Insights into the flow dynamics and rotor performance of a Savonius turbine with dynamic venting using controllable flaps

Abstract

The drag-driven vertical-axis turbines with semicircular rotor blades, commonly known as Savonius turbines, remain potentially beneficial to extract renewable energy from wind and water streams, especially in terms of their practicality to provide low-cost solutions to rural areas. However, they suffer drawbacks due to the negative torques on their returning blades. We propose a novel solution by dynamically venting out these returning blades using controllable flaps, which retained their omnidirectional capability. Results from unsteady numerical simulations showed that the vented rotor attained a maximum average power coefficient ( CP) of 0.275 at the tip-speed ratio (symbolized as λ) of 0.9, which was 21.7% better than that on the unvented rotor. Furthermore, the proposed dynamically vented blades produced large improvements in the average torque coefficient ( CT), with maximum gains of 38.3% on the returning blade at λ = 0.4 and 24.8% on the full rotor at λ = 1.0, relative to those on the unvented rotor. The controlled dynamic venting was beneficial because it modified the pressure distributions surrounding the returning blade and changed the flow structure downstream of the vented blade that improved the torque on the subsequent blade entering the returning side. Elucidation on the flow dynamics revealed that the flows were vented outward through the flap aperture onto the convex side of the returning blade, rather than inward into its concave side, as initially hypothesized.