 Solution

Scale axial distance with $\lambda \hat{h}$ and normal velocity with $\hat{U}/\lambda$, while the dimensionless axial velocity has scale $\hat{U}$. Let $r=R+y$; keep $R>0$ fixed on the asymptotic scales. In the core, the ratio of transverse viscous terms to axial convective terms is of order $\lambda /Re$. A small indentation and the associated displacement create tangential slip of order $ϵ$ at the edges of the wall layers.

For a layer of relative thickness $\delta \ll ϵ$, that slip makes axial convection of order ${ϵ}^2/\lambda$, while transverse viscous diffusion is of order $ϵ/(Re{\delta }^2)$. Balancing them givesUnsteady diffusion gives the equivalent estimate $\delta \sim (ReSt{)}^{-1/2}$ when $\lambda St=O(ϵ)$. Thus a consistent regime supporting thin viscous boundary layers and an inviscid core isAt a location where both imposed unsteadiness and slip vanish, a shear-dominated entrance balance can instead have thickness $(\lambda /Re{)}^{1/3}$; the same strict Reynolds-number condition keeps even that scale below $ϵ$. The reduction below assumes that no wall-layer thickening invalidates the stipulated small thickness. The normal flux displaced by a slip layer is of order $ϵ\delta$, smaller than the retained ${ϵ}^2$ core flux. Tangential no slip is imposed within these layers, not directly on the leading inviscid core.