Viscous effects on the attenuation of a plane wave by an acoustic lining in shear flow.

The attenuation of a plane acoustic wave incident on a flat impedance surface in a sheared and viscous fluid is investigated numerically and asymptotically. Predictions of various boundary models of impedance surfaces in shear flow are tested by comparing their predicted reflection coefficient. It is found that viscosity has a significant effect, reducing the reflection of upstream propagating sound while increasing the reflection of cross-stream propagating sound. The classical Ingard-Myers boundary condition is shown to incorrectly predict the damping rate of sound in many cases, and in some cases viscous effects are shown to be comparable to shear effects.