Muijres Florian T, Christoffer Johansson L, Winter York, Hedenström Anders
Department of Biology, Lund University, Ecology Building, SE-223 62 Lund University, Sweden. Department of Biology, Box 351800, 24 Kincaid Hall, University of Washington, Seattle, WA 98195-1800, USA.
Bioinspir Biomim. 2014 Jun;9(2):025006. doi: 10.1088/1748-3182/9/2/025006. Epub 2014 May 22.
Slow and hovering animal flight creates high demands on the lift production of animal wings. Steady state aerodynamics is unable to explain the forces required and the most commonly used mechanism to enhance the lift production is a leading edge vortex (LEV). Although LEVs increase the lift, they come at the cost of high drag. Here we determine the flow above the wing of lesser long-nosed bats at slow and cruising speed using particle image velocimetry (PIV). We find that a prominent LEV is present during the downstroke at slow speed, but not at cruising speed. Comparison with previously published LEV data from a robotic flapper inspired by lesser long-nosed bats suggests that bats should be able to generate LEVs at cruising speeds, but that they avoid doing so, probably to increase flight efficiency. In addition, at slow flight speeds we find LEVs of opposite spin at the inner and outer wing during the upstroke, potentially providing a control challenge to the animal. We also note that the LEV stays attached to the wing throughout the downstoke and does not show the complex structures found in insects. This suggests that bats are able to control the development of the LEV and potential control mechanisms are discussed.
缓慢且悬停的动物飞行对动物翅膀产生升力提出了很高要求。稳态空气动力学无法解释所需的力，而增强升力产生的最常用机制是前缘涡（LEV）。尽管前缘涡会增加升力，但它们是以高阻力为代价的。在这里，我们使用粒子图像测速技术（PIV）确定了长鼻蝠在慢速和巡航速度下翅膀上方的气流。我们发现，在慢速下拍过程中存在一个明显的前缘涡，但在巡航速度下则不存在。与之前发表的受长鼻蝠启发的机器人扑翼的前缘涡数据进行比较表明，蝙蝠应该能够在巡航速度下产生前缘涡，但它们避免这样做，可能是为了提高飞行效率。此外，在慢速飞行时，我们发现在上拍过程中，内翼和外翼的前缘涡旋转方向相反，这可能给动物带来控制挑战。我们还注意到，前缘涡在整个下拍过程中都附着在翅膀上，并且没有显示出昆虫中发现的复杂结构。这表明蝙蝠能够控制前缘涡的发展，并对潜在的控制机制进行了讨论。
