Collaborative Research: Role of Turbulence Structure in Bedload Transport

足够快的流体流过可侵蚀的沙砾层，夹带着表面的颗粒，然后这些颗粒作为层积物滚动和跳跃。该项目旨在建立流体流动和层质输运之间的定量预测关系，更精确地结合湍流引起的波动流速的作用，湍流总是存在的。这种关系将适用于在溪流和波浪下普遍存在的不均匀或不稳定的流动，并将为从波纹和沙丘到沙洲和河道到分选和分级的侵蚀和沉积特征的形成提供新的见解。该项目将研究(1)简单河床（如河床板和低沙丘）上的湍流结构和河床输运；(2)河床粗糙度沿水流方向突变的湍流结构；(3)湍流结构和固定天然河床颗粒的波动力；(4)湍流结构和夹带过程中作用于河床颗粒的流体力；(5)移动的河床颗粒对流体速度和湍流结构的改变。此外，该项目将进一步发展我们的泥沙输运离散颗粒计算模型，方法是：(1)更好地处理河床和河床颗粒的升力以及颗粒加速时流体的反作用力，(2)将其与现有的三维河床湍流数值模拟相结合。提出的研究解决了三个相互交织的问题：施加在床上和在床上移动的颗粒上的力如何与近床流体速度的时空分布相关，这些颗粒的夹带和运动如何与力相关，以及近床流体速度如何被移动的床载的存在所改变。

Sufficiently fast fluid flows over erodible beds of sand and gravel entrain surface particles, which then roll and hop along as bedload. This project aims to develop quantitative predictive relations between fluid flow and bedload transport that more precisely incorporate the role of the fluctuating flow velocities due to turbulence, which always is present. Such relations will apply to the non-uniform or unsteady flows that are ubiquitous in streams and beneath waves and will provide new insight into the formation of erosional and depositional sedimentary features from ripples and dunes to bars and channels to sorting and grading. The project will investigate (1) turbulence structure and bedload transport over simple bedforms such as bedload sheets and low dunes, (2) turbulence structure over abrupt streamwise changes in bed roughness, (3) turbulence structure and the fluctuating forces on fixed natural bed particles, (4) turbulence structure and the fluid forces that act on bedload particles during entrainment, and (5) the modification of fluid velocity and turbulence structure by moving bedload particles. In addition, the project will further develop our discrete-particle computational model for sediment transport by (1) incorporating into it better treatment of the lift forces on bed and bedload particles and of the reaction forces on the fluid as the particles accelerate and (2) coupling it with an existing numerical simulation of turbulent flow over three-dimensional bedforms. The proposed research addresses three intertwined issues: how the forces exerted on particles on the bed and moving over it are related to the temporal and spatial distribution of the near-bed fluid velocities, how the entrainment and motion of those particles are related to the forces, and how the near-bed fluid velocities are modified by the presence of the moving bedload.