Flow and Viscous Effects on Impedance Eduction

In principle, the impedance of a locally reacting acoustic liner should be independent of the structure of the sound field above the liner. Some experimental studies, however, have found that the impedance inversely determined by available eduction techniques is actually dependent on the direction of incident waves relative to the mean flow. This dependence has mainly been attributed to the incomplete description of flow physics in the eduction techniques, particularly to the neglect of the viscous effects. The purpose of the paper is to examine whether such a wave-direction dependence can be removed by including the flow physics that is absent in previous eduction techniques. To this end, two eduction methods using the full linearized Navier-Stokes equations are proposed. Possible turbulent effects are considered by using available turbulence models. The proposed eduction methods are applied to new measurement data and data available in the literature, and are compared with another eduction method that has been commonly used in the community. The results show that including viscous effects does not resolve the controversy on the apparent wave-direction dependence.