Hydrodynamic independence and passive control application of twist and flapwise deformations of tidal turbine blades

The load-induced deformations experienced by axial-flow rotor blades can result in significant hydrodynamic impacts on rotor operation. These changes in hydrodynamics are dominated by the flapwise and twist deformation components, affecting blade loading and performance. This work uses blade-resolved computational fluid dynamics simulations to explore the hydrodynamic interactions of coupled flapwise and twist deformations, and their potential for use in passive control strategies. The rotor blades were simulated under parametrically prescribed flapwise-only, twist-only and coupled flapwise-twist deformations. The results show that the hydrodynamic effects are adequately described by blade-element theory for twist deformations regardless of the presence of flapwise deformations, whereas flapwise deformations induce changes in the local lift and drag coefficients that are independent of twist. For moderate blade deflections, the hydrodynamic changes generated by the two deformation components can be approximated to be independent from each other. The observed hydrodynamic independence between the two deformation components is used to explore passive deformation strategies for a tidal rotor. By extrapolating an existing dataset containing CFD simulations of twist-only and flapwise-only deformation cases at different tip-speed ratios, control paths are designed within a tip-speed ratio, flapwise and twist deformation parameter space. These control paths demonstrate passive control strategies as a potential alternative to active pitch control on tidal turbines, showing similar performance and maximum loading, compared with an active pitch strategy, over a full tidal cycle. In particular, it is shown that flapwise deformations have an important role in power capping above rated flow speed.