3D Aerodynamic Effects and Circulation Control of Vertical Axis Wind Turbines

Abstract

The vertical axis wind turbine (VAWT) concept in its current, fixed-pitch iteration is hindered by lower aerodynamic efficiency compared to existing horizontal axis wind turbines. Implementing a form of circulation control on the VAWT by actively pitching the blades throughout the turbine’s rotation could potentially enhance power capture efficiency and reduce loads variability. In this thesis, circulation control is investigated in 3D, since the 3D flow field differs significantly from the 2D equivalent.After a review of available 2D and 3D aerodynamic models for the VAWT, a suitable candidate emerges in the form of HAWC2 NW, which is a lifting-line based extension to popular aeroelastic modelling suite HAWC2 designed to model trailed vorticity in the near wake of the blade. HAWC2 NW is used to conduct fast 3D aerodynamic simulations of a highly simplified vertical axis wind turbine geometry to demonstrate the influence of 3D effects on the flow field surrounding the VAWT. The slenderness of the turbine geometry, expressed using the turbine aspect ratio H/D, turns out to be a major factor in determining the performance losses due to 3D effects.Subsequently, HAWC2 NW is used to analyze optimized pitch sequences with various objectives, such as maximum power or a certain normal loading distribution. A relationship is found between the ratio of upwind and downwind normal loading, the turbine aspect ratio H/D and the 3D power coefficient CP,3D. The most important conclusion is that when considering the design of a vertical axis wind turbine with a turbine aspect ratio H/D < 5, losses due to 3D effects can be minimized by implementing a circulation control strategy in such a way that the normal loading is uniformly distributed between the upwind and downwind half-cycles.