  The optimal control of a globally unstable two-dimensional separated boundary
layer over a bump is considered using augmented Lagrangian optimization
procedures. The present strategy allows of controlling the flow from a fully
developed nonlinear state back to the steady state using a single actuator. The
method makes use of a decomposition between the slow dynamics associated with
the baseflow modification, and the fast dynamics characterized by a large scale
oscillation of the recirculation region, known as flapping. Starting from a
steady state forced by a suction actuator located near the separation point,
the baseflow modification is shown to be controlled by a vanishing suction
strategy. For weakly unstable flow regimes, this control law can be further
optimized by means of direct-adjoint iterations of the nonlinear Navier-Stokes
equations. In the absence of external noise, this novel approach proves to be
capable of controlling the transient dynamics and the baseflow modification
simultaneously.
