Effect of confinement on the hydrodynamic performance of a fully-passive oscillating-foil turbine

An experimental study was conducted to assess the effects of flow confinement on the hydrodynamic performance of a fully-passive oscillating-foil turbine at a Reynolds number of 19,000. The experiments were performed using a National Advisory Committee for Aeronautics (NACA) 0015 foil with an aspect ratio of 7.5 in a water tunnel equipped with adjustable lateral walls. The kinematic parameters of the foil oscillations and its energy harvesting performance were measured at eight blockage ratios, ranging from 21 % to 60 %. Quantitative flow imaging was performed using particle image velocimetry (PIV) to observe the timing of the leading-edge vortex (LEV) formation and shedding. Loading on the foil was related to the flow structure by calculating the moments of vorticity with respect to the pitching axis of the foil. The efficiency and the power coefficient increased with increasing confinement and constant upstream velocity. At the highest level of confinement, the proximity of the foil to the walls during parts of the oscillation cycle resulted in a change in the phase lag between the pitching and the heaving components of the foil motion. In turn, this shift in the kinematic parameters led to a sharp decrease in the energy-extraction performance of the turbine.