Aerodynamic performance of variable pitch vertical axis wind turbines at low TSR: a transient URANS investigation

• Variable pitch control enhances low-TSR aerodynamic performance. • Phase-resolved torque analysis reveals stall mitigation mechanisms. • Variable pitch maintains near-optimal angle of attack throughout rotor rotation. • Variable pitch reduces negative torque intervals in stall-dominated regimes. • Increased solidity limits pitch authority and aerodynamic gains. Vertical Axis Wind Turbines (VAWTs) are increasingly considered for distributed renewable energy generation in built environments, where wind direction variability and low wind speeds challenge conventional horizontal-axis turbines. This study investigates the aerodynamic performance of VAWTs equipped with variable pitch control for improving energy capture under low tip speed ratio (TSR) operation. Transient URANS simulations were conducted using ANSYS Fluent with the SST k–ω turbulence model and validated against a Double Multiple Stream Tube (DMST) model to ensure predictive reliability. The analysis examines the coupled effects of blade count (1–4 blades) and TSR on torque production, power coefficient, and unsteady flow phenomena such as dynamic stall and blade–wake interaction within a two-dimensional framework. Results indicate that variable pitch control significantly enhances aerodynamic performance at low TSR. The single-bladed configuration achieved a relative increase in power coefficient of up to 885.6% at TSR = 2 compared to fixed-pitch operation, while the two-bladed turbine reached a maximum power coefficient of 0.322 at TSR = 3.5, representing more than a 50% improvement over its fixed-pitch counterpart. However, the aerodynamic benefits diminish with increasing blade count due to reduced pitch authority and intensified blade–wake interaction. The findings identify aerodynamic trade-offs between blade number and pitch effectiveness under controlled inflow conditions and provide insight into the aerodynamic potential of variable pitch control in low-TSR operation. Application to realistic urban wind environments would require further investigation incorporating atmospheric boundary-layer and three-dimensional effects