Aeronautical Engineering Branch, Israeli Air Force, Tel Aviv 6473428, Israel.
Department of Physics and Engineering Science, Coastal Carolina University, Conway, SC 29528, United States of America.
Bioinspir Biomim. 2022 Oct 31;17(6). doi: 10.1088/1748-3190/ac9bb5.
The leading-edge vortex (LEV) is a common flow structure that forms over wings at high angles of attack. Over the years, LEVs were exploited for augmenting the lift of man-made slender delta wings aircraft. However, recent observations suggested that natural flyers with high-aspect-ratio (high-AR) wings, such as the common swift (), can also generate LEVs while gliding. We hypothesize that the planform shape and nonlinear sweep (increasing towards the wingtip) enable the formation and control of such LEVs. In this paper, we investigate whether a stationary LEV can form over a nonlinear swept-back high-AR wing inspired by the swift's wing shape and evaluate its characteristics and potential aerodynamic benefit. Particle image velocimetry (PIV) measurements were performed in a water flume on a high-AR swept-back wing inspired by the swift wing. Experiments were performed at four spanwise sections and a range of angles of attack for a chord-based Reynolds number of20000. Stationary LEV structures were identified across the wingspan by utilizing the proper orthogonal decomposition (POD) method for angles of attack of 5-15. The size and circulation of the stationary LEV were found to grow towards the wingtip in a nonlinear manner due to shear layer feeding and spanwise transport of mass and vorticity within the LEV, thus confirming that nonlinear high-AR swept-back wings can generate stationary LEVs. Our results suggest that the common swift can generate stationary LEVs over its swept-back wings to glide slower and at a higher rate of descent, with the LEVs potentially supporting up to 60% of its weight.
前缘涡（LEV）是一种常见的流动结构，在大迎角下形成于机翼上。多年来，LEV 被用于增强人造细长三角翼飞机的升力。然而，最近的观察表明，具有高展弦比（高-AR）的天然飞行动物，如普通雨燕（），在滑翔时也可以产生 LEV。我们假设，翼型形状和非线性后掠（向翼尖逐渐增加）使这些 LEV 的形成和控制成为可能。在本文中，我们研究了是否可以在受雨燕翅膀形状启发的非线性后掠高-AR 机翼上形成静止的 LEV，并评估其特性和潜在的空气动力效益。在受雨燕翅膀形状启发的高-AR 后掠机翼上，在水槽中进行了粒子图像测速（PIV）测量。在基于弦长的雷诺数为 20000 的范围内，针对四个展向截面和一系列攻角进行了实验。通过利用适当的正交分解（POD）方法，在 5-15 的攻角范围内，确定了整个机翼跨度上的静止 LEV 结构。由于剪切层馈送和 LEV 内质量和涡量的展向输运，静止 LEV 的大小和环流以非线性方式向翼尖增长，从而证实了非线性高-AR 后掠翼可以产生静止 LEV。我们的结果表明，普通雨燕可以在其后掠机翼上产生静止的 LEV，以更慢的速度滑翔，下降率更高，LEV 可能支撑其重量的 60%。
