Statistical Characterization of the Intermittency of Bedload Transport in Conditions Near the Threshold of Motion

Abstract:

The dynamics of sediment particles in a flat bed channel is mainly determined by near bed coherent structures of the turbulent boundary layer. These vortices are characterized by intense velocity fluctuations, which produce an instantaneous increase of the bed shear stress and consequently, sediment particle entrainment and deposition. At low shear stress conditions, the sediment flux has an intermittent behavior, with a series of frequent and localized transport events. Although many experimental and computational investigations have addressed the interaction between the flow and the sediment particles, the mechanisms that generate the intermittency in the bedload transport flux are not yet fully understood. In order to give new insights into these processes, we develop a Lagrangian sediment transport model to simulate sediment transport in a flat bed channel. We couple direct numerical simulation (DNS) to solve the 3D Navier-Stokes equations for the flow and the discrete element method (DEM) to solve the particle dynamics (LIGGGHTS, http://www.cfdem.com/liggghts), using a two-way coupling approach. The objectives of this study are: i) to make a detailed description of the first-order statistics of sediment particles, and ii) to explore the intermittent nature of sediment transport in conditions near the threshold of motion. For low Shields numbers, we show that this intermittency has a fractal behavior, whose characteristics change as the shear stress increases.

This work was supported by Conicyt National-PhD Grant, Fondecyt grant 1130940 and ONR-G NICOP Project N622909-11-1-7041.