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Effect of motion trajectory on the energy extraction performance of a single fully-activated flapping foil
Author(s): A. Boudis, H. Oualli, A. Benzaoui, O. Guerri, A. C. Bayeul-Lainé, O. Coutier Delgosha
Keywords: Flapping foil, Energy extraction, Renewable energy, Non-sinusoidal trajectory, CFD
In recent years, flapping foil energy harvesting systems are considered as competitive devices for conventional turbines. Several research projects have been carried out to improve the performance of these new devices. This paper studies the effects of non-sinusoidal heaving trajectory, non-sinusoidal pitching trajectory, and the effective angle of attack on the energy extraction performances of a flapping foil operating at low Reynolds number (Re=1100). An elliptic function with an adjustable parameter S (flattening parameter) is used to simulate various sinusoidal and non-sinusoidal flapping trajectories. The flow around the flapping foil is simulated by solving the Navier–Stokes equations using the commercial software Star CCM+ based on the finite-volume method. Overset mesh technique is used to model the flapping motion. The study is applied to the NACA0015 foil with the following kinetic parameters: a dimensionless heaving amplitude $h_0 = 1c$, a shift angle between heaving and pitching motions $\phi=90^\circ$, a reduced frequency $f^*=0.14$, and an effective angle of attack $ \alpha_{max} $ varying between $ 15^\circ $ and $ 50^\circ $, corresponding to a pitching amplitude in the range $ \theta_0= 55.51^\circ$ to $90.51^\circ$. The results show that, the non-sinusoidal trajectory affects considerably the energy extraction performances. For the reference case (sinusoidal heaving and pitching motions, $S_h = S_\theta=1$), the best performances are obtained with the effective angle of attack, $ \alpha_{max}=40^\circ $. At small effective angle of attack $ \alpha_{max}< 30^\circ$, the non-sinusoidal pitching motion combined with a sinusoidal heaving motion, greatly improve the energy extraction performance. For $\alpha_{max}=15^\circ $, $S_h =1$ and $ S_\theta=2$, the energy extraction efficiency is improved by 52.22\% and the power coefficient is improved by 70.40\% compared to sinusoidal pitching motion. At high effective angles of attack ( $\alpha_{max}>40^\circ $), the non-sinusoidal pitching motion has a negative effect. The performance improvement is quite limited with the combined motions non-sinusoidal heaving/sinusoidal pitching.