Résumé
When designing and predicting the performance of acoustic liners, it is crucial that the lined wall is described by a correct acoustical boundary condition. Usually, the effect of a lined wall on the sound field is described by the impedance of the liner, i.e. the ratio of sound pressure to the wall normal particle velocity at the wall. This is correct for flexible (impermeable) walls, when the viscosity can be neglected. For perforated (permeable) walls, however, the axial momentum of the mean flow is transmitted to the wall by the acoustic flow through the wall. Then not only wall-normal (pressure) forces but also axial shear forces act on the wall and vice versa on the fluid flow. These shear forces increase more or less proportionally to the mean flow velocity. They exceed the acoustic wall shear stress in the stationary medium by orders of magnitude and cannot be neglected against the effects of the wall impedance when mean flow is superimposed. If at all, the shear stress effects have so far been described by the simplifying assumption of a homogeneous wall which is permeable at every point and at the same time fulfills the no-slip condition. In the study to be presented here, we examine from a physical point of view the mechanisms by which the wall shear stress is generated at a realistic liner wall. The permeable and the no-slip areas are clearly separated from each other and the respective flow acoustic effects must be modelled separately before homogenization. It is shown that the acoustic flow near the wall is controlled by two independent mechanisms: one causes a local reaction of the wall shear stress to the wall-normal particle velocity, while the other causes an interaction between the openings and depends on spatial distribution of the wall-normal particle velocity.