Coupled CFD-DEM model for the direct numerical simulation of sediment bed erosion by viscous shear flow

Debris flows devastate downstream regions with large impulsive forces resulting from the significant increase in volume due to the erosion during their rapid movement. As this increase in volume is primarily derived from the sediment bed, sediment bed erosion characteristics determine the sediment transport rate and the corresponding severity degree. This study presents a coupled model based on computational fluid dynamics (CFD) and discrete element method (DEM) to simulate sediment bed erosion by viscous shear flow at various shear flow velocities. DEM is adopted to simulate the granular bed particles, and CFD is employed to model the shear flow (water and mudflows). The numerical model was verified by comparing the erosion formula and critical erosion shear stress with a pre-existing laboratory experiment. The erosion characteristics of the sediment bed were investigated from macro and micro perspectives. A sediment bed can be divided into four layers from top to bottom: saltating layer, rolling layer, creeping layer, and static layer. The depth profile of the average particle velocity obeyed a linear distribution in the rolling layer, and the velocity was approximately zero in the bottom layer. Dynamic information on the particle loss, trajectory, velocity, and principal stresses was recorded. Velocity traces demonstrated significant fluctuations with several isolated peaks, and the velocity variable displayed Gaussian distributions in the streamwise and vertical direction. The influence of the shear velocity on the drag force and sediment erosion was more obvious than those of the viscosity coefficient and fluid density. The drag force had an effect on the principal stresses and the micromechanical arrangement of the sediment grains. The results provide the insights into micro-mechanisms of the onset and erosion of sediment grains.