Debris flows are among the most deadly natural hazards, causing great destructions to environment and property losses and accounting for a significant portion of annual human fatalities around the world. Our fundamental understanding of debris flow remains limited, which prevents effective design of engineering measures for debris flow mitigation. This thesis presents a novel numerical framework based on coupled Computational Fluid Dynamics and Discrete Element Method (CFD-DEM) to investigate key aspects of debris flows. A debris flow is considered as a mixture of fluid and particles where the fluid and particle phases are modelled by the CFD and the DEM, respectively. Interactions between the fluid and the particles are considered by exchanging interaction forces between the C...[ Read more ]

Debris flows are among the most deadly natural hazards, causing great destructions to environment and property losses and accounting for a significant portion of annual human fatalities around the world. Our fundamental understanding of debris flow remains limited, which prevents effective design of engineering measures for debris flow mitigation. This thesis presents a novel numerical framework based on coupled Computational Fluid Dynamics and Discrete Element Method (CFD-DEM) to investigate key aspects of debris flows. A debris flow is considered as a mixture of fluid and particles where the fluid and particle phases are modelled by the CFD and the DEM, respectively. Interactions between the fluid and the particles are considered by exchanging interaction forces between the CFD and DEM computations. Both the macroscopic and microscopic characteristics of debris flow can be captured by the employed CFD-DEM approach. The study focuses on several topics closely related to the initial mobilization and propagation of a debris flow, its interactions with channel beds and its impact onto a rigid or flexible resisting structure, with major findings outlined below:

(i) A CFD-DEM modelling of dam break of mixtures consisting of non-Newtonian liquids and particles indicates the importance of considering solid-liquid interactions in realistic simulation of the initial mobilization of debris flow. When collapsing, a non-Newtonian liquid is found to form conforming flow with the particles, in contrast to separated profiles of water and particles. In comparison with pure dry and pure liquid cases, the solid-liquid interactions have significant impacts on all around behavior of a non-Newtonian fluid-particle mixture during its collapse, including the initiation of the collapse, the conformity of the flow profile, the evolution of flow front and the energy exchange. The study further reveals the underpinning physical mechanisms related to the macro observations during the collapse.

(ii) Entrainment and mass exchange can significantly affect the kinematics of debris flow. An erodible channel bed is simulated by CFD-DEM to reproduce the erosion patterns observed in experimental tests. It is further found that the slope inclination and solid concentration of a mixture show a positive correlation with the total eroded mass. Breakage of aggregates into fines which are blended into the fluid phase represents an important source of inter-phase mass exchange. The numerical results show that by considering such a mass exchange, both the mobility and the impacts of debris flow can be enhanced. The overall flow patterns can also be greatly changed with the mass exchange.

(iii) Based on coupled CFD-DEM simulations of debris flows impacting on rigid barriers at a wide range of flow regimes (from subcritical flows to supercritical flows), a new, physically based analytical model is proposed for the impact pressure where the total impact pressure is expressed as a combination of hydrostatic and hydrodynamic contributions. The model prediction shows promising consistency with both the numerical data and reported field data from the literature.

(iv) A unified modelling framework based on CFD-DEM is developed to simulate the complicated interactions between debris flows and flexible barriers. A flexible barrier is simulated by the DEM as a network of bonded particles with remote interactions. It is demonstrated that the unified treatment by CFD-DEM enables us to conveniently handle the complicated three-way interactions among the fluid, the debris particles and the flexible barrier. A systematic investigation shows that the impact force, the resultant deformation patterns and the retained mass in a flexible barrier may notably be affected by the channel inclination and the volumetric solid fraction of a debris mixture. Good agreements are found between the present numerical predictions with existing experimental and numerical studies. The coupled CFD-DEM study also reveals possible failure modes of a flexible barrier under various impacting cases. Further modelling of the use of reinforcing components in a flexible barrier, including double twists and reinforcing cables highlights that the loading sharing mechanisms and reinforcing effects can be significant changed with their inclusion. The study provides a feasible analytical tool for practical design of flexible barriers for debris flow mitigation.