Fluid-particle systems can involve particles with very complex shapes in many natural and engineering processes. To model these systems, the particles are often simplified as spheres for simplicity. However, particle shapes can significantly impact the overall mechanical behaviors at the macroscale. To address this challenge, we introduce a coupled scheme between the Metaball Discrete Element Method and the Lattice Boltzmann Method. The complex particle morphology is elegantly described by the Metaball function, and both the collision algorithm between non-convex particles and the coupling with fluid benefit from the use of Metaball functions. The proposed model has the potential to simulate many fluid-particle systems with realistic particle shapes.

This research focuses on obtaining and comparing the soil water retention curve (SWRC) via numerical modelling using three different multiphase extensions of the lattice Boltzmann method (LBM). The SWRC is a fundamental behavioral characteristic of unsaturated soils, that describes the variation of suction given imposed changes in saturation degree or vice-versa. The LBM is a computational fluid dynamics method, suitable for handling the complex geometries inherent to flow through the pore space of soils. Given that most soils exist in an unsaturated state, it is important that the modelling is done for the combined flow of the air and water phases, and thus multiphase or multicomponent extensions have to be employed. In this research the multiphase Shan-Chen, multicomponent Shan-Chen, and He-Chen-Zhang (phase field) extensions are considered. An in-house code for each of the three methods is developed and used to simulate the SWRC for a packing of granular soils. The accuracy of the predictions made by each method is measured by comparing the results with the SWRC obtained by experimental means. The three methods are also compared in terms of required computational power and time. Finally, a list of pros and cons for each method is provided to assist future researchers in choosing a suitable modeling tool.

Geophysical mass flows, such as debris flows, avalanches, and floods, are often observed by the presence of large boulders and driftwood, whose shapes significantly influence the behavior of flow transportation, jamming, impact, and deposition. However, quantitatively assessing how the shapes of these large-sized solids alter flow behavior remains an open question. This challenge arises from complexities involved in capturing the interactions between fluids, arbitrary-shaped boulders or wood, and structures. In this study, we employ a newly developed resolved computational fluid dynamics and discrete-element method (CFD-DEM) framework to simulate the interactions between arbitrary-shaped solids and viscous slurry or water. Specifically, the shapes of boulders, driftwood, and trees are obtained from reduced-scale samples using X-ray computed tomography (CT) techniques, and the CFD and DEM models employ the immersed boundary method (IBM) and signed distance field (SDF) method, respectively. As a result, the proposed CFD-DEM coupling framework offers a unified treatment of fluid-arbitrary-shaped solids-structure interactions in geophysical flows. Through flows carrying wood and boulders against a slit dam and forests, we test the modeling capability and explore the effects of different shapes on flow-structure interactions. The high-fidelity numerical predictions of flow-structure interactions demonstrate reasonable consistency with experimental and real-world observations. Therefore, this physics-based model holds significant potential for geophysical flow hazard assessment and broader applications in nature and engineering scenarios. Acknowledgments: This research was supported by the UGC-PolyU Start-up Fund (Grant No.: A0049544).

Monopiles are commonly used as foundations for offshore wind turbines. The behaviour of the pile depends on the interactions between the pile and the surrounding soil mass. Evaluating and understanding the soil-pile interaction is crucial for the design of monopiles.

Numerical methods such as the finite element method (FEM) are often used for the design and analysis of the behaviour of laterally loaded pile. However, the behaviour depends on the capability of the adopted constitutive model, which poses challenges in accurately reproducing complicated behaviour such as the cyclic response and strain localization. Multiscale numerical approaches, as an alternative to classical phenomenological laws, provide a new perspective to investigate the soil-pile behaviour. In this method, FEM is used to define the boundary value problem, while the discrete element method (DEM) is employed to derive the constitutive relationship at each Gauss point of the FEM mesh. Consequently, the strengths of FEM and DEM are integrated in a concurrent model, which enables to capture the complicated soil behaviour in large-scale boundary value problems.

This study presents a multiscale analysis by coupling FEM and DEM to investigate the complex interactions between laterally loaded piles and dry sands. Insights from the macroscale, such as the p-y curve and displacement field, are provided and compared with the results derived from pure FEM simulations. Moreover, microscopic insights, including contact force chain and particle arrangement close to and far from the pile are presented to interpret and better understand the macroscopic behaviour.

In recent years, there has been increasing interest in predicting fluid and moisture-driven crack propagation in deforming porous media, particularly in the modeling of hydraulic fracturing, also known as "fracking." The phase-field fracture method has become the most popular approach for crack propagation simulations due to its ability to capture complex crack behaviors without the need for explicit tracking of crack surfaces and additional fracture criteria. Currently, almost all phase-field hydraulic fracturing simulations are built within a finite element framework. In this study, we model hydraulic fracturing using an adaptive finite-volume phase-field framework proposed previously. The porous medium is based on classical Biot's poroelastic theory, with fracture behavior controlled by the phase-field model. Crack propagation is driven by elastic energy, where the phase-field value serves as an interpolation function to transition fluid properties between intact and fully damaged regions. The framework is validated using a classic 2D specimen subjected to increasing internal pressure and compared with analytical solutions. Additionally, simulations are conducted for scenarios including single crack propagation, interaction between hydraulic fracturing and pre-existing natural fractures, and interactions among multiple hydraulic fractures. The resulting crack propagation patterns and pressure distributions under different conditions are analyzed and compared, demonstrating the feasibility of this framework.