Efficient control of the fully passive oscillating foil in 2D confined flows with adjustment of the heave damping

A foil that is free to pitch and heave in an upstream flow can oscillate very regularly and with large amplitudes given that its inertial properties and support stiffness in pitch and heave are well adapted to the flow velocity. Useful energy can be extracted from these oscillations via an appropriate damping in heave that models the presence of an electric generator. In recent years, the structural parameters of such a fully passive oscillating-foil turbine (OFT) have been optimized, yielding a maximum energy extraction efficiency of 51.0% under the assumptions of 2D and unconfined flow. However, the turbine is normally deployed in channels with finite cross-sectional area, thus impacting the flow rate passing through the turbine via the blockage effect. In this work, we extend the applicability of the 2D optimized structural parameters to 2D confined scenarios with a simple tuning of the viscous heave damping coefficient. Performance is determined via a fluid-structure interaction solver based on an unsteady Reynolds-averaged Navier–Stokes approach. As expected, confining the turbine increases the heave amplitude and the power, up to a point where the motions become chaotic, and thus require an increase in the heave damping coefficient. This study shows that in all confined 2D scenarios, reasonably good performances of the fully passive OFT can be maintained when using its optimal structural parameters obtained in 2D unconfined conditions, given that the generator is adjusted accordingly.