Large eddy simulation of the effect of blade rotation on laminar separation bubbles in horizontal axis wind turbines

The formation and evolution of laminar separation bubbles on a horizontal axis wind turbine blade in the transitional flow regime are investigated using Large Eddy Simulations. Both rotating and translating cases of a blade element, based on a small horizontal axis wind turbine, are analyzed to distinguish the specific aerodynamic effects introduced by rotation. The results demonstrated that in the rotating case, the roll-up vortices developed through Kelvin-Helmholtz instability appear shorter, inclined at varying angles along the span, and break down at various locations along the span, in contrast to the continuous two-dimensional structures of the Kelvin-Helmholtz vortices observed in the translating case. Furthermore, in the rotating case, the Coriolis force induces a stabilization of the boundary layer by enhancing momentum transfer, promoting an earlier transition to turbulence and facilitating a rapid reattachment of the flow. Centrifugal force drives radially outward flow, displacing the bubble laterally, restricting its growth and limiting its extent along the blade surface. These effects result in a 66.4% thinner bubble, leading to a 26.7% reduction in lift, and a 36.3% reduction in drag compared to the translating case. These results demonstrate that, unlike large wind turbines, rotation in small horizontal axis wind turbines can degrade lift performance due to reduced effective curvature, despite improving aerodynamic efficiency due to the accompanied drag reduction. This study provides insights into laminar separation bubble behavior under rotation and contributes to a better understanding of the physics, aiding investigations and improvements in performance prediction models for the transitional flow regime.