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

塌缩气体粒子射流的影响：火山碎屑流和涌动的初始条件

• 批准号: 1623793
• 负责人: Greg Valentine
• 金额: $ 29.31万
• 依托单位: SUNY at Buffalo
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2016
• 资助国家: 美国
• 起止时间: 2016-08-01 至 2021-07-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1623793&HistoricalAwards=false
• 关键词: Impact; Collapsing; Gas; particle; Jets

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.

火山碎屑流和火山喷发(这里称为火山碎屑密度流，简称PDC)是火山爆发的基本现象。人们对PDDC知之甚少，因为它们涉及气体和颗粒(如火山灰和岩石碎片)在广泛的速度和温度范围内的复杂混合物的流动。这种流动是最具破坏性的火山现象，但对它的理解不足限制了科学家的研究？能够预测他们的行为，以优化缓解危险和后果的方式。由于它们的不可预测性和危险性，直接测量它们在自然界中的动态是罕见的。相反，我们对给定火山的PDC的理解在很大程度上依赖于以前喷发的沉积物的特征，以及实验和计算模型。许多火山碎屑密度流是由气体和碎片喷流的后退或坍塌造成的。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)，这种转变因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。撞击过程严重影响所产生的侧向流动的特征(速度、颗粒浓度)，但这是以前的定量研究没有解决的问题。该项目将结合数值模拟和实验，并辅之以现场研究，以解决以下假设：(1)冲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、撞击地点地面的坡度和坍塌持续时间；(2)垂直脉冲射流的坍塌倾向于通过喷射细粉和气体来供给稀释的PDC(火山碎屑涌流)；(3)持续射流的坍塌可以在撞击后立即供给浓缩和/或稀释的PDC，这取决于坍塌混合物的碎屑大小和密度群体；(4)冲击动力学确定了港口和港口设施的初始条件，并应为危险评估提供简化的流动模型。这项工作将确定喷流/喷泉崩塌提供的流动的初始条件，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中的任何人后续使用。预计这项研究的流体动力学方面将应用于涉及气固两相流动的工业过程。这项工作的各个方面将构成博士论文的关键部分。此外，还将为大学生准备一个基于视频的教学模块，该模块将演示综合方法，并有助于直观地理解多相流动力学作为地质流体动力学的一个关键方面。