Evaluation of the radial flow effects on micro HAWTs through the use of a transition CFD 3D model - Part I: State of the art and Numerical model review

The radial flow along a rotating blade is a fluid dynamic behavior that specifically affects the flow field of HAWTs. The physical effects of such flow on the rotor performance are not yet fully understood due to the complexity of the phenomenon and its high dependence on three dimensionality and Reynolds numbers. In the first part of this paper the authors reviewed the State of the Art of physics and modeling of radial flows. Some researchers have proposed empirical models to take into account the centrifugal pumping inside 1D codes. It was found in general, that the radial flow acts on the blades, increasing the forces and delaying the stall. Compared to a simple 2D condition, the aerodynamic coefficients are hence increased. Obviously, this phenomenon is heavily dependent on rotational speed as the centrifugal force increases with the square of the angular velocity and only linearly with the radial distance. So, due to higher rotational speed, the aerodynamics of mini and micro rotors is mostly influenced by the radial flow rather than the large rotors. The combined effects of both transitional and radial flow were evaluated in the present work using an accurate CFD 3D model as there was no specific literature in this particular field. This model, developed by the authors, was based on a RANS, four equations, transition turbulence model and it was calibrated and validated on a suitably designed micro rotor. The rotor was tested in the subsonic wind tunnel owned by the University of Catania. A review of the modeling and validation strategy is presented in the first part of this paper while the extrapolated data and the post-processing is presented in the second part, thus finding results of significant interest.