  Vortex-induced vibrations (VIV) of flexible cables are an example of
flow-induced vibrations that can act as energy harvesting systems by converting
energy associated with the spontaneous cable motion into electricity. This work
investigates the optimal positioning of the harvesting devices along the cable,
using numerical simulations with a wake oscillator model to describe the
unsteady flow forcing. Using classical gradient-based optimization, the optimal
harvesting strategy is determined for the generic configuration of a flexible
cable fixed at both ends, including the effect of flow forces and gravity on
the cable's geometry. The optimal strategy is found to consist systematically
in a concentration of the harvesting devices at one of the cable's ends,
relying on deformation waves along the cable to carry the energy toward this
harvesting site. Furthermore, we show that the performance of systems based on
VIV of flexible cables is significantly more robust to flow velocity
variations, in comparison with a rigid cylinder device. This results from two
passive control mechanisms inherent to the cable geometry : (i) the
adaptability to the flow velocity of the fundamental frequencies of cables
through the flow-induced tension and (ii) the selection of successive vibration
modes by the flow velocity for cables with gravity-induced tension.
