Boundary-layer thickness effects of the hydrodynamic instability along an impedance wall

The Ingard-Myers condition, modelling the effect of an impedance wall under a mean flow by assuming a vanishingly thin boundary layer, is known to lead to an ill-posed problem in time domain. By analysing the stability of a linear-then-constant mean flow over a mass-spring-damper liner in a two-dimensional incompressible limit, we show that the flow is absolutely unstable for h smaller than a critical h(c) and convectively unstable or stable otherwise. This critical h(c) is by nature independent of wavelength or frequency and is a property of liner and mean flow only. An analytical approximation of h(c) is given, which is complemented by a contour plot covering all parameter values. For an aeronautically relevant example, h(c) is shown to be extremely small, which explains why this instability has never been observed in industrial practice. A systematically regularised boundary condition, to replace the Ingard-Myers condition, is proposed that retains the effects of a finite h, such that the stability of the approximate problem correctly follows the stability of the real problem.