Acoustic Properties of Perforated Plates with Bias Flows

Acoustic properties of micro-perforated plates with bias flows are numerically evaluated using the Lattice Boltzmann method. Predictions are carried out for different porosity and length to diameter ration of orifices ranging 3% = s = 20% and 0.2 = lw/D = 5. The flow Mach number through orifices is up to 0.1. Particular attention is paid to the combined effect of interaction between orifices and finite plate thickness on the orifice impedance. Perforated plates are modeled as a rigid plane with a circular orifice placed in the middle of duct (three dimensional computational domain) with square cross section. The periodic boundary condition is imposed to the duct lateral walls to model the periodic arrangement of perforation. The acoustic wave is produced by a sinusoidal velocity fluctuation of flow provided at the upstream end of the duct. Obtained acoustic properties are compared with previous results given in analytical models and experimental results. The rate of acoustical energy absorbed by vortices is calculated from the predicted strength of vorticity at the edge of orifice, and is compared with the acoustic absorption coefficient to confirm that the physical mechanism of absorption is the conversion of acoustical energy into vortical energy.