Active flow control for power enhancement of vertical axis wind turbines: Leading-edge slot suction

Vertical axis wind turbines (VAWTs) suffer from a poor power performance at low tip speed ratios, where their blade aerodynamics are dominated by unsteady separation and dynamic stall. Therefore, to enhance their aerodynamic performance, separation control is highly desired. The present study intends to suppress the flow separation on VAWTs using boundary layer suction through a slot located near the blade leading edge. High-fidelity computational fluid dynamics simulations extensively validated with experiments are employed. A characterization of the impact of the suction amplitude, 0.5% ≤ AS ≤ 10%, and the suction location, 8.5 ≤ XS/c ≤ 28.5, is performed. The dependency of the obtained power gain on operating conditions, i.e. tip speed ratio, 2.5 ≤ λ ≤ 3.5, Reynolds number, 0.51 × 105 ≤ Rec ≤ 2.78 × 105, and turbulence intensity, 1% ≤ TI ≤ 25%, is studied. The results show that applying suction along the chordwise extent of the laminar separation bubble (LSB) can prevent its bursting, eliminate/postpone its formation, avoid the formation of the dynamic stall vortex and trailing-edge roll-up vortex, and delay the incipient trailing-edge separation. This will significantly increase the blade lift force, decrease the drag force, delay the stall angle and suppress the aerodynamic load fluctuations. For the reference turbine and for AS = 0.5% and XS/c = 8.5%, the power coefficient at λ of 2.5, 3.0 and 3.5 is enhanced by 247%, 83% and 24%, respectively. The suction location is critical while a minimum amplitude, e.g. AS = 0.5%, suffices. The optimal suction location is insensitive to TI, weakly sensitive to λ while comparatively more sensitive to Rec.