An analytical model for airfoil aerodynamic characteristics over the entire 360° angle of attack range

The design of wind turbines requires rapid and accurate evaluation of aerodynamic loads. High-fidelity methods based on Computational Fluid Dynamics (CFD) are considered as most promising but remain expensive for predictions of aerodynamics. It is still imperative to use low or medium-fidelity methods that depend strongly on the knowledge of 360° polars. Polars can be accurately predicted by wind tunnel measurements or CFD techniques in the range of small values of AoAs up to a couple of degrees beyond the critical angle of stall; this AoA range is denoted by Dms. For higher values of AoAs, in the deep stall regime Dds, 3D effects play an important role and the determination of accurate polars is still challenging. This paper presents a semi-empirical model of force coefficients for both ranges Dms and Dds, with an appropriate transition between the two. For the range Dds, the modeling is based on assuming a nearly flat plate behavior and some symmetry properties for the force coefficients with respect to the angle of attack. The parameters of the model equations are determined from the analysis of selected experiments on airfoils associated with the characteristics of the airfoil geometry. For the range Dms, a modified formulation of the dynamic stall model of Beddoes–Leishman has been proposed. The model has been tested successfully on two airfoils, NACA 643-618 and DU97-W-300. The new approach of elaborating the polars for the whole range of AoAs provides a foundation for the application of a dynamic stall model.