THE EFFECT OF SHEAR-STRESS ON THE PROPAGATION AND SCATTERING OF SOUND IN FLOW DUCTS

The scattering of plane sound waves at a lined section of an otherwise rigid flow duct with circular cross-section is investigated theoretically and experimentally. Measurements have been carried out at low Mach numbers and at moderate frequencies. The lining consists of a pervious screen with a very low flow resistance backed by narrow cavities. Therefore, in the first instance, the liner can be modelled by a locally reacting yielding wall. Additionally, the wall shear stress originating from the interaction between the turbulent flow and the pervious wall affects the sound propagation in the lined duct and the sound scattering at the discontinuities of the wall compliance. The influence of the lateral gradients of the mean flow velocity and of the sound pressure have also been considered. The sound field is characterized by the flux of mass and momentum. Concerning the scattering of the plane wave at the discontinuities, it turns out that the plane-wave acoustical flux of mass and momentum are continuous at the interface: i.e., the effect of the near field at the discontinuity on the plane wave is negligibly small. © 1992.