Effect of wall shear layers on the sound attenuation in acoustically lined rectangular ducts

Abstract

This paper deals with the manner in which a shear layer proximate to the wall of an acoustically treated rectangular duct modifies the attenuation spectra. The restriction of this shear layer to the region near the lined duct walls is aimed at simulating boundary layer effects on the attenuation. Theoretical results show that shear significantly changes the peak attenuation, causing a frequency shift of this peak. For the inlet mode, i.e. flow against the direction of sound propagation, both results are a strong function of Mach number and layer thickness. For the exhaust mode, i.e. flow in the direction of propagation, these effects are relatively weak. The theory set forth in this paper is also compared with experimental data obtained from flow duct facilities. Inclusion of shear definitely improves agreement with experimental data of the attenuation for the inlet mode but slightly worsens agreement with the experiments for the exhaust mode.