Abstract
An asymptotic structure is developed for a linear, high-frequency, unsteady disturbance superimposed upon a steady, possibly separated, nonlinear flow. The unsteady viscous sublayer is found to split into a two-region structure. The leading-order flowfield is driven primarily by the upper region, which coincides with the region of non-parallel flow in the original steady viscous sublayer. It is found that introducing a viscous-inviscid interaction into the unsteady problem drastically alters the domain of dependence of the unsteady flow throughout the entire viscous sublayer. The determination of the correct domain of dependence is found to involve a subtle interplay between the convective terms, the pressure-displacement interaction and the non-parallel base flow. Preliminary extensions to fully nonlinear unsteady interactive boundary layers are noted.