Confinement effects on energy harvesting by a heaving and pitching hydrofoil

Wall confinement effects on the energy harvesting performance by a flapping hydrofoil (aspect ratio = 4.5) have been investigated in a circulating water flume at a Reynolds number of 50,000. Measurements of hydrodynamic forces are taken for three different confinement configurations (unconfined, one-wall and two-wall confinement) and a series of confinement levels for each configuration. Compared with the unconfined situation, a significant improvement of efficiency performance is obtained for strong two-wall confinement due to the enhancement of the hydrodynamic forces, while only a modest increase is observed in the one-wall confinement configuration. Results show that the heave component of efficiency is the primary contributor to the performance improvement. A parametric study, varying reduced frequency and pitching amplitude, shows that as the confinement increases, the optimal energy harvesting occurs at larger values of the reduced frequency and pitching amplitude, and can reach a Lanchester–Betz efficiency as high as 50%.