An insight into the rotational augmentation on HAWTs by means of CFD simulations - Part II: Post-processing and force analysis

The interesting results obtained in the first part of this work, led the authors to further study the fluid dynamic behavior of rotating blades. This was done in order to improve the physical comprehension of the rotational augmentation and to provide new ideas and explanations on its onset and development. Three operative conditions were analyzed in-depth: an attached flow condition, at low AoAs; a maximum augmentation condition at medium AoAs; and a fully separated condition at high AoAs. A comparison between 2D airfoil and 3D results was made as well. Post-processing images of streamlines, pressure contours, pressure coefficient trends and helicity contours were thus obtained, demonstrating the presence of a coherent helical structure in the inner blade at medium AoAs. The development of such a structure explained the strong flow acceleration and thus the depressurization of the suction side of the blade. Therefore the helical vortex was found to be mainly responsible for the lift and drag augmentation. Moreover, from an accurate analysis of the forces acting on a fluid element, it was shown that the radial component of the Coriolis forces was the main factor, responsible for the helical structure onset. This Coriolis component acted in an imbalance with the centrifugal force and the spanwise pressure gradient inside the entire separated layer, triggering the 3D flow tangle. The streamwise component of the Coriolis force, instead, was finally found to be of minor importance. Further confirmations derived from the 3D CFD simulations of the rotor fixed wing.