A parametric study on the hydrodynamics of tidal turbine blade deformation

Abstract

The blades of axial-flow rotors, which are typically made of flexible composite materials, can experience significant deformations through their operation. However, the impact of these deformations on rotor hydrodynamics is not well understood. Blade deformations can be separated into three main components: flapwise, a thrust-driven deformation in the direction of the flow; edgewise, relatively small deformations in the plane of rotor motion and driven by rotor torque; and twist deformations that affect each blade’s angle of attack distribution and are generated by the torsional moment acting about the blade’s spanwise axis as well as by structural interactions. This work evaluates the hydrodynamic effects of the decoupled flapwise and twist deformations on a tidal turbine blade, using blade-resolved Computational Fluid Dynamics (CFD) simulations with the objective of identifying and quantifying the associated flow phenomena. The deformation cases were generated by scaling the static deformation shapes from a structural model of the turbine blade. The dataset used in this work, part of a larger research project, consists of 48 blade-resolved Reynolds-Averaged Navier–Stokes CFD steady-state simulations. It was found that twist deformation effects are significant, and can be adequately described using blade element theory. Flapwise deformations, on the other hand, produce different phenomena affecting the rotor loading and performance in ways that cannot be explained on a blade sectional basis through blade element theory. It is found that the hydrodynamic impact of flapwise deformations can be explained through two different mechanisms: a pressure drop on the suction side of the blade that generates inboard load augmentation, and an increase in near-tip radial flow effects that moves the onset of tip-loss effects, and shedding of thrust and power, further inboard. The studied cases show a significant impact of flapwise deformation on integrated power (up to a 20% drop), while the impact on integrated thrust remains limited, with variations between -4% to +2% in thrust coefficient observed depending on blade bending.