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.

诸多昆虫物种（包括大蚊与豆娘）的翅膀均为具柄翅（以柄状结构连接），其翼型平面并非始于翼铰链处，而是距翼铰链存在一定距离。目前学界对拍动具柄翅的空气动力学效应仍知之甚少，尤其是对增升前缘涡（leading-edge vortex, LEV）的形成机制——这是诸多昆虫、鸟类与蝙蝠用以提升升力系数的关键流场结构。本研究针对展弦比为3、柄化参数P取值1至3的矩形翼阵列，采用粒子图像测速法（particle image velocimetry）开展流场测量，探究了柄化参数P的空气动力学影响。实验采用名为“Flapperatus”的机械装置驱动翼面，实现高度可重复的类昆虫运动学模式。所有测试工况下，翼面均保持恒定的雷诺数1400，以及无量纲拍幅Λ*（翼尖划过的弦长倍数）为6.5。研究结果表明，柄化程度更高的翼面，其前缘涡通常尺寸更大、环流更强，且覆盖翼面的面积更广，尤其在拍动周期早期的展向中部与翼内侧区域。各类工况下，前缘涡初始均呈拱状，其外侧端终止于翼面的焦点-汇点结构，随后逐渐与翼尖涡融合为一体。在拍动周期的后半段，柄化程度更高的翼面会出现更早脱落的前缘涡，脱落位置分别约在展向70%（P=1）与50%（P=3）处。由此带来的结果是，具柄翅在拍动周期前半段的基于前缘涡的升力系数更高，但在后半段二者差异则有所收窄。在所测试的参数范围内，时均前缘涡升力系数随柄化参数提升整体呈上升趋势。