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

The objective of this research is to develop a coupled Computational Fluid Dynamicsand Discrete Element Method (CFD-DEM) approach to simulate the behaviour of fluid-particleinteractions in granular media for applications relevant to geotechnical andmining engineering. This research consists of several major components: (1) formulationof 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, includingthe sandpile formation in water, granular flow impacting on a water reservoir, impactingbehaviour 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 andthen coupled with DEM, to consider the movement of free surface and the interfacebetween two fluids. The extended coupled CFD-DEM tool was first benchmarked bytwo classic geomechanics problems where analytical solutions are available. It was thenemployed to investigate the characteristics of sand heap formed in water through hopperflow. It is shown in particular that a sand pile formed in water is more homogeneous interms of void ratio, contact force and fabric anisotropy. The central pressure dip ofvertical stress profile at the base of sandpile is moderately reduced, as compared to thedry case.;The CFD-DEM program was further applied to the simulation of impacting behaviour ofa granular flow falling from an inclined slope into a water reservoir. The surging waveinduced by the impacting granular flow into the reservoir was investigated. Theimpacting force on the basin dam exerted by the wave and the granular deposit wasexamined. By investigating the influence of debris falling height and the water level, anoptimal water level in the reservoir was recommend which leads to least impactingforces. Two different mechanisms of energy dissipation were found for the dry and thewet cases. In the dry case, the interparticle/particle-wall frictions and collisions are thedominating factors. In the wet case, the granular flow first transfers the majority of itskinetic 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 allenergy and eventually settles down. The Savage number depicts a peak at the transitionpoint 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 behaviourof debris flow by using the Bingham rheological model. The major parameters thatinfluence the impacting behaviour of dry granular flow are calibrated by the experiment.Two major sets of fluid properties in Bingham fluid model, including the conventionalslurry and the thickened slurry, were employed in the coupled simulations and werecarefully compared with each other. The evolutions of the impacting forces exerted bythe particle phase and the fluid phase were investigated. The velocity profile and theanimations of the debris flow were analysed.;The proposed CFD-DEM model was also employed to study the particle sizesegregation in a mixed particle-fluid flow. To investigate the influence of fluid density onthe segregation process, three density values were used. The centroid evolutions of fourcases were presented to show the segregation procedure, and the mechanisms ofsegregation in the particle-fluid flow were related with the dimensionless contact force.The segregation influences on the flowing mobility in granular flow and particle-fluidflow were also investigated.