turbulent flow boundary layer

The acoustic impedance of a duct orifice in laminar or turbulent boundary layer flow is investigated as a model of perforated plate acoustic liner performance. A modification of the impedance tube technique was used to determine the impedance of an orifice in a flate plate mounted between the top and bottom walls of a rectangular duct at Strouhal numbers from 0.026 to 9.4. For low Strouhal numbers (less than 0.125) the resistance of the normalized end correction is found to approach the dc flow resistance and the flow resistance is found to depend primarily on the ratio between boundary layer thickness and orifice diameter, with boundary layer profile shape (laminar or turbulent) having little influence. For plate-like configurations with up to nine orifices, it is observed that hole impedance is not affected by interactions between the holes, however other effects on flow configuration are apparent. Finally, it is found that rounding off the flowside edge of the orifice leads to a smaller flow resistance, while effects on reactance can not be explained by simple parameters.