Department of Mechanical and Materials Engineering, Queen's University, Kingston, ON, K7L 3N6, Canada.
Center for Turbulence Control, Harbin Institute of Technology, Shenzhen 518055, China.
Bioinspir Biomim. 2022 May 17;17(4). doi: 10.1088/1748-3190/ac68bb.
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 () 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 () 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 (*) by systematically increasing the rotor moment of inertia (). 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.
具有旋转能力的翅果在风的帮助下进化到可以成功传播到其萌发地。反过来，这种风在大气边界层的广泛尺度内本质上是不稳定的。为了产生升力，翅果依靠在其机翼的吸力侧形成一个稳定附着的前缘涡（LEV）。为了深入了解导致成功传播的空气动力学机制，实验研究了经历阵风的旋转翅果的运动学。使用能够产生垂直阵风的小型非定常风洞研究了 7 个成熟的白蜡枫（）翅果的阵风响应。有趣的是，发现翅果在阵风阶段具有稳定的尖速比（），因此表明 LEV 仍然稳定附着。受翅果启发，我们设计了一个三叶转子，它包含翅果的关键空气动力学和几何特性，以表现出稳定附着的 LEV。使用拖曳水槽设施研究了受翅果启发的转子的阵风响应。通过从初始稳定速度加速到最终稳定速度来在拖曳水槽中模拟阵风。通过系统地增加转子转动惯量（）来改变转子的归一化惯性数（*），从而测试不同的阵风强度。与天然翅果类似，转子在所有模拟阵风下都表现出稳健的尖速比。转子的尖速比在最慢和最快的模拟阵风期间分别增加了最大 11%和 6%。通过在阵风期间保持稳定的尖速比，受翅果启发的转子被认为保持稳定的 LEV，从而保持稳定的自转。因此，通过从受翅果启发的转子中学习，我们认为翅果之所以能够在不稳定（现实）的环境中从其母体树成功传播，部分原因是它们具有稳健的自转特性。
