Control mechanism of unsteady flow separation on vertical axis wind turbine blades via double-slotted flap

Aiming at the core issue of flow separation on vertical-axis wind turbine (VAWT) blades under high angles of attack during periodic rotation—which degrades aerodynamic performance and energy capture efficiency—this paper proposes a VAWT blade configuration equipped with double-slotted flaps, drawing on mature high-lift technology from aircraft wings. Numerical simulations are conducted using the Unsteady Reynolds-Averaged Navier–Stokes method with the Shear Stress Transport k–ω turbulence model, covering static aerodynamic characteristics of a single airfoil as well as two-dimensional and three-dimensional full-turbine models. Optimal design parameters of the double-slotted flaps are identified via the control variable method, and the aerodynamic performance and flow-field characteristics of passive vs active controlled double-slotted flap VAWTs are compared. The underlying flow mechanism of unsteady separation control is revealed: flap deflection increases airfoil camber, while high-pressure airflow from the pressure side is injected through the primary and secondary slots into the suction-side boundary layer, re-energizing the stalling boundary layer, and thereby delaying dynamic stall and flow separation. Results show that the optimally configured single airfoil achieves a significantly improved lift-to-drag ratio and delayed flow separation within the 0°–20° angle-of-attack range. Under active control, the power coefficient of the double-slotted flap VAWT at a tip-speed ratio λ = 2.33 is 32% higher than that of the reference wind turbine. Its wake is laterally wider with an extension length reduced to 7R, and both the intensity and spatial extent of vortex shedding on blade surfaces in the three-dimensional flow field are effectively suppressed. This study elucidates the inherent mechanism by which double-slotted flaps enhance VAWT aerodynamic performance—namely, through active unsteady separation control leading to energy capture improvement—and provides theoretical guidance for small vertical-axis wind turbines in distributed and building-integrated wind energy applications.