Fragmentation and flow of gas-particle mixtures in volcanic systems

气体和颗粒混合物的输送是工业和自然界中普遍存在的现象。在地球科学中，这些气体-颗粒混合物存在于风搬运沉积物、陨石撞击、岩石和雪崩以及火山爆发等过程中。在这项研究计划中，我们将重点放在火山爆发期间产生的气体-颗粒流上，尽管我们希望发现的物理学将可移植到这些其他学科，并计划举办一个多学科研讨会以促进知识转移。世界上百分之十的人口（即几亿人）居住在距离活火山100公里的范围内。此外，随着全球人口的增加，这个数字将会上升。在所有的火山爆发中，火山碎屑密度流（PDCs）都可以形成——岩石和气体的高温多相混合物，迅速从火山口流出。这些现象是所有火山灾害中最致命的，造成了超过三分之一的火山相关死亡。然而，尽管火山碎屑密度流具有致命的性质，但我们目前缺乏准确的模型来预测这些流所达到的跳动距离，因此任何危险图和缓解策略都是有限的。为了改进我们的数值预报模型，我们需要了解这些“不透明”和危险的流动中复杂的内部流动动力学。直接的内部观察是不可能的，但受控的实验室实验提供了一种严格研究这些隐藏现象的方法。具体和有针对性的实验旨在为数值流动模拟提供关键输入参数提供了明确的前进方向。在本研究计划中，我们将使用两种规模的实验室实验，以可控和可观察的方式来研究这些现象。首先，使用流变仪的实验将测量天然火山气体-颗粒混合物的流动特性（如粘度、流动性）。我们将量化颗粒大小分布和气体流动条件如何影响这些流动的流动性。其次，在与火山系统相关的一系列条件下，天然火山颗粒（如气泡火山玻璃、晶体、岩石碎片）将悬浮在空气中形成气-颗粒混合物。实验产品的大小、形状和丰度将被表征为时间的函数（即传输距离）。这将使我们能够量化火山颗粒在流动过程中破裂、变圆和改变相对丰度的速度。这将导致一个全面的了解如何颗粒的质地性质（如大小，形状）在流动过程中变化，以及如何，反过来，这些变化改变了火山碎屑密度电流的流动性。这些关于流动条件和颗粒破碎的实验数据来自最新的技术，将被纳入现有的3D数值流动模型中。将对一个个案火山（厄瓜多尔通古拉瓦）进行数值模拟。这将使我们能够说明并定量评估这种“新物理学”对现有火山碎屑密度电流模型的影响，重要的是，它们预测的耗尽距离。在与厄瓜多尔地球物理研究所合作下，这些新结果将用于批判性地评价目前的危险地图和减轻风险战略。因此，这项研究计划将提供一个平台，在此基础上可以为全球火山爆发建立可靠的PDC模型和预测。最终，这将使这些危险的地球物理流动的人力和经济成本降到最低。

The transport of gas and particle mixtures are a widespread phenomenon, both in industry and in the natural world. In the geosciences these gas-particle mixtures exist during sediment transport by the wind, meteorite impacts, rock and snow avalanches and explosive volcanic eruptions. In this research proposal we focus on gas-particle flows produced during volcanic eruptions, although we expect the physics uncovered will be portable to these other disciplines and plan to host a multidisciplinary workshop to facilitate knowledge transfer. Ten percent of the world's population (i.e. 100's of millions) live within 100 km of an active volcano. Furthermore, this number is set to rise with the increasing global population. During all explosive volcanic eruptions pyroclastic density currents (PDCs) can form - high temperature multiphase mixtures of rock and gas that rapidly flow away from the volcanic vent. These phenomena are the most lethal of all volcanic hazards and are responsible for more than a third of volcanic related fatalities. However, despite the lethal nature of pyroclastic density currents we currently lack accurate models to forecast the runout distance achieved by these flows and thus any hazard maps and mitigation strategies are inherently limited. To improve our numerical forecasting models, we need to understand the complex internal flow dynamics within these 'opaque' and hazardous flows. Direct internal observation is not possible, but controlled laboratory experiments offer a way to rigorously study these otherwise hidden phenomena. Specific and targeted experiments aimed at providing key input parameters for numerical flow simulations offer a clear way forward.During this research programme, we will use two types of scaled laboratory experiments to investigate these phenomena in a controlled and observable manner. Firstly, experiments using a rheometer will measure the flow behaviour (e.g. viscosity; mobility) of natural volcanic gas-particle mixtures. We will quantify how the particle size distribution and gas flow conditions effect the mobility of these flows. Secondly, natural volcanic particles (e.g. bubbly volcanic glass, crystals, rock fragments) will be suspended in air to form a gas-particle mixture at a range of conditions relevant to the volcanic system. The experimental products will be characterised for their size, shape, and abundance as a function of time (i.e., transport distance). This will allow us to quantify the rate at which volcanic particles break, round and change their relative abundance during flow. This will lead to a comprehensive understanding of how the particle textural properties (e.g. size, shape) change during flow and how, in turn, these changes alter the mobility of pyroclastic density currents. These experimental data, on flow conditions and particle breakage, derived from new state-of-the-art techniques will be incorporated into existing 3D numerical flow models. Numerical simulations will be performed on a case volcano (Tungurahua, Ecuador). This will allow us to illustrate, and quantitatively assess, the impact this 'new physics' has on existing pyroclastic density current models and importantly, their predicted run out distances. In collaboration with the Geophysical Institute of Ecuador these novel results will be used to critically evaluate current hazard maps and risk mitigation strategies. This research programme will therefore provide a platform upon which reliable PDC models and forecasts can be built for eruptions worldwide. Ultimately this will minimise the human and economic cost of these hazardous geophysical flows.