On the importance of the root-to-hub adapter effects on HAWT performance: A CFD-BEM numerical investigation

This research presents the results of an investigation about the effects of the transition region shape, between the hub and the full blade, on the overall performance of horizontal axis wind turbine rotors. The analysis was carried out by developing a new methodology based on the joint use of accurate CFD 3D simulations and an inverse 1D BEM code. The CFD 3D model was specifically developed taking advantage of the new powerful generalized k-ω turbulence model and the pseudo-transient coupled solver, available in Ansys Fluent. Particularly, the surface of one of the blades was split in the same number of BEM elements already at the CAD level, in such a way to easily obtain the local axial and tangential aerodynamic forces of the blade sections. These forces were then directly used in the inverse BEM code as input to derive local aerodynamic coefficients and angles of attack. The entire procedure was automated in Matlab by using specific command lines within the BEM code to launch an Ansys Fluent journal that drove the CFD simulation. The methodology was validated using the NREL Phase VI geometry and the related experimental measurements. The results demonstrated that the shape of the root to hub adapter of the Phase VI rotor strongly affected the performance as the wind speed increased. This was found to be due to the effects of the centrifugal pumping which extended well beyond the inner section of the full blade. The comparison with the simulation of an experimental micro rotor, with a different root-to-hub shape, confirmed the findings. These results, on the one hand evidenced the importance of taking into account the root-to-hub transition region in 1D codes; on the other hand, they led the way for the possibility of increasing rotor performance by improving the aerodynamic exploitation of the blade root.