Comparative Study of Thin and Thick Symmetric Airfoils with Plain, Gurney, and Hybrid Plain-Gurney Flaps for Low-Re Darrieus VAWT Performance

The rising demand for decentralised renewable energy has increased interest in hybrid solar-wind systems for rural low-wind regions. Small Darrieus VAWTs are suitable for such settings but suffer from low-Reynolds-number (Re) limitations and poor self-starting. This study investigates how passive trailing-edge flap (TEF) configurations affect the aerodynamic performance of thin and thick symmetric aerofoils, focusing on NACA 0015 and NACA 0021. Three TEF designs are examined: plain flaps (PF), Gurney flaps (GF), and hybrid plain-Gurney flaps (PGF). A systematic comparison evaluates their individual and combined effects on lift enhancement, torque generation, stall behaviour, and dynamic response under low-Re conditions. 2D unsteady CFD simulations using URANS with the SST k-ω model are performed across Re = 5.4 × 10 4 -2.7 × 10 5 and tip speed ratios (TSR) = 0.2-4.5. PFs placed between 0.4c and 0.6c increase lift, delay stall, and improve torque, with the 0.5c, 10° configuration offering the best balance. The 0.015c hybrid PGF yields the highest gains, boosting power coefficient (C p ) by 53% for NACA 0015 and reaching C p = 0.393 for NACA 0021 at TSR 1.5. It also enhances low-TSR torque, improving self-starting. Prototype PF testing at 4.5-5.5 m/s confirms performance improvements through rotor-speed measurements.