Energy harvesting from passive oscillation of inverted foil

A numerical study is carried out to investigate the energy harvesting from an inverted foil undergoing flow-induced pitching oscillation for reduced velocity Ur  =  1-45 and damping ratio ζ = 0-0.295. The benchmark results with undamped foil (ζ = 0) indicate that the foil does not oscillate for Ur  ≤ 27 but does oscillate with increasing amplitude for 27 < Ur < 34 and with constant amplitude for Ur  ≥ 34. Lissajous diagrams of moment coefficient against the foil displacement are linked to the energy harvesting, showing how Ur and ζ affect the oscillating amplitude, reduced frequency, wake structures, and power exchange between the foil and the flow. The energy harvesting efficiency η up to 15.06% is achieved at Ur  =  37 and ζ = 0.130 with a reduced frequency f* = 0.151 that is used by the cruising aquatic animals. The foil oscillation with negative power enhances the growth of vortices while that with positive power weakens the growth.