Experimental investigation of an optimized airfoil for vertical‐axis wind turbines

The present study addresses the experimental verification of the performance of a new airfoil design for lift-driven vertical-axis wind turbines (VAWTs). The airfoil is obtained through a genetic-algorithm optimization of an objective function which maximizes the aerodynamic performance of airfoils having a larger thickness, providing better structural stiffness compared to more slender NACA design. The work presents an experimental analysis of such improved performance of a 26% thick VAWT-optimized airfoil (DU12W262). The 2D flow velocity, pressure and aerodynamic loads are measured by combined use of particle image velocimetry, wall-pressure sensors and wake rakes. Additionally, the airfoil surface pressure is determined by integrating the pressure equation from the experimental velocity field. Results are initially obtained with the airfoil in steady conditions, at Reynolds 3.5*105, 7.0*105 and 1.0*106 with both free and forced (1%c) boundary layer transition. Xfoil simulations are employed for comparison with the experimental results, showing a good agreement in the linear range of angle of attack and a consistent lift/drag overestimation in the separated one. The airfoil performance is further assessed under pitching conditions (oscillatory, ramp up), at Reynolds 7.0*105 with reduced frequencies ranging from 0.07 to 0.11 and the aerodynamic load behaviour compared with the steady case. The experimental data are used as input for a numerical simulation of a 2D VAWT; while the performance are compared with those for NACA 4-series airfoils commonly used for VAWTs, showing a significantly higher maximum power coefficient for the optimized airfoil. All the data presented in the manuscript are made available in the Supporting information. Copyright © 2014 John Wiley & Sons, Ltd.