Dragonfly flight:: free-flight and tethered flow visualizations reveal a diverse array of unsteady lift-generating mechanisms, controlled primarily via angle of attack
Dragonfly flight:: free-flight and tethered flow visualizations reveal a diverse array of unsteady lift-generating mechanisms, controlled primarily via angle of attack
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DOI：
10.1242/jeb.01262
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发表时间：
2004-11-01
影响因子：
2.8
通讯作者：
Bomphrey, RJ
中科院分区：
文献类型：
--
作者：
Thomas, ALR;Taylor, GK;Bomphrey, RJ
在这里，我们表明，定性自由和系绳飞行流动可视化，蜻蜓飞使用非定常空气动力机制，以产生高升力，前缘涡流。在正常的自由飞行中，蜻蜓使用逆流运动学，在前翼下冲程上有前缘涡（LEV），在前翼上冲程上有附着流，在后翼上始终有附着流。加速的蜻蜓切换到同相的翼拍，具有高度分离的下冲程流，单个LEV连接在前后翼上。我们使用烟雾可视化来区分三个最简单的局部解析解的Navier-Stokes方程产生流分离导致的LEV。LEV是一个开放的U形分离，连续穿过胸部，平行于机翼前缘运行，并在尖端弯曲，形成翼尖涡流。随着LEV的增长，空气螺旋进入中心线上方的自由滑动临界点。展向流不是流场的主要特征-展向流有时从翼尖流向中心线，或相反，这取决于侧滑的程度。LEV的形成总是与迎角的快速增加相一致，并且烟雾可视化清楚地显示了每当迎角快速增加时LEV的形成。没有离散的起始涡流。相反，当机翼处于非零迎角时，在后缘后面形成剪切层，并在开尔文-亥姆霍兹不稳定性下卷起成一系列横向涡，其环流与机翼和LEV周围的环流符号相反。蜻蜓产生的流场与蜻蜓、果蝇和天蛾的机械模型不同，这排除了自然的翅膀相互作用。然而，受控参数实验表明，只要Strouhal数合适，并且左右翅膀之间可以发生自然的相互作用，即使是一个简单的俯冲板也可以重现蜻蜓中所见的流动的详细特征。在我们的模型中，在蜻蜓中，它似乎是通过一个通用的机制，从而扑动运动学配置，使一个LEV将有望形成自然的翅膀，并保持连接的中风的持续时间来实现的LEV的稳定。然而，LEV的实际形成和脱落是由机翼迎角控制的，蜻蜓可以通过两个极端变化，从零到导致在机翼行程期间的任何时间立即流动分离的范围。
Here we show, by qualitative free- and tethered-flight flow visualization, that dragonflies fly by using unsteady aerodynamic mechanisms to generate high-lift, leading-edge vortices. In normal free flight, dragonflies use counterstroking kinematics, with a leading-edge vortex (LEV) on the forewing downstroke, attached flow on the forewing upstroke, and attached flow on the hindwing throughout. Accelerating dragonflies switch to in-phase wing-beats with highly separated downstroke flows, with a single LEV attached across both the fore- and hindwings. We use smoke visualizations to distinguish between the three simplest local analytical solutions of the Navier-Stokes equations yielding flow separation resulting in a LEV. The LEV is an open U-shaped separation, continuous across the thorax, running parallel to the wing leading edge and inflecting at the tips to form wingtip vortices. Air spirals in to a free-slip critical point over the centreline as the LEV grows. Spanwise flow is not a dominant feature of the flow field - spanwise flows sometimes run from wingtip to centreline, or vice versa depending on the degree of sideslip. LEV formation always coincides with rapid increases in angle of attack, and the smoke visualizations clearly show the formation of LEVs whenever a rapid increase in angle of attack occurs. There is no discrete starting vortex. Instead, a shear layer forms behind the trailing edge whenever the wing is at a non-zero angle of attack, and rolls up, under Kelvin-Helmholtz instability, into a series of transverse vortices with circulation of opposite sign to the circulation around the wing and LEV. The flow fields produced by dragonflies differ qualitatively from those published for mechanical models of dragonflies, fruitflies and hawkmoths, which preclude natural wing interactions. However, controlled parametric experiments show that, provided the Strouhal number is appropriate and the natural interaction between left and right wings can occur, even a simple plunging plate can reproduce the detailed features of the flow seen in dragonflies. In our models, and in dragonflies, it appears that stability of the LEV is achieved by a general mechanism whereby flapping kinematics are configured so that a LEV would be expected to form naturally over the wing and remain attached for the duration of the stroke. However, the actual formation and shedding of the LEV is controlled by wing angle of attack, which dragonflies can vary through both extremes, from zero up to a range that leads to immediate flow separation at any time during a wing stroke.