Response of Bedload and Bedforms to Near-Bed Flow Structures

In this study, the large eddy simulation (LES) under the Eulerian method is used to solve the Navier-Stokes equations for turbulent flow simulation. The Lagrangian point-particle model is applied to track particle trajectories and to calculate the forces exerted by the flow on the particles, and the particle–wall and particle–particle collisions are also accounted for. Nine simulations cases were carried out along the line of previous experiments that considered different bedform regimes, namely, ripples and dunes. The resulting bedload intensity parameter and the simulated bedforms for all the cases agree with the results obtained from the existing classical formulas. The three-dimensionality of sediment transport randomly occurs due to the turbulent flow. Coherent structures are formed as the near-bed low-speed fluid streaks entrain into the mainstream over the stoss-side of the ripples, and the high-speed fluid streaks from the mainstream rush toward the bed over the leeside. As a result, kolk–boil and hairpin vortices develop nearby. Ejection and sweep prevail near the bed, where the particles transport. The phenomenon disappears as the flow intensity increases. The presence of bedload particles also modifies the propagation angle and range of velocity fluctuation, especially in the streamwise direction. To conclude, a logistic regression formula for bedload intensity parameters, accounting for the fluid rotation, deformation, and translation terms that signify the fluid vortical motions, is obtained. It reveals that as long as these three terms are accurately quantified, the bed shear stress and bedload transport rate can be effectively estimated.