Conformally Decambered Natural Laminar Flow Blades for Vertical Axis Wind Turbines

Abstract

A two-element natural laminar flow (NLF) airfoil – commonly used on medium-altitude, long-endurance, unmanned air vehicles – was conformally decambered for application on a two-bladed, H-rotor, vertical axis wind turbine. Blade kinematics were used to determine the virtual camber-line, which was then used to conformally map the original profile onto the chord-line, thereby virtually representing the original profile. Both decambered and original blade profiles were evaluated experimentally using large chord-radius ratios (0.6 and 0.75) that exploit the phenomenon of dynamic stall to produce the driving torque. Decambered blades showed substantially greater power and torque coefficients than the original blades, up to 60% and 27% respectively, which represents the first experimental validation of conformal decambering. Relatively large peak power coefficients of 28% were attained, despite maximum chord-based Reynolds numbers being less than 2x10⁵.Depending upon the chord-radius ratio, light or deep dynamic stall occurs in the second upstream quadrant, and the flap flow remains attached virtually throughout. In contrast, on the original profiles, massive separation was observed on the blades, and the flap flow remained separated due to outer surface flow separation. Future research should consider surface pressure and flowfield measurements, significantly higher Reynolds numbers, and variable intra-cycle flap deflection mechanisms should be implemented to optimize performance and minimize unsteady loads.