Investigation of a composite flow control technique for vertical axis wind turbine: A slotted blade integrated with the active flap control

Slotting is a commonly used passive flow control technique that can improve the aerodynamic performance of vertical axis wind turbines (VAWTs) under low tip speed ratio (TSR) conditions. However, conventional slotted blades often cause flow separation at medium-to-high TSRs, reducing overall efficiency. To address this, a composite control mechanism integrating a slot with a dynamically actuated flap is proposed. By lifting or retracting the flap, the slot state is adaptively adjusted, while the flap also suppresses flow separation. Two-dimensional simulations combined with orthogonal experimental design were used to determine optimal slot-flap (OSF) parameters. A flap control strategy was then developed, enabling dynamic actuation. Quasi three-dimensional simulations were performed to validate the aerodynamic performance of the OSF configuration. Results show that across the investigated TSR range (1.43 ∼ 3.29), the OSF-VAWT consistently outperforms baseline, conventional slotted, and flap-only models in power coefficient and torque output. Flow field analyses reveal that these improvements stem from three synergistic mechanisms: effective camber modification at stall onset, vortex fragmentation during deep stall, and wake stabilization via the history effect during recovery. This study demonstrates that the slot-flap mechanism effectively overcomes the limitations of traditional passive control methods and provides a potential design strategy for improving VAWT blade performance.