曲折、转弯和相遇:大气小颗粒的轨迹解密

Taraprasad Bhowmick, Yong Wang, Jonas Latt, Gholamhossein Bagheri
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摘要

大气中的每一种固体颗粒,从冰晶和花粉到灰尘、灰烬和微塑料,都是非球形的。这些粒子在地球气候系统中扮演着重要角色,影响着温度、天气模式、自然生态系统、人类健康和污染程度。然而,我们对这些微粒的了解主要基于极小微粒的理论和在液体介质中进行的实验。在这项研究中,我们利用创新的实验装置和粒子分辨数值模拟来研究空气中形状各异的亚毫米椭圆体的行为。我们的研究结果揭示了这些粒子中涉及无数曲折的复杂衰减振荡模式,这与它们在液体介质中的动力学形成了鲜明对比。我们发现,这些振荡的频率和衰减率与颗粒形状有很大关系。有趣的是,圆盘状粒子的振荡频率几乎是棒状粒子的两倍,但它们的振荡衰减速度也更快。在振荡过程中,即使是微妙的非球形粒子也会发生横向漂移,漂移幅度可达其体积等效球形直径的十倍。与体积相当的球体相比,这种行为能使粒子横向和纵向扫过的空气量增加四倍,从而大大提高了粒子的相遇率和聚集可能性。我们的发现为雪花和火山灰等高度非球形颗粒的长程飘移和自然形成聚集体提供了解释。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
Twist, turn and encounter: the trajectories of small atmospheric particles unravelled
Every solid particle in the atmosphere, from ice crystals and pollen to dust, ash, and microplastics, is non-spherical. These particles play significant roles in Earth's climate system, influencing temperature, weather patterns, natural ecosystems, human health, and pollution levels. However, our understanding of these particles is largely based on the theories for extremely small particles and experiments conducted in liquid mediums. In this study, we used an innovative experimental setup and particle-resolved numerical simulations to investigate the behaviour of sub-millimetre ellipsoids of varying shapes in the air. Our results revealed complex decaying oscillation patterns involving numerous twists and turns in these particles, starkly contrasting their dynamics in liquid mediums. We found that the frequency and decay rate of these oscillations have a strong dependence on the particle shape. Interestingly, disk-shaped particles oscillated at nearly twice the frequency of rod-shaped particles, though their oscillations also decayed more rapidly. During oscillation, even subtly non-spherical particles can drift laterally up to ten times their volume-equivalent spherical diameter. This behaviour enables particles to sweep through four times more air both vertically and laterally compared to a volume-equivalent sphere, significantly increasing their encounter rate and aggregation possibility. Our findings provide an explanation for the long-range transport and naturally occurring aggregate formation of highly non-spherical particles such as snowflakes and volcanic ash.
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