A generalized method to extend airfoil polars over the full range of angles of attack. (August 2020)

• Record Type: Journal Article
• Authors: Battisti, Lorenzo; Zanne, Luca; Castelli, Marco Raciti; Bianchini, Alessandro; Brighenti, Alessandra
• Is Part Of: Renewable energy. Volume 155(2020)
• Page Start: 862
• Page End: 875
• Publication Date: 2020-08
• Subjects: Airfoil polars -- Post-stall behavior -- Lift and drag coefficients -- Stall-controlled wind turbine -- Vertical-axis wind turbine -- Vertical-axis water turbine; Renewable energy sources -- Periodicals; Power resources -- Periodicals; Énergies renouvelables -- Périodiques; Ressources énergétiques -- Périodiques; 333.794
• DOI: 10.1016/j.renene.2020.03.150
• Languages: English
• ISSNs: 0960-1481

Abstract

Abstract: The accurate prediction of airfoil polars over the whole range of angles of attack and for different Reynolds numbers is pivotal to ensure an accurate prediction of wind turbine aerodynamic performance. In particular, modeling correctly the deep-stall and post-stall airfoil behavior is essential to reproduce reliable power and thrust curves for high wind conditions, especially as far as stall-controlled wind turbines are concerned. This need is even more urgent in vertical-axis wind turbines, in which the airfoils do experience a continuous variation of the angle of attack, exceeding the stall limit even in normal operating points. This paper presents a new method for extending an existing database of airfoil aerodynamic coefficients, e.g. in the common ranges of aeronautical references, over the ±180° range of angles of attack for an arbitrary airfoil shape. Moving from a few basic geometric data, as maximum thickness, camber and leading-edge radius, the whole curves of lift and drag coefficients are reconstructed starting from deep-stall conditions. The proposed correlation is compared to different models for the post-stall extrapolation of the airfoil behavior and experimental data, showing a good agreement with experimental polars for both symmetrical and cambered airfoils. Highlights: New method to extend existing 2D airfoil polars in the range ± 180° AoA proposed. Maximum drag estimation based on a simple airfoil geometry (half ellipsis + wedge). Particular focus on the accuracy in the post-stall region. Good agreement with experimental data for both symmetric and cambered airfoils. Better accuracy than existing methods.