Enhancing start-up and torque in Darrieus VAWTs through a novel plain gurney flap design

Enhancing aerodynamic efficiency and self-starting torque in Darrieus-type Vertical Axis Wind Turbines (VAWTs) is essential for power generation under low and intermittent wind conditions. This study presents a systematic evaluation of Plain Flap (PF), Gurney Flap (GF), and hybrid Plain Gurney Flap (PGF) modifications applied to the NACA 0015 aerofoil. Two-dimensional Unsteady Reynolds-Averaged Navier-Stokes (URANS) simulations using the SST k-ω model were conducted across a tip-speed-ratio (TSR) range of 0.2-4.5 and Reynolds numbers ((Re = 5.4 × 10⁴-2.7 × 10⁵), with results validated by prototype testing. The optimal 0.5c, 10° PF configuration demonstrated the most robust and stable performance across the full operational range. At low TSR (0.2-0.5), the PF significantly improved mean Cm by 30-40% and Cp by 40% by increasing effective camber. While the 0.015c PGF generated higher instantaneous torque, it incurred larger downwind losses. At moderate TSR (0.8-1.5), PF and PGF achieved similar high Cp gains (40-50%). Beyond TSR = 2.0, the PF maintained superior aerodynamic stability and consistent efficiency, whereas the PGF performance declined due to drag penalties. Prototype testing conclusively confirmed these numerical trends, with PF blades increasing shaft speed by up to 51% at 5.5 m/s compared to the baseline. Overall, PF and PGF configurations effectively mitigate weak self-starting and low efficiency, providing a validated passive-flap design pathway for efficient small-scale and hybrid solar-wind energy systems.