Birch James M, Dickson William B, Dickinson Michael H
Department of Integrative Biology, University of California, Berkeley, CA 94720, USA.
J Exp Biol. 2004 Mar;207(Pt 7):1063-72. doi: 10.1242/jeb.00848.
The elevated aerodynamic performance of insects has been attributed in part to the generation and maintenance of a stable region of vorticity known as the leading edge vortex (LEV). One explanation for the stability of the LEV is that spiraling axial flow within the vortex core drains energy into the tip vortex, forming a leading-edge spiral vortex analogous to the flow structure generated by delta wing aircraft. However, whereas spiral flow is a conspicuous feature of flapping wings at Reynolds numbers (Re) of 5000, similar experiments at Re=100 failed to identify a comparable structure. We used a dynamically scaled robot to investigate both the forces and the flows created by a wing undergoing identical motion at Re of approximately 120 and approximately 1400. In both cases, motion at constant angular velocity and fixed angle of attack generated a stable LEV with no evidence of shedding. At Re=1400, flow visualization indicated an intense narrow region of spanwise flow within the core of the LEV, a feature conspicuously absent at Re=120. The results suggest that the transport of vorticity from the leading edge to the wake that permits prolonged vortex attachment takes different forms at different Re.
昆虫卓越的空气动力学性能部分归因于一种被称为前缘涡（LEV）的稳定涡度区域的产生和维持。对LEV稳定性的一种解释是，涡核内的螺旋轴向流将能量排入梢涡，形成一个类似于三角翼飞机产生的流动结构的前缘螺旋涡。然而，虽然螺旋流是雷诺数（Re）为5000时扑翼的一个显著特征，但在Re = 100时进行的类似实验未能识别出类似的结构。我们使用一个动态缩放的机器人来研究在Re约为120和约1400时，经历相同运动的翅膀所产生的力和流动。在这两种情况下，以恒定角速度和固定攻角运动都会产生一个稳定的LEV，没有脱落的迹象。在Re = 1400时，流动可视化显示在LEV的核心内有一个强烈的窄展向流区域，这一特征在Re = 120时明显不存在。结果表明，在不同的Re下，从前缘到尾流的涡度传输（这使得涡能够长时间附着）采取不同的形式。
