COLLABORATIVE RESEARCH: The statistical mechanics of bed load sediment transport: Meshing theory, experiments and advanced computations of coupled fluid-particle behavior

粒子运动的概率描述，借用统计力学的关键思想，为将粒子运动的统计描述与耦合流体-粒子行为的动力学公式联系起来提供了一种令人信服的策略。本研究旨在将这种分析方式纳入河床泥沙输运的研究中，在输运的小规模机械描述与河流形态动力学和示踪剂运动的大规模问题之间建立联系。这项工作包括：(1)以类似于经典统计力学公式的方式正式定义粒子构型和速度的集合，特别是澄清在计算输运率时活跃粒子斑块的影响；(2)从统计力学论证中寻求粒子速度指数分布的理论表述；(3)开展一系列物理和计算实验，以充分阐明粒子扩散率与系综平均粒子速度之间的关系；(4)利用我们的泥沙输运数值模拟系统，第一个具有四向耦合（固体和流体之间的质量和动量交换）的离散颗粒输运模型，探索湍流结构与泥沙运动之间的详细相互作用；(5)开展理论、实验和计算工作，重点研究粒子活动和速度协方差对计算泥沙通量的影响，以及混合粒径运动对湍流响应的时空组织。人类对河流的改变所造成的负面影响，如筑坝、撤除灌溉和采矿活动，仍然难以预测和缓解。这部分是因为目前对沉积物运动的了解仍然不足以预测许多河床的几何变化，以及在不同的河流流量期间发生的污染河流沉积物的运输和命运。这项工作的目的是通过实验和先进的计算来阐述一个理论框架，用于描述床载沉积物、示踪颗粒和沉积物携带物质的运输。这项工作将阐明流动湍流和相关的颗粒夹带和沉积的斑块如何导致输运率的变化。该理论的概率基础，连同耦合流体-颗粒系统的先进计算，代表了理解沉积物输运物理的重要技术融合。此外，这项工作涉及：(1)学生教育的协作结构，这将极大地丰富两所大学学生的智力经验；(2)开发基于实验可视化和数值模拟的引人注目的教学工具；(3)为科学界提供独一无二的数据集。

Probabilistic descriptions of particle motions, borrowing key ideas from statistical mechanics, offer a compelling strategy for connecting statistical descriptions of particle motions with dynamical formulations of coupled fluid-particle behavior. This research is aimed at incorporating this style of analysis in studies of bed load sediment transport, growing the connection between small-scale mechanistic descriptions of transport and larger-scale problems of river morphodynamics and tracer motions. The work involves: (1) formally defining an ensemble of particle configurations and velocities in a manner similar to formulations from classic statistical mechanics, notably to clarify effects of active particle patchiness in calculating transport rates; (2) pursuing a theoretical formulation of the exponential-like distribution of particle velocities from statistical-mechanics arguments; (3) pursuing a set of physical and computational experiments designed to fully clarify the relationship between the particle diffusivity and the ensemble-average particle velocity; (4) using our numerical sediment transport modeling system, the first discrete-particle transport model to have four-way coupling (mass and momentum exchange between the solids and fluid), to explore detailed mutual interactions between turbulence structures and sediment motions; and (5) pursuing theoretical, experimental and computational work focused on the effects of covariance in particle activities and velocities in computing sediment fluxes, and the spatiotemporal organization of motions of mixed particle sizes in response to turbulence.The negative consequences of human modifications to rivers, such as by dams, irrigation withdrawal, and mining activities, remain difficult to forecast and mitigate. This is partly because current understanding of sediment movement remains insufficient to predict many of the river bed geometric changes, and the transport and fate of contaminated river sediments that occur during varying river discharges. The work is aimed at elaborating a theoretical framework, informed by experiments and advanced computations, for describing the transport of bed load sediment, tracer particles, and sediment-borne substances. The work will clarify how flow turbulence and the associated patchiness in particle entrainment and deposition contribute to variability in transport rates. The probabilistic basis of the theory, together with the advanced computations of coupled fluid-particle systems, represents an important melding of techniques for understanding the physics of sediment transport. In addition, the work involves: (1) a collaborative structure of student education that will greatly enrich the intellectual experiences of students from two universities; (2) developing compelling teaching tools deriving from visualizations of experiments and numerical simulations; and (3) providing one-of-a-kind data sets to the science community.