Flow characteristics of aerial swimmers: flapping wing propulsion of tiny insects flying at extremely low Reynolds numbers

研究复杂生物体中力产生的流体动力学过程对于深入了解飞行和游泳动物的运动至关重要。在过去的十年中，昆虫和鸟类产生升力的不寻常和强大的机制吸引了许多生物流体动力学领域的研究人员。大多数关于昆虫空气动力学的研究都认为动物的翅膀是简单的，大部分是刚性的平板，在大约200到10,000之间的高雷诺数下拍打。相比之下，翼展小于一毫米的小昆虫的飞行被排除在这一分析之外，因为小昆虫完全放弃了机翼的作用，而是在空中游泳。此外，与传统的昆虫和鸟类相比，小型昆虫表现出不同寻常的梳状平台翅膀，高达80%的翅膀表面被长毛所取代。我们对刚毛翼对爬行流推进的空气动力学影响的理解存在明显的差距，包括基于阻力的推进系统如何产生垂直升力以支撑体重的问题。因此，这一建议涉及在雷诺数低于大约10的情况下飞行的最小昆虫的流体动力学现象。我们的目标是量化刚毛翼飞行器的流动结构，用高速视频技术记录翅膀的运动学，用时间分辨数字粒子图像测速（TRPIV）技术量化流动结构。利用机器人翅膀，我们将确定刚毛翼设计在尾迹结构和力产生方面的功能相关性，随后将扑动模型翅膀的流动结构与从飞行昆虫中获得的数据进行比较。这项工作的结果可能会给我们提供一些关于动物运动的新见解，即通过估计小型生物飞行系统中的边界层结构和粘性阻力推进，并通过提供飞行的气动关键值，可用于确定高粘性环境中的动力消耗和飞行效率。

Investigating the fluid dynamic processes of force production in complex organisms is central for any in-depth understanding of locomotion in flying and swimming animals. Within the past decade, the unusual and potent mechanisms of lift production in insects and birds have attracted numerous researchers in the field of biological fluid dynamics. Most studies on insect aerodynamics considered animal wings as simple, mostly rigid, flat plates, flapping at high Reynolds numbers between approximately 200 and 10,000. Flight of small insects with wingspans of less than a millimetre, by contrast, had been excluded from this analysis because tiny insects have abandoned altogether airfoil action and literally swim in the air. In contrast to conventional insects and birds, moreover, small insects exhibit unusual wings of comb-like planform, where up to 80% of the wing surface is replaced by long hairs. There is a pronounced gap in our understanding of the aerodynamic consequences of bristle wings for creeping flow propulsion including the question of how drag-based propulsion systems may generate vertical lift for body weight support. This proposal is thus concerned with the fluid dynamic phenomenon in smallest insects flying at Reynolds numbers below approximately 10. We aim to quantify flow structures of bristle wing fliers, scoring wing kinematics by high-speed video technique and quantifying flow structures by time-resolved digital particle image velocimetry (TRPIV). Employing robotic wings, we will determine the functional relevance of bristle wing design on both wake structure and force generation, and subsequently compare flow structures at flapping model wings with the data obtained from the flying insect. The results of this work might give us several new insights into animal locomotion, i.e. by estimating boundary layer structure and viscous drag propulsion in a miniaturized biological flight system and by providing aerodynamic key values for flight that may be used to determine power expenditures and flight efficiency in highly viscous environments.