  A variational technique is used to derive analytical expressions for the
sensitivity of recirculation length to steady forcing in separated flows.
Linear sensitivity analysis is applied to the two-dimensional steady flow past
a circular cylinder for Reynolds numbers $40 \leq Re \leq 120$, both in the
subcritical and supercritical regimes. Regions which are the most sensitive to
volume forcing and wall blowing/suction are identified. Control configurations
which reduce the recirculation length are designed based on the sensitivity
information, in particular small cylinders used as control devices in the wake
of the main cylinder, and fluid suction at the cylinder wall. Validation
against full non-linear Navier-Stokes calculations shows excellent agreement
for small-amplitude control. The linear stability properties of the controlled
flow are systematically investigated. At moderate Reynolds numbers, we observe
that regions where control reduces the recirculation length correspond to
regions where it has a stabilising effect on the most unstable global mode
associated to vortex shedding, while this property does not hold any more at
larger Reynolds numbers.
