THESIS
2015
xxii, 206 pages : illustrations (some color) ; 30 cm
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 conside...[
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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.
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