An Experimental and Numerical Assessment of Airfoil Polars for Use in Darrieus Wind Turbines-Part II: Post-stall Data Extrapolation Methods

Accurate post-stall airfoil data extending to a full range of incidences between -180 deg and +180 deg are important to the analysis of Darrieus vertical-axis wind turbines (VAWTs), since the blades experience a wide range of angles of attack, particularly at the low tip-speed ratios (TSRs) encountered during startup. Due to the scarcity of existing data extending much past stall and the difficulties associated with obtaining post-stall data by experimental or numerical means, wide use is made of simple models of post-stall lift and drag coefficients in wind turbine modeling (through, for example, blade element momentum (BEM) codes). Most of these models assume post-stall performance to be virtually independent of profile shape. In this study, wind tunnel tests were carried out on a standard NACA 0018 airfoil and a NACA 0018 conformally transformed to mimic the "virtual camber" effect imparted on a blade in a VAWT with a chord-to-radius ratio c/R of 0.25. Unsteady computational fluid dynamics (CFD) results were taken for the same airfoils both at stationary angles of attack and at angles of attack resulting from a slow VAWT-like motion in an oncoming flow, the latter to better replicate the transient conditions experienced by VAWT blades. Excellent agreement was obtained between the wind tunnel tests and the CFD computations for both the symmetrical and cambered airfoils. Results for both airfoils also compare favorably to earlier studies of similar profiles. Finally, the suitability of different models for post-stall airfoil performance extrapolation, including those of Viterna-Corrigan, Montgomerie, and Kirke, was analyzed and discussed.