Flow curvature effects on the aerodynamic polars of a NACA 0015 airfoil

The blades of vertical-axis turbines (VAT) operate in curved and nearly circular flow. Compared to a uniform flow, the curved streamlines alter the aerodynamic coefficients of the airfoil. As a result, measuring and predicting the aerodynamic coefficients of an airfoil in curved flow poses a challenge. In this work, we first present a methodology to properly determine the aerodynamic coefficients (lift, drag, and moment) of a NACA 0015 airfoil attached at the quarter-chord in steady curved flow at Rec=6×106 using blade-resolved CFD with a “key-hole mesh domain”. By varying the airfoil's angle of attack and the airfoil's chord to turbine's radius ratio (c/R), the aerodynamic coefficient curves are obtained and presented. The results show that the curvature effects on the aerodynamic coefficients are significant. It is observed that the drag coefficient is closely related to the moment coefficient of the airfoil and the moment coefficient about the airfoil's rotation axis. Furthermore, the Coriolis effect is shown to be responsible for reducing the drag coefficient values compared to those in a uniform flow. The findings of this investigation will help develop improved models based on actuator lines (ALM) for the prediction of VAT's performance.