School of Mechanical and Aerospace Engineering, Oklahoma State University, Stillwater, OK 74078, United States of America.
Bioinspir Biomim. 2021 Jul 26;16(5). doi: 10.1088/1748-3190/ac050a.
The smallest flying insects, such as thrips (body length < 2 mm), are challenged with needing to move in air at a chord-based Reynolds number (Re) of the order of 10. Pronounced viscous dissipation at such a low Rerequires considerable energetic expenditure for tiny insects to stay aloft. Thrips flap their densely bristled wings at large stroke amplitudes, bringing both wings in close proximity to each other at the end of upstroke ('clap') and moving their wings apart at the start of downstroke ('fling'). From high-speed videos of free take-off flights of thrips, we observed that their forewings remain clapped for approximately 10% of the wingbeat cycle before the start of downstroke (fling stroke). We sought to examine if there are aerodynamic advantages associated with pausing wing motion after upstroke (clap stroke) and before downstroke (fling stroke) at Re= 10. A dynamically scaled robotic clap and fling platform was used to measure lift and drag forces generated by physical models of solid (non-bristled) and bristled wings in single wing and wing pair configurations, for pause times ranging between 0% to 41% of the cycle. For solid and bristled wing pairs, pausing before the start of downstroke (fling stroke) dissipated vorticity generated at the end of upstroke (clap stroke). This resulted in a decrease in the drag coefficient averaged across downstroke (fling stroke) and in turn reduced power requirements. Also, increasing the pause time resulted in a larger decrease in the dimensionless power coefficient for the wing-pair configurations compared to the single-wing configurations. Our findings show that wing-wing interaction observed in the clap and fling motion of tiny insect wings is necessary to realize the aerodynamic benefits of pausing before fling, by reducing the power required to clap and fling for a small compromise in lift.
最小的飞行昆虫，如蓟马（体长 < 2 毫米），在以弦为基础的雷诺数（Re）为 10 左右的空气中移动时面临挑战。在如此低的 Re 下，明显的粘性耗散需要微小的昆虫付出相当大的能量才能保持在空中。蓟马以大冲程幅度拍打其长满刚毛的翅膀，使两个翅膀在上升冲程结束时（拍打）非常接近，然后在下降冲程开始时（甩动）分开。从蓟马自由起飞的高速视频中，我们观察到它们的前翅在开始下降冲程（甩动冲程）之前，拍打冲程大约持续了翅膀拍动周期的 10%。我们试图研究在 Re=10 时，在上冲程（拍打冲程）结束后和下冲程（甩动冲程）开始前暂停翅膀运动是否存在气动优势。一个动态缩放的机器人拍打和甩动平台用于测量物理模型的升力和阻力，这些模型为固体（无刚毛）和刚毛翅膀，在单翼和双翼配置中，暂停时间范围为 0%到 41%的周期。对于固体和刚毛双翼，在下冲程（甩动冲程）开始前暂停可以耗散在上冲程（拍打冲程）结束时产生的涡旋。这导致跨下冲程（甩动冲程）的阻力系数平均减小，从而降低了功率要求。此外，与单翼配置相比，增加暂停时间会导致翼对配置的无量纲功率系数下降更大。我们的研究结果表明，在微小昆虫翅膀的拍打和甩动运动中观察到的翅膀-翅膀相互作用，通过减少拍打和甩动所需的功率，以较小的升力损失为代价，是实现甩动前暂停的气动优势所必需的。
