Analysis of flow curvature and unsteady effects on the aerodynamic coefficients of a one-bladed vertical-axis turbine

To obtain realistic predictions of vertical-axis turbines (VAT), simplified models must use correct aerodynamic coefficients based on the local flow conditions. As the blades of a VAT operate in flows with curved streamlines, and in an unsteady manner, predicting realistic coefficients based on the local angle of attack becomes challenging. Indeed, the flow curvature and its unsteadiness both alter the aerodynamic coefficients of the blade. In this work, we first present a methodology to properly measure the aerodynamic coefficients (lift, drag and moment) of a NACA 0015 airfoil in steady curved flow at Re_c = 6 x 10⁶ using CFD with a "keyhole mesh domain", with varying the airfoil’s angle of attack, arm radius to airfoil's chord ratio (R/c), as well as the position of the airfoil's connection point to the arm. In particular, it is seen that the pressure drag is much affected by the flow curvature which we show to be related to a Coriolis effect. Furthermore, when performing CFD with added upstream cross-flow, it is also seen that the effect of flow unsteadiness is significant, producing a hysteresis curve of the aerodynamic coefficients over the rotation cycle. It is also shown that a simple quasi-steady correction model can predict the hysteresis effect fairly well. The findings of this investigation will help develop improved models for actuator line methods (ALM).