Applicability of Conventional Blockage Corrections to a Fully-Passive Oscillating-Foil Turbine

The influence of flow confinement on the kinematics and hydrodynamic performance of a fully-passive oscillating-foil turbine prototype was investigated experimentally at a Reynolds number of 19,000. The positions of the heave and pitch degrees of freedom were measured using rotary encoders, and quantitative flow patterns were obtained using particle image velocimetry. Quantifying confinement effects is important for comparison between model-scale confined experiments and full-scale unconfined turbine performance. Existing methodologies for correcting blockage effects on turbines are based on applying the conservation laws to flow around an actuator disk, to adjust the confined performance to unconfined performance. The kinematic parameters and energy harvesting performance of the turbine were measured at eight blockage ratios, ranging from 22% to 60%. The turbine performance metrics were then adjusted using two existing blockage correction models, which are based on the actuator disk theory. Both corrections were deemed unacceptable for the case of a fully-passive oscillating-foil turbine. The average corrected power coefficient gave up to 33.8% error compared with the projected unconfined power coefficient from linear extrapolation of the data, and the corrected value for zero confinement was not consistent among the different tested confinement levels. In contrast to the case of a kinematically-constrained turbine, where these corrections were successful, the kinematics of the oscillating foil changed with the changing confinement due to the passive nature of the turbine, making the different operating conditions incomparable.