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，提供了一个令人信服的策略，连接粒子运动的统计描述与耦合流体-粒子行为的动力学公式。 这项研究的目的是将这种风格的分析在推移质泥沙输移的研究，增长之间的连接小规模的机械运输和大规模的河流形态动力学和示踪剂运动的问题的描述。 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系综，特别是为了澄清在计算输运率时活动粒子斑块的影响;（2）从统计力学参数出发，寻求粒子速度的指数状分布的理论公式;（3）进行一系列物理和计算实验，以充分阐明颗粒扩散率与总体平均颗粒速度之间的关系;（4）利用我们的泥沙输运数值模拟系统，首次建立了具有四向耦合的离散颗粒输运模式（固体和流体之间的质量和动量交换），探索湍流结构和沉积物运动之间的详细相互作用;（5）开展理论、实验和计算工作，重点研究颗粒活动和速度的协方差在计算泥沙通量中的影响，以及响应湍流的混合颗粒运动的时空组织。例如水坝、灌溉用水和采矿活动造成的灾害仍然难以预测和减轻。 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，以及在不同的河流排放过程中发生的污染河流沉积物的运输和命运。 这项工作的目的是制定一个理论框架，通知实验和先进的计算，用于描述推移质泥沙，示踪粒子和沉积物携带的物质的运输。 这项工作将澄清如何流动湍流和相关的斑块颗粒夹带和沉积有助于运输率的变化。 该理论的概率基础，以及耦合流体-颗粒系统的先进计算，代表了理解泥沙输运物理学的重要技术融合。 此外，这项工作还包括：（1）学生教育的合作结构，这将极大地丰富来自两所大学的学生的知识经验;（2）开发来自实验和数值模拟可视化的引人注目的教学工具;以及（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.