  This work applies a combined approach a reduced-order model (ROM) together
with experiments and direct numerical simulations to investigate the optimal
efficiency of fluid-flow energy harvesting from transverse vortex-induced
vibration (VIV) of a circular cylinder. High resolution efficiency maps were
predicted over wide ranges of flow reduced velocities and structural damping
ratios, and the maximum efficiency and optimal settings of damping ratio and
reduced velocity were then examined for different mass ratios and Reynolds
numbers. Efficiencies predicted by the ROM were also validated against either
experiments or direct simulations. The present work indicates that: (i) the
maximum efficiency is controlled by both the incoming reduced velocity and the
product of mass ratio and structural damping ratio, which is similar to the
maximum amplitude of VIV; (ii) the maximum efficiency at a relatively high
Reynolds number ($Re \approx 6 \times 10^3$) in subcritical regime is higher
than that of a low Reynolds number ($Re = 150$) in laminar regime; (iii) the
energy harvesting efficiency from VIV of a circular cylinder with a low mass
ratio is more robust than that with a high mass ratio. This finding suggests
that the VIV harvester performs better in water than in air.
