A numerical study on the response of a 2D bluff body wake subjected to harmonic
forcing imposed by two rear pitching flaps is performed. The wake is generated by a
rectangle at a height-based Reynolds number Re = 100, characterised by laminar vortex
shedding. Two forcing strategies are examined corresponding to in-phase “snaking” and
out-of-phase “clapping”. The effects of the bluff body aspect ratio (AR = 1, 2, 4), flapping
frequency, flapping amplitude, flap length and Reynolds number are investigated. For the
snaking motion, a strong fundamental resonance of the root mean square (RMS) drag is
observed when the wake is forced near the vortex shedding frequency. For the clapping
motion, a weak subharmonic resonance is observed when the forcing is applied near
twice the vortex shedding frequency resulting in an increase of the lift RMS whereas
the drag RMS remains unaffected. Both resonances intensify the vortex shedding and a
concomitant mean drag increase is observed for the snaking motion. Forcing away from
the resonant regimes, both motions result in considerable drag reduction through wake
symmetrisation and propulsion mechanisms. The formation of two vortex dipoles per
oscillation period due to the flapping motion, which weaken the natural vortex shedding,
has been identified as the main symmetrisation mechanism. A single scaling parameter is
proposed to collapse the mean drag reduction of the forced flow for both motions over a
wide range of flapping frequencies, amplitudes and flap lengths. Finally, the assessment
of the performance of the forcing strategies has revealed that clapping is more effective
than snaking.