Three-dimensional vortical structures of a vertical-axis wind turbine and their effects on the aerodynamic performance and wake*

We conduct large eddy simulation of flow past a vertical-axis wind turbine (VAWT) to investigate the flow structures around the blades affecting aerodynamic performance and wake. At the Reynolds number of 80,000 based on the free-stream velocity and turbine diameter, we consider the tip-speed ratios of 1.0, 1.2, and 1.7. We compare the flow fields with those from a VAWT with infinite span blades and examine the three-dimensional flow structures at the blade tips. Dynamic stall and large leading-edge vortices (LEVs) develop on the inner blade surfaces during rotation, while strong blade tip vortices (BTVs) form at the blade tips. As the tip-speed ratio increases, the strength of LEV decreases and that of BTV increases. Near the blade tips, the formation of LEV is suppressed by BTVs. The power generation near the tips decreases, but the area affected by BTVs reduces with increasing tip-speed ratio. In the wake region, the tip-speed ratio of 1.7 has broader streamwise velocity deficit transversely but wake recovery is faster than those at lower tip-speed ratios.