The present study is a prelude to applying different flow control devices on pitching and plunging airfoils with the intention of controlling the growth of the leading edge vortex (LEV); hence, the lift under unsteady stall conditions. As a pre-requisite, the parameters influencing the development of the LEV topology must be fully understood, and this constitutes the main motivation of the present experimental investigation. The aims of this study are twofold. First, an approach is introduced to validate the comparability between flow fields and LEV characteristics of two different facilities using water and air as working media by making use of a common baseline case. The motivation behind this comparison is that with two facilities the overall parameter range can be greatly expanded. This comparison includes an overview of the respective parameter ranges, control of the airfoil kinematics and careful scrutiny of how post-processing procedures of velocity data from time-resolved particle image velocimetry (PIV) influence the integral properties and topological features used to characterise the LEV development. Second, and based on results coming from both facilities, the appearance of secondary structures and their effect on LEV detachment over an extended parameter range is studied. A Lagrangian flow field analysis, based on finite-time Lyapunov Exponent (FTLE) ridges, allows precise identification of secondary structures and reveals that their emergence is closely correlated to a vortex Reynolds number threshold computed from the LEV circulation. This threshold is used to model the temporal onset of secondary structures. Further analysis indicates that the emergence of secondary structures causes the LEV to stop accumulating circulation if the shear layer angle at the leading edge of the flat plate has ceased to increase.

中文翻译：

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深入了解俯仰和俯冲平板上前缘涡旋的形成和分离

本研究是在俯仰和俯冲翼型上应用不同流量控制装置的前奏，目的是控制前缘涡 (LEV) 的增长；因此，在不稳定失速条件下的升力。作为先决条件，必须充分了解影响 LEV 拓扑发展的参数，这构成了本实验研究的主要动机。这项研究的目的是双重的。首先，引入了一种方法，通过使用通用基线案例来验证使用水和空气作为工作介质的两个不同设施的流场和 LEV 特性之间的可比性。这种比较背后的动机是，使用两个设施可以大大扩展整体参数范围。这种比较包括对各自参数范围的概述、翼型运动学的控制以及对来自时间分辨粒子图像测速 (PIV) 的速度数据的后处理程序如何影响用于表征 LEV 的积分特性和拓扑特征的仔细审查发展。其次，基于来自两个设施的结果，研究了二级结构的出现及其在扩展参数范围内对 LEV 脱离的影响。基于有限时间李雅普诺夫指数 (FTLE) 脊的拉格朗日流场分析允许精确识别二级结构，并揭示它们的出现与从 LEV 循环计算的涡流雷诺数阈值密切相关。该阈值用于对二级结构的时间开始进行建模。