Large Eddy Simulation of Sediment Deformation in a Turbulent Flow by Means of Level-Set Method

Abstract

Sediment transport in a turbulent channel flow over the sediment bed with a ripple structure is numerically simulated by means of a large eddy simulation. The filtered Navier-Stokes equations for the channel flow and the filtered advection-diffusion equation with a settling term for the suspended sediment are numerically solved, in which the unresolved subgrid-scale processes are modeled by the dynamic subgrid-scale model of Germano et al. The migration and deformation of the interface between the sediment bed and the fluid flow is captured by the level-set method. The sediment erosion is approached by means of three different pickup relations postulated by van Rijn, Einstein, and Yalin, respectively, partly modified by the authors. Generally, the sediment is entrained into the flow from locations where the shear stress exceeds a critical value—on the upstream slopes of ripple crests—and is advected downstream in suspension by the flow, until it settles again when the local flow condition cannot further transport it, e.g., on the lee sides of ripples. A global effect of these local processes is the migration of ripples. The numerical results on the fluid flow field and the sediment concentration distribution are discussed. The computed migration speed of the ripples, which is only a fraction of the free stream velocity, is compared with known experimental data and a good agreement is demonstrated.