Tidal turbines operate in adverse flow environments, resulting from high turbulence, waves, and shear, which can lead to large unsteady forces acting on the blades and thus to structural fatigue. To mitigate these unsteady loads, and to cap the maximum power when the flow speed is above rated, turbines use active pitch control systems that adapt the blade pitch to the flow condition. Active pitch control systems require additional gearboxes and motors that increase capital and operational costs, require additional maintenance and may decrease system reliability. To address these challenges, Viola et al. (2022) introduced a novel passive pitching blade concept. The blades passively adjust their pitch in response to the oncoming flow conditions. One way of achieving this is to mount each blade onto a torsional spring. Liu et al. (2024) showed numerically that passive pitching foils can mitigate lift fluctuations by at least two-thirds. Otomo et al. (2024) developed a mathematical framework for optimising the pivot point location to maximise unsteady load mitigation. Gambuzza et al. (2025) demonstrated experimentally that passively pitching blades can be used to keep the power constant above rated conditions while reducing thrust loads. Most previous studies have focused on the effect of varying the tip speed ratio and the freestream speed on power and thrust loads. Instead, in the present work, we study the effect of spring preload on the system performance by conducting experiments on a 1.39-metre-diameter turbine equipped with three independently passively pitching blades. Experiments were conducted at the University of Edinburgh’s Flowave Ocean Energy Research Facility. Results indicate that a turbine equipped with passively pitching blades achieves design-point performance similar to that of a turbine with fixed pitch blades. It is shown that there is an optimum spring preload that enables the turbine to operate at rated power despite a change in tip speed ratio, while reducing both mean and fluctuating thrust by nearly 20% and 30%, respectively. By changing the spring preload, the tip speed ratio at which the power is matched can be tuned - lower preload gives matched performance at a lower tip speed ratio. Passive pitch blades are also shown to have higher hydrodynamic efficiency at the design point(78% as opposed to 55% for fixed pitch blades). References1. I. M. Viola, G. Pisetta, W. Dai, A. Arredondo-Galeana, A. M. Young, A. S. M. Smyth, Morphing blades: Theory and proof of principles, IMEJ, 2022.https://doi.org/10.36688/imej.5.183-1932. S. Otomo, S. Gambuzza, Y. Liu, A. M. Young, R. Broglia, E. D. McCarthy, and I. M. Viola, A general framework for the design of efficient passive pitch systems, Physics of Fluids, 2024.https://doi.org/10.1063/5.02126263. Y. Liu, R. Broglia, A. M. Young, E. D. McCarthy, and I. M. Viola, Unsteady load mitigation through passive pitch, J. Fluids Struct, 2024. https://doi.org/10.1016/j.jfluidstructs.2024.1042164. S. Gambuzza, P. Sunil, M. Felli, A. M. Young, R. Broglia, E. D. McCarthy, and I. M. Viola, Power and thrust control by passive pitch for tidal turbines. Renewable Energy, 2025.https://doi.org/10.1016/j.renene.2024.121921
Sunil et al. (Mon,) studied this question.