Collaborative Research: Role of Turbulence Structure in Bedload Transport

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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.