Core mechanisms of drag enhancement on bodies settling in a stratified fluid
Core mechanisms of drag enhancement on bodies settling in a stratified fluid
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分层流体中沉降物体阻力增强的核心机制
DOI：
10.1017/jfm.2019.524
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发表时间：
2019-07
影响因子：
3.7
通讯作者：
Jacques Magnaudet
中科院分区：
文献类型：
--
作者：
Jie Zhang;Matthieu J.Mercier;Jacques Magnaudet
由盐或热梯度引起的分层极大地影响了海洋和低层大气中惰性粒子和生物体的分布。考虑线性分层流体中球体沉降的实验室研究证实，分层可能会显著增加对物体的阻力，但未能确定导致这种增加的一般物理机制。我们提出了一个严格的分裂计划的各种贡献的阻力上的沉降体，这使得他们能够适当地解开惯性，粘性，扩散和浮力效应的相对大小。我们应用这种分裂过程中获得的数据，通过直接数值模拟的流动过去的沉降球在一系列的参数，涵盖了各种情况下的实验室和地球物理的兴趣。与普遍的看法相反，我们表明，在模拟所涵盖的参数范围内，阻力增强一般不是主要由于额外的浮力所产生的轻流体拖动的身体，而是设置在浮力效应的涡量场的特定结构。模拟还揭示了如何不同的浮力引起的阻力的贡献随流动参数而变化。为了解开这些变化的起源，我们分析了不同的可能的领导阶平衡的控制方程。由于这个程序，我们确定了几个不同的制度，不同的长度尺度与分层，粘度和扩散率的相对大小。我们推导出在这些制度的浮力引起的阻力贡献的比例律。考虑到有形的例子，我们展示了如何将这些比例律与数值结果相结合，以获得可靠的预测超出了模拟所涵盖的参数范围。
Stratification due to salt or heat gradients greatly affects the distribution of inert particles and living organisms in the ocean and the lower atmosphere. Laboratory studies considering the settling of a sphere in a linearly stratified fluid confirmed that stratification may dramatically enhance the drag on the body, but failed to identify the generic physical mechanism responsible for this increase. We present a rigorous splitting scheme of the various contributions to the drag on a settling body, which allows them to be properly disentangled whatever the relative magnitude of inertial, viscous, diffusive and buoyancy effects. We apply this splitting procedure to data obtained via direct numerical simulation of the flow past a settling sphere over a range of parameters covering a variety of situations of laboratory and geophysical interest. Contrary to widespread belief, we show that, in the parameter range covered by the simulations, the drag enhancement is generally not primarily due to the extra buoyancy force resulting from the dragging of light fluid by the body, but rather to the specific structure of the vorticity field set in by buoyancy effects. Simulations also reveal how the different buoyancy-induced contributions to the drag vary with the flow parameters. To unravel the origin of these variations, we analyse the different possible leading-order balances in the governing equations. Thanks to this procedure, we identify several distinct regimes which differ by the relative magnitude of length scales associated with stratification, viscosity and diffusivity. We derive the scaling laws of the buoyancy-induced drag contributions in each of these regimes. Considering tangible examples, we show how these scaling laws combined with numerical results may be used to obtain reliable predictions beyond the range of parameters covered by the simulations.
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DOI：
10.1073/pnas.1012319108
发表时间：
2010-12-21
影响因子：
11.1
作者：
Kindler, Kolja;Khalili, Arzhang;Stocker, Roman
通讯作者：
Stocker, Roman
影响因子：
4.5
作者：
H. Ploug;M. Iversen;G. Fischer
通讯作者：
H. Ploug;M. Iversen;G. Fischer
影响因子：
56.9
作者：
Farmer, D;Armi, L
通讯作者：
Armi, L
影响因子：
3.7
作者：
BATCHELOR, GK
通讯作者：
BATCHELOR, GK
影响因子：
2.2
作者：
J. Beckers;M. Rixen
通讯作者：
J. Beckers;M. Rixen