On the robust autorotation of a samara-inspired rotor in gusty environments

Abstract

Autorotating samaras have evolved to propagate successfully to their germination sites with the help of wind. This wind, in turn, is inherently unsteady across an extensive range of scales in the atmospheric boundary layer. To generate lift, samaras rely on the formation of a stably-attached leading-edge vortex (LEV) on the suction side of their wings. The kinematics of autorotating samaras experiencing gusts were examined experimentally in order to provide insights into the aerodynamic mechanisms responsible for successful propagation. The gust response of seven mature Boxelder Maple (\textit{Acer negundo}) samaras was investigated using a small unsteady wind tunnel able to create vertical gusts. Interestingly, the samaras were found to have a stable tip-speed ratio ($\lambda$) during the gust phase, thus suggesting that the LEV remained stably-attached. Inspired by samaras, we designed a three-bladed rotor that incorporates key aerodynamic and geometric properties of samaras so as to exhibit a stably-attached LEV. The gust response of the samara-inspired rotor was examined using a towing-tank facility. The gust was emulated in the towing tank by accelerating the rotor from an initial steady speed to a final steady speed. Different gust intensities were tested by varying the rotor's normalized inertia number ($I^*$) by systematically increasing the rotor moment of inertia ($I$). Similar to the natural samaras, the rotor exhibited a robust tip-speed ratio during all simulated gusts. The rotor's tip-speed ratio increased by a maximum of 11\% and 6\% during the slowest and fastest simulated gusts, respectively. By maintaining a stable tip-speed ratio during the gust, the samara-inspired rotor is thought to maintain stable LEVs resulting in stable autorotation. Therefore, by learning from the samara-inspired rotor, we suggest that samaras propagate successfully from their parent trees in unsteady (realistic) environments in part due to their robust autorotation properties.