Collaborative Research: Role of Turbulence Structure in Bedload Transport

流速足够快的流体流过可侵蚀的沙和砾石床时，会夹带表面颗粒，然后这些颗粒作为推移质沿着滚动和跳跃。 该项目旨在开发流体流动和推移质输运之间的定量预测关系，更精确地将湍流引起的波动流速的作用结合起来，湍流总是存在的。 这种关系将适用于溪流和波浪下普遍存在的不均匀或不稳定流动，并将为从涟漪和沙丘到沙洲和水道到分选和分级的侵蚀和沉积沉积特征的形成提供新的见解。 该项目将研究（1）简单底形（如推移质片和低沙丘）上的湍流结构和推移质输运，（2）床面粗糙度沿流向突变的湍流结构，（3）湍流结构和作用在固定天然床颗粒上的脉动力，（4）湍流结构和夹带过程中作用在推移质颗粒上的流体力，推移质颗粒运动对水流速度和紊流结构的改变。 此外，该项目还将进一步发展我们的离散颗粒泥沙输运计算模型，方法是：（1）在模型中更好地处理作用在床面和推移质颗粒上的升力以及颗粒加速时作用在流体上的反作用力;（2）将模型与现有的三维底形紊流数值模拟相结合。 拟议的研究解决了三个相互交织的问题：如何施加在床上的颗粒上的力，并在它上面移动有关的时间和空间分布的近床流体速度，如何夹带和运动的这些颗粒是有关的力，以及如何近床流体速度被修改的存在下，移动推移质。

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.