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首页> 外文期刊>The Journal of Experimental Biology >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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Dragonfly flight: free-flight and tethered flow visualizations reveal a diverse array of unsteady lift-generating mechanisms, controlled primarily via angle of attack

机译：蜻蜓飞行：自由飞行和束缚流可视化显示各种不稳定的升力产生机制，这些机制主要通过迎角来控制

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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.
机译：在这里，我们通过定性的自由飞行和束缚飞行可视化显示，蜻蜓通过使用不稳定的空气动力学机制来产生高升力的前沿涡流而飞行。在正常的自由飞行中，蜻蜓使用逆冲程运动学，在前冲程的下冲程具有前沿涡流（LEV），在前冲程的上冲程具有附加的气流，在整个后翼具有附加的气流。加速的蜻蜓切换到具有高度分离的下行冲程流的同相机翼节拍，并且在前机翼和后机翼上都连接了一个LEV。我们使用烟雾可视化来区分Navier-Stokes方程的三个最简单的局部解析解，从而产生导致LEV的流分离。 LEV是一个开放的U形分隔，贯穿胸腔，平行于机翼前缘延伸，并在尖端弯曲以形成翼尖涡旋。随着LEV的增加，空气在中心线上盘旋至临界点。翼展方向流动不是流场的主要特征-翼展方向流动有时会从翼尖流向中心线，反之亦然-取决于侧滑程度。 LEV的形成总是与迎角的快速增加相吻合，并且烟雾可视化图像清楚地表明，只要迎角的快速增加就会形成LEV。没有离散的起始涡旋。取而代之的是，每当机翼处于非零迎角时，剪力层就会在后缘后面形成，并在开尔文-亥姆霍兹不稳定性的作用下卷成一系列横向涡旋，其旋流与机翼周围的旋流符号相反和LEV。蜻蜓产生的流场与蜻蜓，果蝇和天蛾的力学模型所公布的流场在质量上有所不同，这些模型排除了自然机翼的相互作用。但是，受控参数实验表明，只要Strouhal数合适，并且左右机翼之间会发生自然相互作用，即使是简单的下沉板也可以重现蜻蜓中流动的详细特征。在我们的模型和蜻蜓中，似乎LEV的稳定性是通过通用机制实现的，通过该机制可以配置襟翼运动学，从而可以预期LEV在机翼上自然形成并在整个行程期间保持附着状态。但是，LEV的实际形成和脱落受机翼迎角的控制，蜻蜓在两种极端情况下都可能发生变化，从零到直至在机翼冲程中随时可立即进行气流分离的范围。

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