Design Implications of Chord Length and Number of Blades on Self-Starting Process in Vertical-Axis Wind Turbines

Abstract

Self-starting remains a key limitation of lift-driven vertical-axis wind turbines and is strongly influenced by geometric design choices that also govern steady-state performance. This work quantifies the roles of chord length and blade number on startup dynamics and the attained steady tip-speed ratio using two-dimensional URANS simulations of freely rotating Darrieus-type rotors. Two configuration families are examined, an equal-chord set in which three and five bladed turbines share the same chord length, and an equal-solidity set in which the chord length is reduced for the five blade turbines to match solidity with the three blade counterparts. Results are analyzed using the time evolution of tip-speed ratio, reduced-frequency measures to identify sustained unsteady intervals, vorticity-field diagnostics of dynamic stall vortex formation and detachment, and a torque decomposition into pressure and viscous moments. The results show that increasing number of blades can enhance early stage acceleration but generally lowers the steady tip-speed ratio by intensifying blade-vortex interaction in the downstream half cycle. Increasing chord length promotes self-starting by strengthening unsteady loading during the transition out of the low-speed regime, but also increases viscous losses and wake interaction, leading to lower the steady tip-speed ratio for self-starting high chord configurations. The role of viscous moments is also analyzed to quantify their contribution to self-starting behavior and to assess their influence on limiting the attainable operating state after self-starting. These findings provide design-relevant guidance on the startup--performance trade-off associated with the chord length and number of blades in freely accelerating vertical-axis wind turbines.