dc.description.abstract | The main topic in this thesis is to investigate granular dynamics under gravity-driven forces. The research topics include particle size segregation and flow regime map in a slurry rotating drum, cohesive force between particles in wet rotating drums, and shock behavior of rapid granular flow down an inclined chute.
Firstly, in binary-mixture slurry granular systems, we find that the time revolution of the segregation index indicates that an increase in liquid viscosity and a decrease in filling degree will cause the segregation index to decrease. When the liquid viscosity is above a critical value, many systems rapidly decay to a local minimum after a short period of time (called “overmixing”). A new dimensionless flow variable is proposed to be used to distinguish the flow regimes. We also find that the flow regime changes from rolling to cascading when the dimensionless flow variable is below a critical value. Furthermore, the change of the segregation index occurs during the transition of the granular flow regime.
In mono-disperse slurry-granular systems, the distance between the centroid of all particles and the center of the rotating tank is the same as in the initial configuration at lower rotation speed or liquid viscosity, but the distance decreases rapidly when the liquid viscosity is above a critical value. The mean flow velocity will decrease with an increase of the liquid viscosity. When the liquid viscosity is above a critical value, the mean flow velocity will increase, and the granular flow behavior will transform into a suspension regime. Furthermore, the experimental results indicate that the liquid viscosity and the flow rate per unit width have a significant influence on the dynamic properties and flow behavior of the immersed granular matter.
To quantitatively determine the effect of the cohesive force on the dynamic properties of wet granular systems is one research topic in this thesis. A series of experiments is performed for wet granular matter in a rotating drum. The results indicate that when only very small amounts of liquid are added, no liquid bridges are formed. This is because the liquid is first trapped on the surface of the particles due to the particle roughness. When the volume fraction of the fluid becomes larger, liquid bridges formed on almost every particle. Once the liquid bridges are formed between all particles, the average energy dissipation due to the hysteretic formation and rupturing of the liquid bridges increases with an increase in the liquid content.
When a rapid avalanche flow is deflected by an obstacle, this usually causes abrupt changes in the flow thickness and velocity and exhibits characteristics like oblique shock waves in the aerodynamic system or oblique hydraulic jumps in the gravity-driven granular flows. We use the Discrete Element Method (DEM) to simulate the motion of granular materials impinging on a wedge obstacle in an adjustable inclined chute. The results of the simulations are compared with the classical oblique shock theory. We note that there is good agreement between the theoretical calculations and the DEM simulation results. Moreover, the microdynamic variables, related to the flow structure, such as the packing density and coordination number, are also discussed in the present study
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