Effect of grazing-bias flow interaction on acoustic impedance of perforated plates

In this paper, the effect of grazing-bias flow interaction on the acoustic behavior of a perforated plate is investigated. In the experiment, bias flow into or out of orifices (inflow or outflow) is used to change the acoustic impedance of the test perforated plates subjected to grazing flow, and the acoustic impedance is measured as a function of both grazing and bias flow speeds. A comparison is made between the present measured orifice acoustic resistance and the previous results for steady state orifice resistance. Qualitatively, both results show similar characteristics. It is shown that there is some difference between grazing-inflow and grazing-outflow interactions. In the outflow case, grazing flow results in the drop of the acoustic resistance below the non-grazing flow value at high bias flow speed, whereas it generally has the effect of increasing the acoustic resistance in the inflow case. Furthermore, a simple grazing-bias flow interaction model is set up in an attempt to explain the observed phenomena. This model shows that the acoustic resistance of an orifice in a thin plate is proportional to bias flow Mach number, and is inverse to the square of the effective discharge coefficient which depends only on the ratio of bias flow speed to grazing flow speed. Finally, based on the theoretical analysis and the experimental data, an empirical equation is presented for the effective discharge coefficient.