Generation of air lubrication within pyroclastic density currents
Generation of air lubrication within pyroclastic density currents
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DOI：
10.1038/s41561-019-0338-2
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发表时间：
2019-05-01
影响因子：
18.3
通讯作者：
Wang, Ting
中科院分区：
文献类型：
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作者：
Lube, Gert;Breard, Eric C. P.;Wang, Ting
火山碎屑密度流是一种非常危险的火山流，它导致了全球超过50%的火山死亡。这些火山颗粒和气体的热混合物具有非凡的流动性，使它们能够将数千吨至数百万吨的火山物质运送到数十公里至数百公里的地球表面，绕过曲折的流动路径，忽略粗糙的基底和平坦和向上倾斜的地形。它们的流动性归因于对抗颗粒摩擦的内部过程。然而，很难测量内部火山碎屑密度流来量化这种摩擦机制。在这里，我们表明，通过大规模的实验和数值多相建模，火山碎屑密度流产生自己的空气润滑。这就形成了一个近乎无摩擦的基底区域。在高基底剪切下，当空气被反向压力梯度局部向下推动并向上移动颗粒时，空气润滑发展。我们表明，空气润滑是通过一个正反馈机制，解释如何火山碎屑密度流能够传播的斜坡比任何天然颗粒材料的休止角浅得多。这一发现需要重新评估的危险模型，旨在预测的速度，跳动和蔓延的火山碎屑密度流。
Pyroclastic density currents are highly dangerous ground-hugging currents from volcanoes that cause >50% of volcanic fatalities globally. These hot mixtures of volcanic particles and gas exhibit remarkable fluidity, which allows them to transport thousands to millions of tonnes of volcanic material across the Earth's surface over tens to hundreds of kilometres, bypassing tortuous flow paths and ignoring rough substrates and flat and upsloping terrain. Their fluidity is attributed to an internal process that counters granular friction. However, it is difficult to measure inside pyroclastic density currents to quantify such a friction-defying mechanism. Here we show, through large-scale experiments and numerical multiphase modelling, that pyroclastic density currents generate their own air lubrication. This forms a near-frictionless basal region. Air lubrication develops under high basal shear when air is locally forced downwards by reversed pressure gradients and displaces particles upward. We show that air lubrication is enhanced through a positive feedback mechanism, explaining how pyroclastic density currents are able to propagate over slopes much shallower than the angle of repose of any natural granular material. This discovery necessitates a re-evaluation of hazard models that aim to predict the velocity, runout and spreading of pyroclastic density currents.