Discrete element modelling of bedload transport | Modélisation par éléments discrets du transport solide par charriage

Discrete element method (DEM) is a numerical method to simulate an assembly of par- ticles, which has been widely used in mechanics (soil, rock) and granular physics. DEM consists in considering undeformable particles and modelling the intergranular interactions with simple laws (e.g. linear elastic and Coulomb friction law). The expression of the equation of motion on each particle considering the nearest neighbor interactions allows then to solve the dynamical behavior of the system explicitely. Since its introduction more than thirty years ago, this type of model has proven its ability to describe well the behavior of granular media in several dirent situations, from quasi-static system to ow of granular media. Bedload transport is a phenomenon observed in a stream and is characterized by particle trans- port restricted to the interface between uid ow and immerged granular media, where particles are rolling, sliding or in saltation over the bed. This situation corresponds to the larger particles transported on the bed in stream channels and has a great inuence on geomorphology. Physical mechanisms and processes ruling bedload transport and more generally coarse-particle/uid sys- tems are poorly known. This is partly due to the small attention given to the role of granular interactions. Starting from these considerations, we used DEM to reproduce experiments carried out with spherical glass beads in an experimental steep and narrow ume. This was done in order to focus on the granular interactions and to have easily access to parameters not available in the experiment. DEM open-source code Yade was coupled with a simplied uid model, taking into account the dirent hydrodynamical interactions (buoyancy, drag, lift...) experienced by the par- ticles. Numerical results obtained from the simulation are compared with an experimental data set es- tablished previously at the laboratory. It consists in monodisperse and bidisperse mixtures of coarse spherical glass beads entrained by a shallow turbulent and supercritical water ow down a steep channel with a mobile bed. The particle diameters were 4 and 6mm, the channel width 6.5mm (about the same width as the coarser particles) and the channel inclination was typically 10%. The water ow rate and the particle rate were kept constant at the upstream entrance and adjusted to obtain bedload transport equilibrium. Flows were lmed from the side by a high-speed camera. Using image processing algorithms made it possible to determine the position, velocity and trajectory of both smaller and coarser particles. The comparison between numerical and experimental results focus on streamwise velocity, concen- tration and sediment rate normal prole. For bidisperse mixture, a particular attention is given to the segregation phenomenon. We observe in both cases a fair agreement between DEM and experiments.