Acoustic impedance characterization of sub-millimeter orifices exposed to grazing and grazing-bias flow

Abstract

This paper characterizes the linear flow-acoustic impedance of circular sub-millimeter orifices exposed to a grazing and grazing-bias flow configuration using a multi-microphone three-port measurement framework. Measurements are performed for fully developed turbulent flows and the static aerodynamic pressure difference over the perforated plate is monitored and controlled. By counteracting any static pressure difference over the perforations, the effect of a purely grazing flow is studied and quantified using existing scaling laws. The increase in perforation resistance is observed to scale with Mach number, whereas a frequency dependent skin friction velocity correlation is seen to better fit the decreasing trend in equivalent length. However, for both impedance variables, the coefficients of the empirical macro-perforated models are revisited to match the measured data for sub-millimeter dimensions. In a grazing-bias flow configuration, the bias flow component is seen to dominate the perforation’s acoustic behavior for pressure differences larger than $100\text{Pa}$. Furthermore, a pressure difference as low as $25\text{Pa}$ is observed to already significantly increase the resistance and decrease the equivalent length compared to the purely grazing flow situation and results in a different frequency dependency with respect to the orientation of the bias flow component. A such, the pressure difference over the perforations also becomes an important parameter to monitor in grazing flow measurements, especially for perforated plates with sub-millimeter dimensions and low percentage of open area.