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鉴了统计力学的关键思想，为粒子运动的统计描述与流体-颗粒耦合行为的动力学公式提供了一种引人注目的策略。这项研究的目的是将这种分析风格纳入到推移质泥沙研究中，加强对输沙的小尺度力学描述与河流地貌动力学和示踪运动的大尺度问题之间的联系。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集合，特别是为了阐明活跃粒子斑块在计算输运率时的影响；(2)从统计力学论证中寻求粒子速度的类指数分布的理论公式；(3)进行一系列物理和计算实验，以充分阐明粒子扩散率和集合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，探索湍流结构和泥沙运动之间的详细相互作用；以及(5)开展理论、实验和计算工作，重点研究颗粒活动和速度的协方差对泥沙通量的影响，以及混合颗粒尺寸运动的时空组织对湍流的响应。人类对河流的修改，如大坝、灌溉取消和采矿活动，仍然难以预测和缓解。这在一定程度上是因为目前对泥沙运动的了解仍然不足以预测河床的许多几何变化，以及在不同的河流排放过程中受污染的河流沉积物的运移和命运。这项工作的目的是阐述一个理论框架，以实验和先进的计算为基础，描述推移质泥沙、示踪颗粒和泥沙物质的输送。这项工作将阐明流动湍流以及颗粒夹带和沉积中的相关斑块如何导致传输速率的变化。该理论的概率基础，加上流体-颗粒耦合系统的高级计算，代表了理解泥沙运移物理的重要技术融合。此外，这项工作涉及：(1)学生教育的合作结构，这将极大地丰富两所大学学生的智力经验；(2)开发基于实验和数值模拟的可视化的引人注目的教学工具；以及(3)向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.