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With flow, the acoustical effect of a lined wall cannot be described by asingle quantity like the wall impedance. At least two quantities are required.In addition to the impedance, the unsteady tangential force exerted by the wallon the flow has to be taken into account. This force is due either to viscouseffects or to the unsteady transfer of axial momentum from the flow into thelined wall. The paper describes a Stress--Impedance model where the twovariables used are the impedance and the friction factor that links thepressure to a tangential stress at the wall. The use of a wall stress helps tobetter understand the mechanisms of momentum transfer between the flow and thewall in the vicinity of an acoustic treatment.

On the use of a Stress-Impedance Model to describe sound propagation in a lined duct with grazing flow