发表机构
Univ. Lille, ULR 7512–Unité de Mécanique de Lille Joseph Boussinesq (UML)（里尔大学，ULR 7512——约瑟夫·傅立叶力学单元）
机构由 AI 辅助整理，请以论文原文为准。AI 中文总结
本研究通过直接数值模拟揭示，在两层流体模型中，弱浮力颗粒的垂直分布受密度分层与混合层湍流共同控制，并提出了随机模型解释其积累层厚度变化。
AI 中文摘要
理解微塑料和浮游生物等颗粒如何扩散或在海洋表面以下局部区域积累，对于制定海洋污染控制策略和海洋生态学都至关重要。然而，由于海洋中密度分层的垂直结构导致的垂直非均匀湍流的作用仍然知之甚少。我们通过直接数值模拟，研究了弱惯性、准中性浮力颗粒在一个理想化的两层流体模型中的动力学，该模型旨在代表海洋对流混合层及其下方更稳定分层的上部温跃层。我们发现，如果颗粒的密度与混合层内选定深度的流体平均值相匹配，它们最终会上升到表面。但有趣的是，对于所有其他密度值，它们会在温跃层内的中性浮力深度附近积累，在那里分层阻碍了垂直输运。我们的分析表明，积累层的厚度是浮力驱动的限制、内波运动和从混合层渗入的湍流波动之间竞争的结果。较小的颗粒对流动波动响应更快，形成更宽的层；随着其平衡深度接近混合层，层也会变宽。然后，我们推导了一个简化的随机模型，解释了颗粒惯性和参考深度对这种围绕平衡位置扩散的依赖性。这些结果表明，分层和衰减的混合层湍流的耦合作用控制了次表层颗粒层的形成，这可能与海洋生物学导向的研究相关，并有助于改进海洋塑料污染的采样。
英文摘要
Understanding how particles such as microplastics and plankton spread, or accumulate in localized regions below the ocean surface is crucial both for the development of sea pollutant control strategies and for marine ecology. Yet the role of vertically inhomogeneous turbulence, due to the vertical structure of density stratification in the ocean, remains poorly understood. We study, by means of direct numerical simulations, the dynamics of weakly inertial, quasi-neutrally buoyant particles in an idealized two-layer fluid model aimed to represent the ocean convective mixed layer and the more stably stratified upper thermocline beneath. We find that particles eventually rise at the surface if their density matches the mean value of the fluid at a selected depth within the mixed layer. Interestingly, however, for all other values of density, they accumulate around their neutral-buoyancy depth within the thermocline, where stratification hinders vertical transport. Our analysis shows that the thickness of the accumulation layer results from the competition between buoyancy-driven confinement, internal-wave motions, and turbulent fluctuations penetrating from the mixed layer. Smaller particles, which respond more rapidly to flow fluctuations, form broader layers; layers also broaden as their equilibrium depth approaches the mixed layer. We then derive a reduced stochastic model that explains the dependence on particle inertia and reference depth of such spreading around the equilibrium position. These results show how the coupled action of stratification and decaying mixed-layer turbulence controls subsurface particle-layer formation, which may be relevant to ocean-biology oriented studies and to improve the sampling of plastic pollution in the sea.
Comments22 pages, 12 figures
