  Many experimental and numerical studies report a large reduction of the
recirculation bubble in Backward-Facing Step flows or airfoils in stall
situation when excited at the natural shedding frequency $f_0$. Through a
simple experiment using Dielectric Barrier Discharge actuator, we find a
different result. For a given Reynolds number, the frequency of the
perturbation is varied for a fixed duty-cycle dc = 27%. Through phase-averaging
of Particle Image Velocimetry measurements, we show that the actuation creates
a forced vortex which interacts with the natural shedding with a different
phase velocity than the unforced one. The largest reduction of the
recirculation bubble (-35%) is obtained in a very narrow frequency range around
$0.73 f_0$ where early vortex pairing occurs between forced and unforced
vortices. Phase averaging shows that in this case, the actuation clearly forces
the vortex pairing in the shear layer. On the contrary, when the forcing
frequency is higher, the shear layer behaves like an amplifier synchronized on
the forced frequency, leading to a constant 10% reduction of the recirculation
bubble.
