Collaborative Research: Multi-scale Dynamics in Explosive Volcanic Eruptions

爆炸性火山喷发是地球上能量最大的流动之一，其中最大的一次可能会对全球产生影响。更常见、规模较小的地震是一种近距离的危险，仍然包括几公里的规模。尽管这些流动的大小和持续时间很长，但这些流动中的质量和能量转移基本上是由小得多的时空尺度上的过程控制的，在这些过程中，单个粒子相互作用，与气体或流动所经过的表面相互作用。我们预测大规模火山流行为的能力最终可能会受到我们对非常小规模或微物理过程的理解的限制。这项提议在实验室中研究了一套粒子尺度的质量和能量转移机制，目的是了解这些过程的物理过程，并将它们纳入对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、科学家和文职官员提供一种技术，以便更好地了解火山骚乱期间的危险。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级的分辨率也可能永远不可能。考虑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。过去关于蒸汽爆炸的工作表明，从实验中发展出来的亚网格模型可以很容易地与多相数值模拟相耦合。更重要的是，这些次网格关系对于预测火山沉积物中反映的动力学至关重要；忽略次网格过程的模型可能无法在火山沉积物中产生几个数量级的能量转移。这项工作将集中在1)颗粒与气体之间的热传递，2)活跃流动中的粉碎和团聚，以及演变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，特别是床层悬浮颗粒的作用。所有拟议的实验都将使用与自然流动中类似的材料和条件进行，从而最大限度地减少扩展到大规模多相流的潜在困难。在所提出的方法中，数值模型与实验数据有机地联系在一起。DUAL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，而实验的优势是详细了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.