Analytical study of pre-stall hydrofoil experimental data for a cyclorotor-based wave energy converter

Cyclorotor-based wave energy converters (WECs), that utilise lift forces generated on hydrofoils due to their interaction with the wave-induced circular motion of water particles, have received recent attention from various researchers and organisations. Cyclorotor WECs have a number of appealing characteristics, including the potential for fast unidirectional rotation, simple power take-off, and relatively low wave loads by adjusting pitch angles, increasing survivability and capacity factor.The published analytical assessments of cyclorotor performance have shown that a cyclorotor achieves maximum performance in terms of wave cancellation, or mechanical shaft power generation, when its hydrofoils operate in the vicinity of the stall angle of attack. Such performance assessments are based on ideal lift and drag coefficient values obtained for aerofoils in aerodynamic tubes. In this study, we question the equivalence between aerofoils and hydrofoils in waves, in terms of lift and drag coefficients. We analyse the open access data, corresponding to 2D experimental testing of a scale prototype LiftWEC cyclorotor conducted by Ecole Centrale Nantes (ECN), using an analytical control-oriented point-vortex model. The current study includes validation of the point vortex model against experimental results, derivation of the lift and drag coefficients for hydrofoils in waves, and analysis of their pre-stall behaviour.