Segregation in geophysical mass flows

许多人还记得1980年圣海伦斯火山爆发后，翻腾的泥石流沿着下山的道路，或者关于另一场雪崩摧毁了半个村庄，造成20人死亡的报道（冰岛，Flateyri, 1995）。也许不太出名的是火山碎屑流，它在一个“安全”的山脊上杀死了一群科学家和记者（1991年，日本云禅山），或者是历史证据和未来预测，水下滑坡会导致海啸传播数百公里，甚至可能到达美国东海岸（加那利群岛的康伯雷别哈火山）。由于流体、固体和气相之间的持续相互作用，这些地球物理流动是如此强大和具有破坏性。这些地球物理质量流中的分离是不均匀性的驱动力，并导致流的流动性增强。地球物理物质流对社区构成重大危害，并直接影响一个地区的环境、基础设施、经济和旅游。因此，对离析对流动动力学的影响有一个基本和彻底的了解是必要的，以减轻损害和生命损失。地球物理质量流中流体、固体和气相之间的连续相互作用产生了一种难以表征的复杂物质。由于材料的主体是由不同大小和密度的颗粒组成的，材料自然会分离。这种组分的自然筛分导致相之间和相内的不均匀性，并直接影响流动特性。地球物理质量流中涉及分离的一个回归基本的问题是，在重力作用下，干燥的粒状物质沿斜坡向下移动。流体成分不存在，气相是空气，但类似的物理原理适用于固体成分的分离。提出的工作旨在了解干燥颗粒物质中的偏析，为地球物理质量流中的偏析提供核心物理学。我们不用跑很远就能在自己的厨房里发现种族隔离。在家里摇一盒麦片就是所谓的“巴西坚果效应”的一个很好的例证——较大的颗粒最终浮在表面，而较小的颗粒沉到底部。直觉上，人们可能会认为这种（尺寸）筛分或分离仅仅是由于较小的颗粒落入较大颗粒之间的孔。这种解释在性质上是正确的，但事实证明，要建立一个能够准确预测偏析速度和程度的数学理论是极其困难的。同样大小但密度不同的颗粒也会发生偏析，因为密度较大的颗粒会移动到混合物的底部。最近关于现象学模型发展的工作，由于实验数据的匮乏，在验证上一再遇到限制。现在是扩展关于偏析的实验数据量和使用改进的现代仪器来量化这些流中偏析的速度和程度的绝佳时机。本研究计划描述了一个框架，用于对密度、大小和初始配置不同的干颗粒状珠混合物进行实验室实验，从而获得一个大型的分离结果实验数据库。DAMTP卓越的实验设施和颗粒流动数值模拟方面的专业知识，加上申请人强大的研究背景，将确保研究项目的成功完成。该建议的优势在于将新的尖端实验室实验与数值代码中分离机制的模拟相结合。该建议的多学科性质由理论建模工作和实验领域组成部分加以补充，从而形成基于现实的模型，可供科学家和决策者使用。

Many people remember the churning debris flow finding its path downhill after the eruption of Mt. St. Helens in 1980 or reports about another snow avalanche that destroyed half a village and killed 20 people (Flateyri, Iceland, 1995). Perhaps less well-known is the pyroclastic flow that killed a group of scientists and journalists from a ``safe'' ridge (Mount Unzen, Japan, 1991) or the historic evidence and future predictions about underwater slides that cause tsunamis to travel many hundreds of kilometers, possibly even reaching the eastern coast of the USA (Cumbre Vieja volcano, Canary Islands). These geophysical flows are so powerful and destructive because of the continuous interaction between the fluid, solid and gas phases. Segregation within these geophysical mass flows is the driving force of inhomogeneity and results in enhanced mobility of the flow. Geophysical mass flows form a significant hazard for communities and directly influence the environment, infrastructure, economy and tourism of a region. A fundamental and thorough understanding of the influence of segregation on flow dynamics is therefore necessary to mitigate damage and loss of life. The continuous interaction between the fluid, solid and gas phases in geophysical mass flows creates a complicated material that is hard to characterize. As the bulk of the material is composed of particles with different sizes and densities, the material naturally segregates. This natural sieving of the components results in inhomogeneity between and within the phases and directly influences the flow characteristics. A back-to-basic problem involving segregation in geophysical mass flows is the transport of a dry granular material down an incline under the action of gravity. The fluid component is absent and the gas phase is air, but similar physical principles apply with regards to segregation of the solid components. The proposed work aims to understand the segregation in a dry granular material, providing the core physics for segregation in geophysical mass flows. We don't have to travel far to find segregation in our own kitchen. Shaking a box of cereal at home is a good illustration of the so-called 'Brazil nut effect' --- the larger particles end up at the surface while the smaller particles sink to the bottom. Intuitively one may think that this (size) sieving, or segregation, is simply due to the smaller particles falling into the holes between the larger particles. This explanation is qualitatively correct, but is has proved extremely difficult to construct a mathematical theory that can accurately predict the speed and degree of segregation. Segregation also occurs for same size but different density particles, as the denser particles move to the bottom of the mixture. Recent work on the development of phenomenological models encounters over and over again limitations on validation because of the meager amount of experimental data. Now is an excellent time to extend the amount of experimental data on segregation and use improved modern instrumentation to quantify the speed and degree of segregation within these flows. This research proposal describes a framework to perform laboratory experiments on dry granular mixtures of beads where the density, size and initial configuration are varied so as to obtain a large experimental database of segregation outcomes. The excellent experimental facilities and the expertise in numerical modeling of granular flows at DAMTP combined with the strong research background of the applicant will ensure the successful completion of the research project. The strength of the proposal is the combination of new cutting-edge laboratory experiments with simulations of segregation mechanisms in a numerical code. The multidisciplinary nature of this proposal is complemented by a theoretical modeling effort and an experimental field component resulting in reality-based models that may be used by scientists and policymakers alike.