We explore numerically and theoretically the capability of flexible foils elastically
mounted to translational springs and dampers at the leading edge to extract energy from
low-speed winds through its passive heave motion. Given the spring and foil stiffnesses,
for each damper constant the theory (which is valid for high Reynolds numbers and
small foil deflection amplitudes, i.e., in absence of separation) provides analytically a
minimum wind velocity for flutter instability, above which energy can be harvested, that
depends on the thickness-to-chord-length ratio of the foil. Simple analytical expressions
for the flutter frequency are also provided. Minimum wind speeds and corresponding
flutter frequencies are characterized for a carbon fiber foil as the spring stiffness and
damper constant are varied, finding that energy can be extracted from wind speeds
lower than in conventional wind turbines. These theoretical predictions are assessed
from full numerical simulations at Reynolds numbers corresponding to these wind
velocities and for chord lengths of the order of the meter (i.e. about 106
) using
appropriate turbulence models, which allow to compute the power extracted from the
wind that the flutter stability analysis cannot provide
