俯仰和垂荡平板前缘涡的形成和分离
Insights into Leading Edge Vortex Formation and Detachment on a Pitching and Plunging Flat Plate
摘要：
本研究是在俯仰和俯仰翼型上应用不同流动控制装置的前奏，目的是控制前缘涡（LEV）的增长，从而控制非定常失速条件下的升力。作为一个先决条件，必须充分了解的参数影响的LEV拓扑结构的发展，这构成了本实验研究的主要动机。本研究的目的有两个。首先，介绍了一种方法来验证流场和LEV特性之间的可比性的两个不同的设施，使用水和空气作为工作介质，通过使用一个共同的基线情况。这种比较背后的动机是，使用两种设施可以大大扩展整体参数范围。这种比较包括各自的参数范围的概述，控制的翼型运动学和仔细审查如何从时间分辨粒子图像测速（PIV）的速度数据的后处理程序的影响的整体性能和拓扑功能，用于验证LEV的发展。其次，并根据来自两个设施的结果，二级结构的外观和它们的影响LEV脱离在一个扩展的参数范围进行了研究。拉格朗日流场分析，有限时间李雅普诺夫指数（FTLE）的脊的基础上，允许精确识别的二级结构，并揭示了它们的出现是密切相关的涡雷诺数阈值计算从LEV环流。该阈值用于模拟二级结构的时间起始。进一步的分析表明，如果平板前缘的剪切层角停止增加，次级结构的出现会导致LEV停止累积环量。
英文摘要：
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.
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