Coupled CFD-DEM modeling of fluid-particle interaction in geomechanics

Abstract

The objective of this research is to develop a coupled Computational Fluid Dynamics and Discrete Element Method (CFD-DEM) approach to simulate the behaviour of fluid-particle interactions in granular media for applications relevant to geotechnical and mining engineering. This research consists of several major components: (1) formulation of the coupling scheme of the CFD and DEM, (2) benchmark of the CFD-DEM scheme, and (3) application of the developed CFD-DEM approach to various problems, including the sandpile formation in water, granular flow impacting on a water reservoir, impacting behaviour of debris flow and particle segregation in particle-fluid flow. To simulate the debris flow, a two-fluid VOF model was developed for the CFD part and then coupled with DEM, to consider the movement of free surface and the interface between two fluids. The extended coupled CFD-DEM tool was first benchmarked by two classic geomechanics problems where analytical solutions are available. It was then employed to investigate the characteristics of sand heap formed in water through hopper flow. It is shown in particular that a sand pile formed in water is more homogeneous in terms of void ratio, contact force and fabric anisotropy. The central pressure dip of vertical stress profile at the base of sandpile is moderately reduced, as compared to the dry case. The CFD-DEM program was further applied to the simulation of impacting behaviour of a granular flow falling from an inclined slope into a water reservoir. The surging wave induced by the impacting granular flow into the reservoir was investigated. The impacting force on the basin dam exerted by the wave and the granular deposit was examined. By investigating the influence of debris falling height and the water level, an optimal water level in the reservoir was recommend which leads to least impacting forces. Two different mechanisms of energy dissipation were found for the dry and the wet cases. In the dry case, the interparticle/particle-wall frictions and collisions are the dominating factors. In the wet case, the granular flow first transfers the majority of its kinetic energy to the water body, which induces surging waves travelling between the check dam and the slope surface for a rather sustained period before it dissipates out all energy and eventually settles down. The Savage number depicts a peak at the transition point and decreases steadily when the flow continues to travel along the reservoir ground. The extended two-fluid CFD-DEM approach has the capability to model the behaviour of debris flow by using the Bingham rheological model. The major parameters that influence the impacting behaviour of dry granular flow are calibrated by the experiment. Two major sets of fluid properties in Bingham fluid model, including the conventional slurry and the thickened slurry, were employed in the coupled simulations and were carefully compared with each other. The evolutions of the impacting forces exerted by the particle phase and the fluid phase were investigated. The velocity profile and the animations of the debris flow were analysed. The proposed CFD-DEM model was also employed to study the particle size segregation in a mixed particle-fluid flow. To investigate the influence of fluid density on the segregation process, three density values were used. The centroid evolutions of four cases were presented to show the segregation procedure, and the mechanisms of segregation in the particle-fluid flow were related with the dimensionless contact force. The segregation influences on the flowing mobility in granular flow and particle-fluid flow were also investigated.

Date of Award	2015
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology