  Hydrodynamic coupling of flexible flags in axial flows may profoundly
influence their flapping dynamics, in particular driving their synchronization.
This work investigates the effect of such coupling on the harvesting efficiency
of coupled piezoelectric flags, that convert their periodic deformation into an
electrical current. Considering two flags connected to a single output circuit,
we investigate using numerical simulations the relative importance of
hydrodynamic coupling to electrodynamic coupling of the flags through the
output circuit due to the inverse piezoelectric effect. It is shown that
electrodynamic coupling is dominant beyond a critical distance, and induces a
synchronization of the flags' motion resulting in enhanced energy harvesting
performance. We further show that this electrodynamic coupling can be
strengthened using resonant harvesting circuits.
