Layers and internal waves in uniformly stratified fluids stirred by vertical grids
Layers and internal waves in uniformly stratified fluids stirred by vertical grids
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垂直网格搅拌的均匀分层流体中的层和内波
DOI：
10.1017/jfm.2016.121
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发表时间：
2016
影响因子：
3.7
通讯作者：
S. Thorpe
中科院分区：
文献类型：
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作者：
S. Thorpe
实验室实验中，均匀分层的流体搅拌水平移动的垂直网格，或阵列的垂直杆，审查其一致性，并比较他们的研究结果，特别是那些有关层的产生。选定的实验有三种类型，其中（a）湍流扩散从一个水平受限的区域，它是连续产生的振荡网格;（B）网格搅拌保持在整个矩形槽;或（c）一个'云'的湍流衰减后，短期的水平本地化的网格混合。在所有的实验中，湍流产生在整个垂直范围内的坦克。在类型（a）和（c）的实验中，观察到相当规模的层从湍流区侵入周围流体。在类型（B）实验中，仅当网格通过分层流体之间的时间间隔足够长时才形成层，从而允许湍流在网格冲程之间基本上衰减。两种机制被认为是占主导地位的生产层。在实验中的类型（a）和（c）翻转涡的湍流区域的规模显着大于Ozmidov长度尺度崩溃和蔓延，侵入和形成层在相邻的层流区域。在侵入体前方传播的内部剪切波的垂直波长约为层高的两倍。在类型（B）实验中，通过Holford &林登（Dyn.大气层。《海洋》，第30卷，1999年a，第30页。173 - 198）：由栅条脱落的涡流的弯曲。层的高度大约是以网格速度行进的内部剪切波的垂直波长的一半。有人提出，剪切波的流场弯曲的旋涡，导致在diapycnal混合，形成层。因此，实验中各层和内剪切波的关系如下：在（a）和（c）类实验中，内波是由从紊流区侵入静止分层区的各层产生的，但在（B）类实验中，内波驱动并决定各层的高度;各层是由内波引起的涡弯曲而产生的。没有足够的证据来确定在这三种网格实验中的任何一种中，是否是由移动网格脱落的涡对或涡街的锯齿形不稳定性造成的。菲利普斯和Posmentier不稳定性可以加强由其他过程形成的层。饼状涡或涡模运动的产生留待以后讨论。
Laboratory experiments in which uniformly stratified fluids are stirred by horizontally moving vertical grids, or arrays of vertical rods, are reviewed to examine their consistency and to compare their findings, particularly those relating to the generation of layers. Selected experiments are of three types, those in which (a) turbulence spreads from a horizontally confined region where it is continuously generated by an oscillating grid; (b) grid stirring is maintained throughout a rectangular tank; or (c) a ‘cloud’ of turbulence decays after a short period of horizontally localized grid mixing. In all the experiments turbulence is generated over the full vertical extent of the tank. In the experiments of types (a) and (c) layers of comparable scale are observed to intrude into the ambient fluid from the turbulent region. In the type (b) experiments, layers form only when the time interval between the passage of the grid through the stratified fluid is sufficiently long, allowing turbulence to decay substantially between grid strokes. Two mechanisms are found to be dominant in the production of layers. In experiments of type (a) and (c) overturning eddies in the turbulent region of scale significantly larger than the Ozmidov length scale collapse and spread, intruding and forming layers in the adjoining laminar region. Internal shear waves propagating ahead of the intrusions have a vertical wavelength that is approximately twice the layer height. In type (b) experiments, layers are formed through a process described by Holford & Linden (Dyn. Atmos. Oceans, vol. 30, 1999a, pp. 173–198): the bending of vortices shed by the grid bars. The height of the layers is approximately half the vertical wavelength of internal shear waves that travel at the speed of the grid. It is proposed that the flow field of the shear waves bends the vortices, resulting in diapycnal mixing that forms the layers. The relationship of layers and internal shear waves in the experiments is therefore as follows: in type (a) and (c) experiments internal waves are generated by layers intruding from the turbulent region into the quiescent stratified region, but in experiments of type (b) internal waves drive and dictate the height of the layers; layers are generated as a consequence of vortex bending by internal waves. There is insufficient evidence to establish whether zigzag instability, either of vortex pairs or of vortex streets shed by a moving grid, accounts for the layers in any of the three types of grid experiments. The Phillips and Posmentier instability may reinforce layers formed by other processes. The generation of pancake vortices or vortical mode motion is left for later review.