Transport of anisotropic particles under waves
Transport of anisotropic particles under waves
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DOI：
10.1017/jfm.2017.853
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发表时间：
2018-02-25
影响因子：
3.7
通讯作者：
Koseff, Jeffrey R.
中科院分区：
文献类型：
--
作者：
DiBenedetto, Michelle H.;Ouellette, Nicholas T.;Koseff, Jeffrey R.
利用数值模型，我们分析了形状对波浪流中各向异性颗粒的取向和运输的影响。粒子被理想化为长球形和扁球形，我们认为该制度的小斯托克斯和粒子雷诺数。我们发现，颗粒优先对齐到剪切平面的平均取向，这仅仅是一个功能的纵横比。然而，这种对齐方式不同于在没有波的情况下在剩余剪切流（即斯托克斯漂移速度场）中发生的杰弗里轨道。由于各向异性颗粒上的阻力取决于其与流动的对齐，因此这种优选取向决定了颗粒上的有效阻力，这反过来又影响了它们的净下游运输。我们还发现，粒子的对齐率是不恒定的，强烈依赖于它们的初始取向，因此，在初始粒子取向的变化导致各向异性粒子羽流的分散。我们表明，这种分散是一个函数的粒子的偏心率和比率的沉降和波的时间尺度。由于这种优先排列，我们发现，在波的各向异性颗粒羽流平均运输更远，但分散少，如果粒子是随机取向。我们的研究结果表明，准确预测各向异性颗粒在波浪环境中的运输，如海洋中的微塑料颗粒，需要考虑这些优先排列效应。
Using a numerical model, we analyse the effects of shape on both the orientation and transport of anisotropic particles in wavy flows. The particles are idealized as prolate and oblate spheroids, and we consider the regime of small Stokes and particle Reynolds numbers. We find that the particles preferentially align into the shear plane with a mean orientation that is solely a function of their aspect ratio. This alignment, however, differs from the Jeffery orbits that occur in the residual shear flow (that is, the Stokes drift velocity field) in the absence of waves. Since the drag on an anisotropic particle depends on its alignment with the flow, this preferred orientation determines the effective drag on the particles, which in turn impacts their net downstream transport. We also find that the rate of alignment of the particles is not constant and depends strongly on their initial orientation; thus, variations in initial particle orientation result in dispersion of anisotropic-particle plumes. We show that this dispersion is a function of the particle's eccentricity and the ratio of the settling and wave time scales. Due to this preferential alignment, we find that a plume of anisotropic particles in waves is on average transported farther but dispersed less than it would be if the particles were randomly oriented. Our results demonstrate that accurate prediction of the transport of anisotropic particles in wavy environments, such as microplastic particles in the ocean, requires the consideration of these preferential alignment effects.