Influence of Base Flow Modelling and Viscous Effects on Impedance Eduction of Acoustic Liners

This work adopts a mid-fidelity numerical framework, coupling Reynolds-Averaged Navier-Stokes simulations with frequency-domain Linearised Navier-Stokes equations, to assess the impact of conventional flow and acoustic propagation assumptions on educed impedance. First, the approach is validated by simulating a three-dimensional acoustic liner geometry and comparing the results with experimental and high-fidelity scale-resolved simulation data. It is demonstrated that the framework reproduces the frequency-dependent acoustic impedance behaviour, capturing the impedance's directional dependence on acoustic wave propagation, but showing some differences in terms of absolute values. Then, the liner geometry is substituted by an impedance wall boundary condition with a frequency-dependent impedance value equal to the one obtained for the explicit liner configuration. The impact on the acoustic field of assuming uniform flow with the Ingard-Myers boundary condition or a sheared flow with an impedance boundary condition without the thin boundary layer assumption is evaluated. It is found that, in both cases, the predicted transmitted and reflected fields remain direction dependent, but the transmission–reflection partition depends on the selected combination of flow profile and acoustic formulation. Based on this outcome, a FEM-based impedance-eduction method employing a dual-stage optimisation (global followed by local refinement) is introduced and applied with a shear-resolved, viscous forward model. Compared with conventional eduction approaches, this procedure shows a less relevant propagation-direction dependence of the educed impedance.