Aerodynamic performance and robustness of a nature-inspired concept for a micro-scale wind turbine

We present direct numerical simulations of a novel concept for a micro-scale wind turbine, inspired in the mechanics of the auto-rotation of winged seeds. In this nature-inspired concept the turbine blades have two degrees of freedom: the pitch and the elevation (or coning) angles. These allow the blade to vary its attitude with respect to the incoming velocity seen by the blade (i.e., the tip-speed ratio, $\lambda$). In order to validate this new concept, we perform numerical simulations of the coupled fluid-solid problem, solving together the Navier-Stokes equations for the fluid and the Newton equations for the rigid body (i.e., the blade). We characterize a preliminary nature-inspired single-blade rotor over a range of operational conditions (including both uniform and turbulent inflows), demonstrating the ability of the novel rotor to extract power at a very low Reynolds number (i.e., $Re=240$ based on the blade's chord and the freestream velocity), significantly changing its attitude in response to different braking torques and tip-speed ratios. The rotor achieves a peak power coefficient of $C_{P,\max} = 0.026$ at $\lambda \approx 2.0$. This peak value is unchanged between uniform and turbulence-perturbed inflows, demonstrating the robustness of the nature-inspired design. However, performance remains lower than that of fixed-blade configurations, showing that while the concept is feasible and stable, optimization of blade planform and mass distribution is essential to improve efficiency.