Acoustic impedance of micro-perforated panels in a grazing flow

Micro-perforated panels (MPPs) are widely used for broadband sound absorptions. For a MPP exposed to a grazing flow, existing acoustic impedance formulas based on different flow parameters give inconsistent results, thus calling for a systematic investigation of the issue to find more intrinsic flow parameters allowing for a reliable acoustic impedance prediction. In this study, three-dimensional CFD simulations are conducted on a MPP hole with a backing space in a flow duct. Numerical results allow identifying the flow velocity gradient in the viscous sublayer as the intrinsic flow parameter and show its linear relationship with a flow-related term in the acoustic resistance formula. Through a linear regression analysis, an acoustic resistance formula is established within a certain flow range (Mach number up to 0.25) under the linear acoustic regime. The validity of the impedance formula is demonstrated through comparisons with existing results and experimental data reported in the literature, showing good agreement and superiority in terms of the prediction accuracy.