Acoustic impedance of perforated plates in the presence of fully developed grazing flow

Abstract

The perforated components are widely used in the intake and exhaust mufflers. The coupling between instable shear layer and acoustic fluctuation has a significant influence on the acoustic impedance of perforated components. To obtain practical and precise acoustic impedance of perforated plates in the presence of grazing flow, the three-dimensional (3D) time-domain computational fluid dynamics (CFD) approach was employed in the present study, and the extraction method of acoustic impedance was presented. The predicted non-dimensional acoustic impedance of the rectangular slot is consistent with the previous published measurement, which validates the accuracy of the present approach. Characterization of the turbulent boundary layer of flat-plate flow was studied and the relation between the convection velocity of vorticity and mean grazing flow velocity was provided. Numerical evaluations were carried out for different mean grazing flow Mach number Mmean (0.05–0.20), thickness to diameter ratio of orifice t/dh (0.2–0.5), porosity ϕ (4.51%–24.93%), and Reynolds number Re resulted from the orifice diameter dh (2 mm–6 mm), and their effects on the non-dimensional acoustic impedance of circular orifice were investigated in detail. The acoustic impedance formulae are obtained using the piecewise numerical fitting method based on the parameter analyses. As an application of engineering computation, the transmission loss of straight-perforated tube mufflers was predicted by using the present acoustic impedance model and the empirical impedance model of Lee and Ih via finite element computations. Comparison of the predicted and measured transmission loss (TL) results demonstrates that the present model yields highly accurate prediction.