On the robust autorotation and LEV stability of samaras and samara-inspired rotors

Autorotating samaras have evolved to propagate successfully to their germination sites with the help of the unsteady wind. To generate lift, samaras rely on forming a stable leading-edge vortex (LEV) attached to the suction side of their wings. The gust response of mature Boxelder Maple (Acer negundo) samaras was examined using a small unsteady wind tunnel to create vertical gusts. Interestingly, the samaras were found to exhibit a stable tip-speed ratio during the gust. Inspired by samaras, we designed a three-bladed rotor that incorporates the key aerodynamic and geometric properties of samaras to exhibit a stably-attached LEV. The gust response of the samara-inspired rotor was then 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. Furthermore, the flow around the blades was examined using time-resolved PIV. Similar to natural samaras, the rotor exhibited a robust tip-speed ratio during the gust. For instance, the rotor’s tip-speed ratio increased by a maximum of 11% and 6% during the fastest and slowest tested gusts, respectively. Additionally, the samara-inspired rotor exhibited a stably-attached LEV during the gust, suggesting that LEV stability is coupled with stable autorotation during gusts.