Numerical implementation of an impedance boundary condition considering a finite boundary layer

The so-called Ingard-Myers boundary condition is commonly used to represent an acoustic impedance in the presence of flow. However, recent studies have shown that may be inadequate in different conditions. Its main problem has been identified as the assumption of a boundary layer of infinitesimal thickness, and several alternative boundary conditions that address this shortcoming have been proposed. In general, the new boundary conditions performed better than the widely used Ingard-Myers condition, but these studies are somewhat limited to simpler geometries. The final goal of the work is to evaluate the impact of a finite boundary layer on the attenuation provided by acoustic liners in turbofan engines. The procedure to obtain an adequate finite element formulation of the chosen boundary condition is described. The implementation is validated by comparison with a simple circular duct analytical model, and used to predict liner attenuation on a realistic turbofan engine geometry and operating conditions, and on a scaled-down fan rig.