On the influence of twist and taper of HAWT blades on the rotational augmentation phenomenon: the NREL Phase VI – Phase II comparison

Abstract

The physics of 3D flows on rotating blades is currently one of the most important research fields related to wind turbines. Although many authors have studied the phenomenon thoroughly and they have proposed meaningful physical explanation of the mechanism which triggers the onset of the rotational augmentation, a universal correlation between rotor geometry, operating conditions and centrifugal pumping effects has not been derived yet. For instance, all the proposed corrections for rotational stall delay within 1D codes have demonstrated fairly good accuracy but in limited operating range or for specific airfoils or rotor geometries. In the present work the authors applied a consolidated methodology, based on the use of accurate CFD 3D models and of an inverse BEM code, to thoroughly analyze the differences in terms of rotational augmentation effects between the two widely known NREL Phase VI and Phase II HAWTs. This choice was made precisely since both the rotors used the S809 airfoil for the active part of the blade, had equal radial dimensions and the operating conditions were approximately the same. Thus, the substantial difference between the rotors was the fact that the Phase VI blade was twisted and tapered while the Phase II one had constant chord and pitch along the radial direction. In this way, the possible influence due to the twist and taper of the blade on the physics of the centrifugal pumping could be highlighted more easily. The CFD models were developed in Ansys Fluent and validated against experimental measurements available in the literature. The inverse BEM code, already implemented in a previous work, allowed the authors to extrapolate and compare sectional data obtained through the CFD simulations. The post-processing of the results demonstrated the strong influence of the twist and taper of the blade on the dynamics of the rotational augmentation. These results lead the way for a better understanding of the relation between rotor geometry and centrifugal pumping physics.