The evolution of coherent vortical structures in increasingly turbulent stratified shear layers
The evolution of coherent vortical structures in increasingly turbulent stratified shear layers
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DOI：
10.1017/jfm.2022.588
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发表时间：
2022-03
影响因子：
3.7
通讯作者：
Xianyang Jiang;A. Lefauve;S. Dalziel;P. F. L. D. O. A. Mathematics;Theoretical Physics;Centre for Mathematical Sciences;U. Cambridge
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作者：
Xianyang Jiang;A. Lefauve;S. Dalziel;P. F. L. D. O. A. Mathematics;Theoretical Physics;Centre for Mathematical Sciences;U. Cambridge
摘要本文研究了湍流度增大的稳定剪切层中欧拉涡结构的形态及其与密度界面的相互作用。我们分析了三维的，同时在分层倾斜管道实验室实验（SID）中获得的速度和密度场。我们跟踪，在15个数据集，相干结构的演变，从前湍流Holmboe波，通过间歇性湍流，充分的湍流和混合。我们使用的局部涡矢量的rottex剪切分解成一个rottex矢量捕获刚体旋转和剪切矢量。我们描述了无处不在的发夹状涡结构（揭示的rottex）的形态，类似于那些通常观察到的边界层湍流。这些都是作为相对较弱的涡流周围的强三维剪切结构的封闭Holmboe波，并逐渐加强和变形下增加湍流，转化成对向上和向下指向的发夹传播方向相反的剪切层的顶部和底部边缘。每个发夹的一对腿是反向旋转的，并且横向和垂直地夹带流体，而它们的拱形“头”，即横向涡流，垂直地夹带流体。然后，我们阐明了这种大规模的旋涡形态搅拌和混合的密度场。基本上，位于混合层（主要是发夹头）的任一边缘上的尖锐密度界面处的涡流将未混合的流体团吞没到混合层中，而混合层（主要是发夹腿）内部的涡流进一步搅拌它，产生强烈的小尺度剪切，增强混合。这些发现提供了新的见解剪切驱动的分层混合中的湍流相干结构的作用。
Abstract We study the morphology of Eulerian vortical structures and their interaction with density interfaces in increasingly turbulent stably stratified shear layers. We analyse the three-dimensional, simultaneous velocity and density fields obtained in the stratified inclined duct laboratory experiment (SID). We track, across 15 datasets, the evolution of coherent structures from pre-turbulent Holmboe waves, through intermittent turbulence, to full turbulence and mixing. We use the rortex–shear decomposition of the local vorticity vectors into a rortex vector capturing rigid-body rotation and a shear vector. We describe the morphology of ubiquitous hairpin-like vortical structures (revealed by the rortex), similar to those commonly observed in boundary-layer turbulence. These are born as relatively weak vortices around the strong three-dimensional shearing structures of confined Holmboe waves, and gradually strengthen and deform under increasing turbulence, transforming into pairs of upward- and downward-pointing hairpins propagating in opposite directions on the top and bottom edge of the shear layer. The pair of legs for each hairpin are counter-rotating and entrain fluid laterally and vertically, whereas their arched-up ‘heads’, which are transverse vortices, entrain fluid vertically. We then elucidate how this large-scale vortex morphology stirs and mixes the density field. Essentially, vortices located at the sharp density interface on either edge of the mixing layer (mostly hairpin heads) engulf blobs of unmixed fluid into the mixing layer, whereas vortices inside the mixing layer (mostly hairpin legs) further stir it, generating strong, small-scale shear, enhancing mixing. These findings provide new insights into the role of turbulent coherent structures in shear-driven stratified mixing.