Self-starting enhancement of an H-Darrieus VAWT using square and triangular surface dimples

Vertical-axis wind turbines (VAWTs) are promising for decentralized and urban wind-energy applications because of their simple structure, omnidirectional operation, and suitability for low-wind-speed environments. Among VAWTs, H-Darrieus turbines offer favorable aerodynamic efficiency but suffer from poor self-starting capability, particularly under low-wind-speed conditions. The present study experimentally investigates the use of surface dimples as a passive flow-control technique to improve the self-starting and aerodynamic performance of an H-Darrieus VAWT. Square and triangular dimples were incorporated and arranged in single, double, and triple-dimple configurations at 0.25C, 0.50C, and 0.75C on NACA 4412 blades and tested in a wind tunnel at 4.16, 5.07, and 6.11 m/s. The performance parameters such as static torque (C _ts ), transient tip-speed ratio, start-up time, power coefficient (C _p ), and dynamic torque coefficient (C _t ) were evaluated. The square single-dimple configuration at 0.25C provided the best overall performance, reducing the start-up time from 86 to 18 s and the cut-in speed from 5.5 to 4.5 m/s, while achieving the maximum improvement attained in C _ts is 26.49% at 4.16 m/s. The results demonstrate that dimple geometry and chordwise placement strongly influence VAWT self-starting and aerodynamic performance, with the leading-edge square dimple providing an effective passive modification for low-wind-speed environments.