Performance enhancement of Darrieus wind turbines using Plain Flap and Gurney Flap configurations: A CFD analysis

• URANS simulations show PF (Plain Flap) and GF (Gurney Flap) modifications improve Darrieus VAWT performance under varying wind and low Reynolds number conditions. • PFs boost Cp and Cm by up to 20 %, enhancing torque and reducing negative torque, especially at low TSRs. • The 0.7c, 10° PF achieves consistent 15–20 % torque increases, balancing lift and drag across TSRs. • GFs provide steady torque but have 12–17 % lower efficiency due to drag at higher TSRs. • PFs improve self-starting and are suitable for urban wind applications. Adaptive flaps can further optimize performance. Darrieus vertical axis wind turbines (VAWTs) have limited self-starting torque, especially in urban areas with low and changing wind speeds. This study modifies NACA 0015 aerofoils with Plain Flaps (PF) and Gurney Flaps (GF) to improve aerodynamic performance for urban solar wind tree systems. The k-omega Shear Stress Transport (SST) URANS model was used to simulate different configurations at varied Tip Speed Ratios (TSRs). The 0.6c, 10° PF configuration achieved the highest power coefficient (Cp) of 0.46 at TSR 2.5, a 9.8 % increase over the baseline NACA 0015. Additionally, PFs reduced negative torque and achieved the highest peak average torque across configurations, significantly boosting self-starting capability and torque consistency under low Reynolds number ( Re ) conditions. GF configurations, especially the 0.25c, 120° GF, provided stable torque generation, enhancing performance under variable wind conditions. These findings indicate that optimizing VAWTs for urban use requires balancing peak efficiency with torque stability. Future work should focus on adaptive flap designs capable of real-time optimization, alongside experimental validation.