How motion trajectory affects energy extraction performance of a biomimic energy generator with an oscillating foil?

A non-sinusoidal trajectory profile is proposed for the oscillating hydrofoil in the energy generators instead of conventional sinusoidal plunging/pitching motions to seek better energy extraction performance. The novel profile is achieved by combining a specially designed trapezoidal-like pitching motion with a sinusoidal plunging motion and investigated numerically on its output energy coefficient and total output efficiency. Through an adjustable parameter β, the pitching profile can be altered from a sinusoidal (β = 1.0) to a square wave (β → ∞). In this work, a series of β ranging from 1.0 to 4.0 are investigated to examine the effect of combined motion trajectory on the energy extraction performance. The study encompasses the Strouhal numbers (St) from 0.05 to 0.5, nominal effective angle of attacks α0 of 10° and 20° and plunging amplitude h0/c of 0.5 and 1.0. Numerical results show that, for different β pitching motions, a larger α0 always results in a higher extraction power Cop and total efficiency ηT. Compared with the sinusoidal motion (β = 1), significant increment of Cop and ηT can be observed for β > 1 over a certain range of St. The investigation also shows that there exists an optimal pitching profile which may increase the output power coefficient and total output efficiency as high as 63% and 50%, respectively, over a wide range of St. Detailed examination on the computed results reveal that, the energy extraction performance is determined by the relative ratio of the positive and negative contributions from the different combination of lift force, momentum and corresponding plunging velocity and pitching angular velocity, all of which are considerably affected by β.