Lagrangian coherent superstructures in buoyancy driven turbulence

浮力驱动湍流中的拉格朗日相干上部结构

• 批准号: 316160518
• 负责人: Professor Dr. George Haller, Ph.D.
• 金额: --
• 依托单位: Eidg. Forschungsanstalt für Wald, Schnee und Landschaft WSL
• 依托单位国家: 德国
• 项目类别: Priority Programmes
• 财政年份: 2016
• 资助国家: 德国
• 起止时间: 2015-12-31 至 2022-12-31
• 项目状态: 已结题
• 来源: https://gepris.dfg.de/gepris/projekt/316160518?language=en
• 关键词: Lagrangian; coherent; superstructures; buoyancy; driven

Large-scale coherent flow structures, termed superstructures (SSs), play a key role in stratified turbulent flows by controlling the overall rate of mass and momentum transport. However, progress in our understanding of the mechanics of SSs has been hampered by arbitrariness in the detection methods of these coherent flow structures and hence leading to contrasting classifications of the results or even misinterpretations of physical mechanisms. As a result, little is known about how SSs determine global transport and mixing rates of the whole flow. The aim of this project is to develop 3D Lagrangian coherent structure identification methods and apply them to experimental particle tracking and numerical simulation data of shear flows with stable and unstable stratification to determine the role of these structures for the global exchange of mass and momentum. In Phase I of the project, we have shown how Lagrangian coherent structures (LCSs) control the mass transfer in stably stratified gravity currents. For the first time, we extracted rotational 3D LCSs using the co-called Lagrangian-averaged vorticity deviation (LAVD) method from experimental 3D particle tracking data. We have achieved this using a novel extraction algorithm and several simultaneous observation volumes stitched together. This approach has revealed the boundaries of LCSs composed of fluid elements that exhibit the same mean material rotation, thereby allowing only negligible radial filamentation for the boundary. The detected LCSs populating the strongly stratified gravity current boundary were predominantly large, spanwise oriented SSs, reminiscent of Kelvin-Helmholtz rollers. These SSs suppress mixing in their immediate vicinity, while organizing entrainment of mass by deflecting streamlines that cross the boundary of the gravity current at their rear and front sides. We have performed a similar analysis of mass transfer in numerical simulations of the planetary boundary layer that will be finalized in the last stage of Phase I. In the present Phase II proposal, we will complete the development of fully 3D Lagrangian coherent structure methods so that they can be applied broadly to experimental and numerical data of turbulent flows: beyond material barriers to momentum transport, we will also seek material barriers to vorticity transport and we will generalize the stably stratified flow analysis by extending it to the unstably stratified regime. We will extend the experimental analysis to explore the unstably stratified regime and investigate moderately high Reynolds number regimes in line with a primary goal of the SPP for Phase II. This effort will provide a new understanding of fluxes of mass, heat and momentum governed by superstructures across a wide range of flows that are of major relevance in nature and practical applications.

大尺度相干流动结构，称为超结构(SSS)，通过控制总的质量和动量输运速率，在分层湍流中起着关键作用。然而，这些相干流动结构的探测方法的任意性阻碍了我们对SSS机理的理解，从而导致了对结果的不同分类，甚至对物理机制的误解。因此，关于悬浮物如何决定整个水流的全球输运和混合速率，人们知之甚少。本项目的目的是发展三维拉格朗日相干结构识别方法，并将其应用于具有稳定和不稳定层结的切变流的实验粒子跟踪和数值模拟数据，以确定这些结构在全球质量和动量交换中的作用。在项目的第一阶段，我们展示了拉格朗日相干结构(LCS)如何控制稳定分层重力流中的质量传递。我们首次使用拉格朗日平均涡度偏差(LAVD)方法从实验的三维粒子跟踪数据中提取了旋转的三维LCS。我们使用一种新的提取算法和几个同时观察到的体积拼接在一起，从而实现了这一点。这种方法揭示了由表现出相同平均材料旋转的流体元素组成的LCS的边界，从而仅允许对边界进行微不足道的径向成丝。探测到的强层结重力流边界上的LCS主要是大的、跨向定向的SSS，这让人想起Kelvin-Helmholtz滚柱。这些SS抑制其附近的混合，同时通过偏转穿过其后侧和前侧重力流边界的流线来组织质量卷吸。我们已经在第一阶段最后一阶段完成的行星边界层数值模拟中进行了类似的质量传递分析。在目前的第二阶段方案中，我们将完成全三维拉格朗日相干结构方法的发展，以便它们可以广泛地应用于湍流的实验和数值数据：除了动量输送的物质障碍之外，我们还将寻找涡量输送的物质障碍，我们将把稳定分层流动分析推广到不稳定分层区域。我们将根据第二阶段SPP的主要目标，将实验分析扩展到探索不稳定层化区域，并研究中等高雷诺数区域。这一努力将提供对超结构控制的质量、热量和动量通量的新理解，这些流动在本质上和实际应用中具有重要意义。