Influence of grazing flow and dissipation effects on the acoustic boundary conditions at a lined wall

The problem of sound propagation near a lined wall taking into account mean shear flow effects and viscous and thermal dissipation is investigated. The method of composite expansion is used to separate the inviscid part, in the core of the flow, from the boundary layer part, near the wall. Two diffusion equations for the shear stress and the heat flux are obtained in the boundary layer. The matching of the solutions of these equations with the inviscid part leads to a modified specific acoustic admittance in the core flow. Depending on the ratio of the acoustic and stationary boundary layer thicknesses, the kinematic wall condition changes gradually from continuity of normal acoustic displacement to continuity of normal acoustic mass velocity. This wall condition can be applied in dissipative silencers and in aircraft engine-duct systems.