Impact of Collapsing Gas-particle Jets: The Initial Conditions for Pyroclastic Flows and Surges

火山碎屑流和涌（这里称为火山碎屑密度流，或PDC）是火山爆发的基本现象。 人们对PDC的了解很少，因为它们涉及气体和颗粒（如火山灰和岩石碎片）的复杂混合物在很大的速度和温度范围内的流动。 这些流动是最具破坏性的火山现象，但对它们的认识不足却限制了科学家们的研究。能够以最优化的方式减轻危害和后果来预测他们的行为。 由于其不可预测性和危险性，在自然界中对其动态的直接测量是罕见的。 相反，我们对特定火山的PDCs的理解大部分依赖于以前喷发的沉积物的特征，沿着实验和计算模型。 许多火山碎屑密度流是由气体和碎片喷流的回落或塌陷造成的。 当混合物撞击地面时，它们从垂直下落的流动转变为侧向流动（PDC），这种转变由于颗粒和气体对速度和方向突然变化的不同反应方式而变得复杂。撞击过程对产生的侧向流的特性（速度、颗粒浓度）有着至关重要的影响，但这是一个以前的定量研究尚未解决的问题。 该项目将结合联合收割机数值模拟和实验，并辅以实地研究，以解决以下假设：（1）撞击动力学取决于坍塌高度、坍塌颗粒浓度、碎屑大小和密度分布、撞击地点地面坡度以及坍塌持续时间;（2）垂直脉冲射流的崩溃倾向于供给稀释的PDC（火山碎屑涌）通过排出微粒和气体;（3）持续射流的坍缩可以在撞击后立即供给浓缩和/或稀释的PDC，这取决于坍缩混合物的碎屑尺寸和密度群体;（4）撞击动力学确定了初级产品开发中心的初始条件，并应告知用于危险评估的简化流动模型。这项工作将定义射流/喷泉塌陷馈送流的初始条件，以便简化的PDC模型可以更好地用于指导决策者应对火山灾害。该项目开发的实验装置将可供研究社区的任何人使用。 预计该研究的流体动力学方面将应用于涉及气体-颗粒流动的工业过程。工作的各个方面将包括一个博士学位的关键部分。论文 此外，还将为大学生制作一个视频教学单元，演示综合方法，帮助学生直观地了解地质流体动力学的一个重要方面-多相流动力学。

Pyroclastic flows and surges (here called pyroclastic density currents, or PDCs) are a fundamental phenomenon of explosive volcanic eruptions. PDCs are poorly understood because they involve flow of complex mixtures of gasses and particles (such as volcanic ash and rock fragments) at a wide range of speeds and temperatures. The flows are the most destructive of volcanic phenomena, but poor understanding has limited scientists? ability to predict their behavior in a way that optimizes mitigation of hazards and consequences. Direct measurements of their dynamics in nature are rare because of their unpredictability and danger. Instead, much of our understanding of PDCs at a given volcano relies on characteristics of deposits from previous eruptions, along with experiments and computational modeling. Many pyroclastic density currents are caused by the falling back, or collapsing, of jets of gas and debris. As the mixtures impact the ground they change from vertically falling flows to lateral flows (the PDCs), and this transition is complicated by the different ways that particles and gas respond to sudden changes in speed and direction. The impact process critically influences the characteristics of the resulting lateral flows (speeds, particle concentrations), yet this is a topic that has not been addressed by previous quantitative research. The project will combine numerical modeling and experiments, complemented by field studies, to address the following hypotheses: (1) Impact dynamics depend upon collapse height, collapsing particle concentration, clast size and density distribution, slope of the ground at the impact site, and duration of collapse; (2) Collapse of vertical, impulsive jets tend to feed dilute PDCs (pyroclastic surges) by expulsion of fines and gas; (3) Collapse of sustained jets can feed concentrated and/or dilute PDCs immediately upon impact, depending upon the clast size and density population of the collapsing mixture; (4) Impact dynamics define initial conditions for the PDCs and should inform simplified flow models for hazards assessments. The work will define the initial conditions for jet/fountain-collapse fed flows so that simplified PDC models can be better used to guide decision makers coping with volcanic hazards. The experimental setup developed by the project will be available for follow-on used by anyone in the research community. It is anticipated that the fluid dynamic aspects of the research will have applications to industrial processes that involve gas-particle flows. Aspects of the work will comprise a key part of a Ph.D. dissertation. Additionally, a video-based education module will be prepared for university students, which will demonstrate the integrated approach and aid in developing an intuitive understanding of multiphase flow dynamics as a key aspect of geological fluid dynamics.