Phillips Nathan, Knowles Kevin, Bomphrey Richard J
Structure and Motion Laboratory, Royal Veterinary College , University of London , Hatfield AL9 7TA , UK.
Aeromechanical Systems Group, Centre for Defence Engineering , Cranfield University , Defence Academy of the United Kingdom, Shrivenham SN6 8LA , UK.
Interface Focus. 2017 Feb 6;7(1):20160084. doi: 10.1098/rsfs.2016.0084.
The wings of many insect species including crane flies and damselflies are petiolate (on stalks), with the wing planform beginning some distance away from the wing hinge, rather than at the hinge. The aerodynamic impact of flapping petiolate wings is relatively unknown, particularly on the formation of the lift-augmenting leading-edge vortex (LEV): a key flow structure exploited by many insects, birds and bats to enhance their lift coefficient. We investigated the aerodynamic implications of petiolation using particle image velocimetry flow field measurements on an array of rectangular wings of aspect ratio 3 and petiolation values of = 1-3. The wings were driven using a mechanical device, the 'Flapperatus', to produce highly repeatable insect-like kinematics. The wings maintained a constant Reynolds number of 1400 and dimensionless stroke amplitude * (number of chords traversed by the wingtip) of 6.5 across all test cases. Our results showed that for more petiolate wings the LEV is generally larger, stronger in circulation, and covers a greater area of the wing surface, particularly at the mid-span and inboard locations early in the wing stroke cycle. In each case, the LEV was initially arch-like in form with its outboard end terminating in a focus-sink on the wing surface, before transitioning to become continuous with the tip vortex thereafter. In the second half of the wing stroke, more petiolate wings exhibit a more detached LEV, with detachment initiating at approximately 70% and 50% span for = 1 and 3, respectively. As a consequence, lift coefficients based on the LEV are higher in the first half of the wing stroke for petiolate wings, but more comparable in the second half. Time-averaged LEV lift coefficients show a general rise with petiolation over the range tested.
包括大蚊和豆娘在内的许多昆虫物种的翅膀是有柄的（长在柄上），其翼平面从距翼铰链一定距离处开始，而不是在铰链处。拍打有柄翅膀的空气动力学影响相对未知, 尤其是对增强升力的前缘涡（LEV）的形成的影响：这是许多昆虫、鸟类和蝙蝠利用的关键流动结构，以提高它们的升力系数。我们使用粒子图像测速法对流场进行测量，研究了长宽比为3且叶柄值为 = 1-3的一系列矩形翅膀的叶柄化对空气动力学的影响。使用一种名为“Flapperatus”的机械设备驱动翅膀，以产生高度可重复的类似昆虫的运动学。在所有测试案例中，翅膀保持恒定的雷诺数1400和无量纲冲程幅度 *（翼尖穿过的弦数）为6.5。我们的结果表明，对于叶柄化程度更高的翅膀，LEV通常更大，环流更强，并且覆盖翼面的面积更大，特别是在翼冲程周期早期的中跨和内侧位置。在每种情况下，LEV最初呈拱形，其外侧端在翼面上终止于一个焦点汇，之后转变为与尾涡连续。在翼冲程的后半段，叶柄化程度更高的翅膀表现出更分离的LEV，对于 = 1和3的情况，分离分别在大约70%和50%翼展处开始。因此，对于叶柄化翅膀，基于LEV的升力系数在翼冲程的前半段较高，但在后半段更接近。时间平均的LEV升力系数在测试范围内总体上随叶柄化程度增加而上升。
