The so-called Ingard–Myers boundary condition is commonly used to represent an acoustic impedance in the presence of grazing flow. However, recent studies have shown that it may be inadequate under certain conditions. The 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 this work, we present a methodology to implement an alternative boundary condition in the context of the finite element method. The acoustic impedance boundary condition for finite boundary layers in straight circular ducts known as the Brambley boundary condition is selected for this purpose. The procedure to obtain an adequate finite element formulation of the chosen boundary condition is described. The new formulation is implemented in the commercial finite-element solver COMSOL Multiphysics and initially validated by comparison with a simple circular duct analytical model. The alternative boundary conditions is then used to predict liner attenuation in a realistic turbofan engine geometry and operating conditions and in a scaled-down fan rig. Results suggest that the effect of the boundary layer is nonnegligible, specially when higher-order modes are involved, with the Brambley boundary condition outperforming the Ingard–Myers boundary condition.
Seki et al. (2025) studied this question.