  A variational technique is used to derive analytical expressions for the
sensitivity of several geometric indicators of flow separation to steady
actuation. Considering the boundary layer flow above a wall-mounted bump, the
six following representative quantities are considered: the locations of the
separation point and reattachment point connected by the separatrix, the
separation angles at these stagnation points, the backflow area and the
recirculation area. For each geometric quantity, linear sensitivity analysis
allows us to identify regions which are the most sensitive to volume forcing
and wall blowing/suction. Validations against full non-linear Navier-Stokes
calculations show excellent agreement for small-amplitude control for all
considered indicators. With very resemblant sensitivity maps, the reattachment
point, the backflow and recirculation areas are seen to be easily manipulated.
In contrast, the upstream separation point and the separatrix angles are seen
to remain extremely robust with respect to external steady actuation.
