Spontaneous concentrations of solids through two-way drag forces between gas and sedimenting particles
Spontaneous concentrations of solids through two-way drag forces between gas and sedimenting particles
复制标题
通过气体和沉积颗粒之间的双向阻力实现固体的自发浓缩
DOI：
10.1051/0004-6361/201526272
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发表时间：
2016
期刊：
影响因子：
--
通讯作者：
Bodenschatz
中科院分区：
文献类型：
--
作者：
Lambrechts;Johansen;Capelo;Bodenschatz
沉积颗粒的行为取决于流体的尘气比。线性稳定性分析表明，即使尘气比达到统一，在爱泼斯坦阻力状态下沉降的固体仍将均匀分布在非旋转不可压缩流体中。然而，非线性演化此前尚未被探讨过。在这里，我们提出的数值计算表明，在颗粒密集的混合物中，固体会自发地从流体中混合出来，并形成颗粒密度至少为 10 倍的群。这种不稳定性是由质量加载区域局部打破平衡背景分层引起的。驱动机制取决于背景流的非线性扰动，并且与吸积盘中的流动不稳定性有一些相似之处。由此产生的富含颗粒的群可以通过凝结刺激颗粒生长。在原行星盘的背景下，不稳定性可能与帮助小颗粒沉降到外盘的中平面有关。在原行星的气体包层内，增强的沉降可能会导致尘埃不透明度降低，从而促进包层的收缩。我们证明可以在实验室环境中重新创建相关的物理设置。这将使我们的数值计算能够在未来进行实验研究。
The behaviour of sedimenting particles depends on the dust-to-gas ratio of the fluid. Linear stability analysis shows that solids settling in the Epstein drag regime would remain homogeneously distributed in non-rotating incompressible fluids, even when dust-to-gas ratios reach unity. However, the nonlinear evolution has not been probed before. Here, we present numerical calculations indicating that, in a particle-dense mixture, solids spontaneously mix out of the fluid and form swarms that are overdense in particles by at least a factor 10. The instability is caused by mass-loaded regions locally breaking the equilibrium background stratification. The driving mechanism depends on nonlinear perturbations of the background flow and shares some similarity to the streaming instability in accretion discs. The resulting particle-rich swarms may stimulate particle growth by coagulation. In the context of protoplanetary discs, the instability could be relevant for aiding small particles to settle to the midplane in the outer disc. Inside the gas envelopes of protoplanets, enhanced settling may lead to a reduced dust opacity, which facilitates the contraction of the envelope. We show that the relevant physical set up can be recreated in a laboratory setting. This will allow our numerical calculations to be investigated experimentally in the future.