Dynamic Stall and Virtual Camber on High-Solidity Vertical Axis Wind Turbines

The effects of dynamic stall and virtual camber were studied experimentally on 2- and 3-bladed vertical axis wind turbine models, with NACA 0021 blade profiles, corresponding to chord-to-radius ratios of 0.75 and 0.5, respectively, thereby enforcing constant solidity. Open jet wind tunnel measurements were performed in the wind speed range 4.0 m/s to 8.2 m/s, where torque and phase-resolved blade surface pressure measurements were made simultaneously. The 2-bladed average peak torque and power exceeded those of the 3-bladed configuration, by approximately 32%, primarily due to delayed dynamic stall, caused by increased blade pitch rates, which are linearly proportional to the chord-to-radius ratio. However, the unsteady turbine drag loads of the 2-bladed configuration were 92% higher. Virtually all positive torque was developed in the upwind quadrants, consistent with previous studies, and the moments developed around the strut-blade connection point, which are usually neglected, constituted a significant fraction of the total torque developed. Virtual camber was studied by computing the pressure coefficient distributions on a conformally cambered airfoil and then comparing them to measured pressure coefficients at corresponding angles-of-attack. The close correspondence between measurements and calculations represents the first unambiguous experimental confirmation of virtual camber. The strut-blade connection point, between 40% and 60% chord, had a profound effect on the nature of torque generation, but not the cycle-averaged values. A connection point of 50% chord proved to be optimal, because it produced the lowest unsteady and mean, drag and lateral, forces on the turbine.