Unsteady power coefficient evolution in a Savonius wind turbine

Wind energy is an attractive solution for small-scale and urban applications, particularly in low-wind environments where horizontal-axis turbines are less efficient. In this context, drag-based vertical axis wind turbines (VAWTs), especially the Savonius rotor, offer advantages such as simple construction, low cost, omnidirectional operation, and strong self-starting capability. However, their relatively low aerodynamic efficiency requires accurate performance evaluation. This study analyzes the evolution of the power coefficient (Cp) over successive rotor revolutions at different tip speed ratios (TSR) using unsteady numerical simulations. The objective is to distinguish transient startup effects from steady state behavior. Results indicate the presence of an initial transient phase, during which Cp progressively increases as the flow develops around the rotor. After several revolutions, the turbine reaches a quasi-steady regime where Cp variations become minimal and periodic. These findings demonstrate that reliable performance assessment must be conducted only after transient effects have dissipated. Considering unsteady aerodynamic behavior is therefore essential for accurate evaluation and effective optimization of Savonius rotor performance.