Collaborative Research: Multi-scale Dynamics in Explosive Volcanic Eruptions

火山爆发是地球上最具活力的火山活动之一，其中规模最大的火山爆发会对全球产生影响。更常见的、较小的事件是近距离的危害，仍然包括几公里的范围。尽管这些流动的规模很大，持续时间也很长，但它们中的质量和能量传递基本上是由更小的空间和时间尺度上的过程控制的，在这些过程中，单个粒子相互作用，与气体相互作用，或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可能会受到我们对非常小尺度或微物理过程的理解的限制。本提案在实验室中研究了一套粒子尺度的质量和能量传递机制，目的是了解这些过程的物理学原理，并将它们纳入大规模的火山爆发模拟中。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害。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中形成的湍流结构。然而，即使随着计算资源的增加，在大规模三维模拟中实现低于米尺度的分辨率也可能永远不可能。考虑亚网格尺度的物理过程需要发展火山物质和条件的本构关系。过去对蒸汽爆炸的研究表明，从实验中建立的子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，这些亚网格关系对于预测火山沉积物中反映的动力学至关重要；忽略亚网格过程的模型可能无法产生火山沉积物中几个数量级的能量转移。这项工作将集中在1)颗粒和气体之间的传热，2)活跃流动中的粉碎和团聚以及不断变化的颗粒尺寸分布对流动动力学的影响，以及3)颗粒边界相互作用，特别是从床上重悬浮颗粒的作用。所有拟议的实验都将在与自然流动相似的材料和条件下进行，以最大限度地减少扩展到大规模多相流的潜在困难。在提出的方法中，数值模型与实验数据完整地联系在一起。双重方法强调了这两种技术的优势：数值模型的优势在于解决非线性、复杂耦合方程和确定紧急行为的能力，而实验的优势在于详细理解在小尺度上运行的物理过程。

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.