On a modified Myers boundary condition to match lined wall impedance deduced from several experimental methods in presence of a grazing flow

This paper deals with the experimental investigation of the lined wall boundary condition for engineering applications such as aircraft engine systems. The results show that the conventional Myers boundary condition fails to predict with accuracy the acoustic behavior and the lined wall impedance in particular when the acoustic waves propagate against the flow. More relevant, a modified Myers boundary condition based on a mixed condition between continuity of normal acoustic displacement and normal mass velocity over linings is used. It takes into account the effect induced by viscosity in the vicinity of the lined wall and leads to define an effective lined wall impedance unique whatever the direction of the acoustic wave propagation. Both are suitably validated by means of experiments. Finally, with minors modification in the Myers boundary condition and the wave equation, it is shown that the general case of acoustic propagation through a lined duct with an arbitrary flow profile at low Mach number can be treated as a classical uniform mean flow problem.