Full instability of boundary layers with the Navier boundary condition

We consider the problem of the stability of the Navier-Stokes equations in $\mathbb{T}\times {\mathbb{R}}_{+}$ near shear flows which are linearly unstable for the Euler equation. In \cite{greniernguyen}, the authors prove an $L^{\mathrm{\infty }}$ instability result for the no-slip boundary condition which also denies the validity of the Prandtl boundary layer expansion. In this paper, we generalise this result to a Navier slip boundary condition with viscosity dependent slip length: ${\mathrm{\partial }}_{y}u={\nu }^{-\gamma }u$ at $y=0$, where $\gamma >1\mathrm{/}2$. This range includes the physical slip rate $\gamma =1$.