We show an optimal stability result for boundary layer solutions of the Navier–Stokes
equation in a half-plane, under a mild concavity condition on the boundary layer
profile. The key point is the derivation of sharp Gevrey estimates for the
linearized Navier–Stokes equation in vorticity form, on a time interval uniform in
. As
the nonlocal boundary condition on the vorticity prevents us from deriving direct
estimates, we use a novel iteration scheme, similar to a splitting method in
numerical analysis. Our result is a big step forward compared to our previous
work (Duke Math. J.167 (2018), 2531–2631), where we proved stability of
boundary layer expansions of shear flow type. Indeed, the approach of the
present paper is much more robust than the one in that previous work, which
was based on the Fourier transform and hence only adapted to expansions
independent of the tangential variable. Moreover, we are now able to relax the
assumption of strict concavity made in our previous work to obtain the optimal
Gevrey
stability, which was not satisfied by generic boundary layer expansions. We provide in
this way the first justification of unsteady boundary layer theory outside the analytic
setting.