The performance of a new concept of hydrokinetic turbine using oscillating hydrofoils to extract energy from water currents (tidal or gravitational) is investigated using URANS numerical simulations. The numerical predictions are compared with experimental data from a 2 kW prototype, composed of two rectangular oscillating hydrofoils of aspect ratio 7 in a tandem spatial configuration. 3D computational fluid dynamics (CFD) predictions are found to compare favorably with experimental data especially for the case of a single-hydrofoil turbine. The validity of approximating the actual arc-circle trajectory of each hydrofoil by an idealized vertical plunging motion is also addressed by numerical simulations. Furthermore, a sensitivity study of the turbine’s performance in relation to fluctuating operating conditions is performed by feeding the simulations with the actual time-varying experimentally recorded conditions. It is found that cycle-averaged values, as the power-extraction efficiency, are little sensitive to perturbations in the foil kinematics and upstream velocity.

A numerical investigation based on 2D URANS simulations is performed in order to seek an optimal spatial configuration for two oscillating foils within a hydrokinetic turbine. The objective of the study is to maximize the power extraction efficiency of the turbine. Tandem spatial configurations are considered because in such arrangement both hydrofoils are sharing the same flow window, which allows the turbine to reach higher efficiencies. The relative positioning of the downstream foil oscillating in the wake shed by the upstream hydrofoil is seen to be critical. Indeed, favorable interactions between the downstream foil and the wake vortices may lead to unexpectedly high power-extraction efficiencies (up to 64%), while unfavorable interactions may cause the downstream foil to contribute negatively to the total power extracted. A global phase shift parameter is introduced to characterize the tandem configuration. This parameter combines the inter-foil spacing and motion phase-shift into a single term. It is found useful to predict additional favorable configurations based on known results for cases with similar upstream-foil wake behavior. A comparison with experimental data is provided. Numerical predictions are seen to overpredict the power extraction performance in some cases. This is likely due to the broken 2D coherence of vortices in the 3D reality which affects the vortex-induced velocities and the subsequent foil-wake interactions.

The use of oscillating foils to develop turbines to extract energy from an oncoming flow has recently been found competitive with rotor blade designs. Moreover, the rectangular extraction plane of the oscillating-foil turbine presents clear advantages in the field of hydrokinetic turbines. The present study, based on two-dimensional unsteady Reynolds-averaged Navier–Stokes numerical simulations, focuses on the range of operating parameters maximizing power extracted by a single oscillating foil for small-scale application Reynolds number. The power-extraction efficiency is mapped in parametric spaces associated with different motion amplitudes. Efficiency maxima reach about 43% in all the parametric spaces considered, but for different values of pitching amplitude and frequency. It is found that the best parametric zones in all cases exhibit similar characteristics with regards to the effective angle of attack. Indeed, they are all associated with a maximum effective angle of attack of about 33 deg and a ...