Collaborative Research: Multi-scale Dynamics in Explosive Volcanic Eruptions

火山爆发是地球上最具活力的流动之一，其中最大的一次可能会产生全球影响。更常见的较小的事件是近距离危险，仍然包括几公里的规模。尽管它们的尺寸很大，持续时间很长，但这些流动中的质量和能量传递基本上是由小得多的空间和时间尺度上的过程控制的，在这些过程中，单个粒子彼此相互作用，与气体相互作用，或者与流动所经过的表面相互作用。我们预测火山流大规模行为的能力最终会受到我们对非常小规模或微物理过程的理解的限制。该提案在实验室中研究了一套粒子级质量和能量转移机制，目的是了解这些过程的物理学，并将其纳入爆炸性火山爆发的大规模模拟中。这项工作的长期目标之一是为学生、科学家和政府官员提供一种技术，以便更好地了解火山动荡时期的危险。计算能力和算法设计的进步使人们能够详细研究火山爆发时产生的湍流结构。然而，即使增加计算资源，在大规模三维模拟中实现米级以下的分辨率可能永远不可能。核算次网格尺度的物理过程需要建立火山物质和条件的本构关系。过去对蒸汽爆炸的研究表明，从实验中发展起来的亚网格模型可以很容易地与多相数值模拟相结合。更重要的是，这些次网格的关系是至关重要的预测反映在火山沉积物的动态模型，忽略次网格的过程可能无法产生的能量转移表现在火山沉积物的几个数量级。这项工作将集中在1）颗粒和气体之间的热传递，2）粉碎和团聚在活跃的流动和不断变化的粒度分布对流动的动力学的影响，和3）颗粒边界的相互作用，特别是从床的再悬浮颗粒的作用。所有拟议的实验都将使用与自然流相似的材料和条件进行，从而最大限度地减少缩放到大规模多相流的潜在困难。在所提出的方法中，数值模型是完整地连接到实验数据。双重方法强调了这两种技术的优势：数值模型的优势是解决非线性，复杂耦合方程和确定紧急行为的能力，实验的优势是详细了解小尺度下的物理过程。

Explosive volcanic eruptions are some of the most energetic flows on the planet, the largest of which can have global impact. The more common, smaller, events are a proximal hazard and still encompass scales of several kilometers. Despite their large size and long duration, mass and energy transfer in these flows are fundamentally controlled by processes at much smaller spatial and temporal scales, where individual particles interact with each other, with gas, or with the surface over which the flows travel. Our ability to predict large-scale behavior of volcanic flows can ultimately be limited by our understanding of very small-scale, or microphysical, processes. This proposal examines a suite of particle-scale mass and energy transfer mechanisms in the laboratory with the aim of understanding the physics of these processes and to incorporate them into large-scale simulations of explosive volcanic eruptions. One of the long term goals of this effort is to provide a technology for students, scientists and civil officials to better understand hazards during times of volcanic unrest.Advances in computational power and algorithm design enable detailed studies of the turbulent structures that develop in explosive volcanic eruptions. However, even with increases in computational resources, achieving resolution below meter-scale in large-scale three-dimensional simulations may never be possible. Accounting for subgrid-scale physical processes requires developing constitutive relationships for volcanic materials and conditions. Past work on steam explosions has shown that subgrid models developed from experiments can be readily coupled to multiphase numerical simulations. More importantly, these subgrid relations are critical for predicting the dynamics reflected in volcanic deposits; models that neglect subgrid processes can fail to produce the energy transfer manifest in volcanic deposits by several orders of magnitude. This work will focus on 1) heat transfer between particles and gas, 2) comminution and agglomeration in active flows and the impact of a evolving grain size distribution on the dynamics of flows, and 3) particle-boundary interactions, and in particular the role of resuspended particles from the bed. All of the proposed experiments will be conducted with materials and conditions similar to those in natural flows, minimizing the potential difficulties with scaling to large-scale multiphase flows. In the methodology proposed, the numerical models are integrally connected to the experimental data. The dual approach emphasizes the strength of both techniques: the strength of numerical models is the ability to solve non-linear, complexly coupled equations and determine emergent behavior, and the strength of the experiments is to understand in detail the physical processes operating at small scales.