High-resolution numerical investigation into the effects of winglet on the aerodynamic performance for a three-dimensional vertical axis wind turbine

Vertical axis wind turbines can harvest kinetic energy in the air stream from all directions and therefore have distinct advantages in the effective utilization of wind resource. Its aerodynamic performance highly depends on the blade geometrical configuration, especially on the tip shapes due to the negative effects of the shedding vortices. In this paper, spatially high-resolution numerical methods are adopted to study the evolution and mutual interaction process for the vortex system formed behind a vertical axis wind turbine with and without blade tip winglet, aiming at further understand the mechanism of the winglet effects on the turbine aerodynamic performance. For the cases without tip winglet, it has been shown that the high-resolution methods can properly capture the evolution and interaction of the omega-shaped vortices system and revealed the differences of the vortex mutual induction in the near wake region under three tip speed ratios. Then, the region in spanwise dominated by the effects of tip vortex is determined by proper orthogonal decomposition survey on the flow field of vertical profiles and based on which the winglet is installed hoping to maximize its three-dimensional effects on the spanwise flow field. The comparison of power coefficients for cases with and without winglet under the same swept area show that the winglet has significantly improved the effectiveness of energy harvesting in the studied tip speed ratios. Finally, the analysis of tip vortex circulation evolution demonstrates that the enhancement of power coefficient for the winglet case comes from the reduction of the spanwise region affected by tip vortex, instead of the strength decrease of the tip vortex itself. The conclusions can further reveal the flow mechanism of winglet effects on the vertical axis wind turbine aerodynamic performance, and therefore are helpful for guiding the optimized blade winglet design to achieve higher efficiency.