Data from: Petiolate wings: effects on the leading-edge vortex in flapping flight

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 P using particle image velocimetry flow field measurements on an array of rectangular wings of aspect ratio 3 and petiolation values of P = 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 Λ* (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 P = 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.诸多昆虫类群（包括大蚊与豆娘）的翅膀均具柄状结构（petiolate），即其翼面构型并非直接从翅关节处延伸而出，而是与关节之间存在一段柄状间距。目前学界对具柄状拍打翅的空气动力学效应仍知之甚少，尤其是针对增升前缘涡（leading-edge vortex, LEV）的形成过程——该流场结构是众多昆虫、鸟类与蝙蝠用以提升升力系数的核心流场机制。本研究采用粒子图像测速法（particle image velocimetry, PIV）对展弦比为3、柄状参数P取值1至3的矩形机翼阵列开展流场测量，以此探究柄状参数P对空气动力学特性的影响。实验采用名为“Flapperatus”的机械装置驱动机翼，可生成高度可复现的类昆虫扑翼运动学特性，所有测试工况均保持雷诺数恒定为1400，无量纲冲程幅值Λ*（即翼尖划过的弦长倍数）为6.5。研究结果显示，柄状参数更高的机翼，其前缘涡通常尺度更大、环流强度更高，且覆盖翼面的区域更广，尤其在扑翼周期早期的展向中部与翼根区域。所有工况下，前缘涡初始均呈拱形，其翼尖侧末端会在翼面形成一个汇焦点，随后逐渐与梢涡融合为一体。在扑翼周期的后半段，柄状参数更高的机翼前缘涡分离程度更高，分离分别约始于展向70%（P=1）与50%（P=3）位置。由此，在前半扑翼周期内，具柄状翅基于前缘涡的升力系数更高，但在后半周期两者差异趋于缩小。在所测试的参数范围内，时均前缘涡升力系数随柄状参数提升整体呈上升趋势。