Direct Numerical Simulation of Incipient Sediment Motion and Hydraulic Conveying

A theoretical study of the incipient sediment motion and the first stages of hydraulic transport is presented. The fluid flow and the motion of 200 heavy particles in a channel are simulated. The direct numerical simulations of fluid flow and particle motion are based on the distributed Lagrange multiplier/fictitious domain method of Glowinski et al. [Int. J. Multiphase Flow 1999, 25, 755-794). First, the particles are allowed to settle under slow flow conditions. Then, as the flow intensity is increased, which is expressed by a characteristic Reynolds number, initially intermittent and then sustained particle motion is detected at the top of the sediment. A critical Shields number is identified and compared with literature data. Further increase in flow intensity results in the increase of the number of particle layers actively participating in the motion and in their rate of transport. Moreover, the fluid flow and the moving sediment become unstable and display characteristic waves which coarsen during the course of the simulations in a way similar to the coarsening of ripples in sand beds exposed to water or air flows. A final feature of the study is the implementation of a discrete element model (DEM) to account for tangential forces and friction during the particle collisions. The collective behavior remains qualitatively the same as in a simpler model which considers only central elastic forces between the particles.