Towards enhanced startup performance of Darrieus vertical-axis wind turbines: Key flow features at moderate tip speed ratios

The self-starting capability of Darrieus vertical-axis wind turbines (VAWTs) remains a critical challenge, limiting their ability to reach optimal tip speed ratios (TSRs) without external assistance. While previous studies have explored startup aerodynamics, the underlying flow mechanisms governing torque generation at moderate TSRs remain insufficiently understood. This study employs high-fidelity three-dimensional CFD simulations to systematically investigate the aerodynamic behavior of a two-bladed straight Darrieus turbine across a wide TSR range (0.25–4.0). The analysis focuses on the VAWT-X blade profile, a patented design, alongside other widely used profiles, including DU 06-W-200 variants, to assess their impact on startup performance. The RANS equations with the SST k-ω turbulence model are used to capture the unsteady flow physics, including blade-vortex interactions and dynamic stall. The results reveal that leading-edge vortex formation and trailing-edge vortex shedding significantly influence torque production. The VAWT-X profile demonstrates improved vortex control compared to the DU 06-W-200 variant, enhancing average torque by 34% at TSR 0.5. Two primary mechanisms govern downwind torque generation: (i) blade-vortex interaction and (ii) blade recovery during downstroke motion. These findings provide critical insights into the aerodynamic design of next-generation VAWTs, offering optimized strategies for improved startup performance while maintaining efficiency at higher TSRs.