Publications

2024

1. Clogging and unclogging of fine particles in porous media: Micromechanical insights from an analog pore system Yin, Yanzhou; Cui, Yifei; and Jing, Lu Water Resources Research 2024 abstract ▾ | link

   Pore clogging and unclogging in porous media are ubiquitous in subsurface hydrologic processes, which have been studied extensively at various scales ranging from a single pore to porous-medium samples. However, it remains unclear how fluid flow, particle rearrangement, and the arching effect typical of cone-shaped pore geometry interact and how they are captured by a pressure drop model at the macroscopic scale. Here, we investigate the pore-scale feedback mechanisms between fluid flow and pore clogging and unclogging using a fully resolved fluid-particle coupling approach (lattice Boltzmann method-discrete element method). We first propose to use a truncated-cone pore to represent realistic pore geometries revealed by X-ray images of prepared sand packing. Then, our simulations indicate that the pore cone angle significantly influences the pressure drop associated with the clogging process by enhancing particle contacts due to arching. A modified Ergun equation is developed to consider this geometric effect. At the microscale, clogging can be explained by the interparticle force statistics; a few particles in an arch (or a dome) take the majority of hydrodynamic pressure. The maximum interparticle force is positively proportional to the particle Reynolds number and negatively associated with the tangent of the pore cone angle. Finally, a formula is established utilizing fluid characteristics and pore cone angle to compute the maximal interparticle force. Our findings, especially a modified pressure drop model that accounts for pore geometry resistance, provide guidance for applying pore-scale models of clogging and unclogging to large-scale subsurface fines transportation issues, including seepage-induced landslides, stream bank failure, and groundwater recharge.

2023

1. Efficient lattice Boltzmann simulation of free-surface granular flows with μ(I)-rheology Yang, G. C.; Yang, S. C.; Jing, L.; Kwok, C. Y.; and Sobral, Y. D. Journal of Computational Physics 2023 abstract ▾ | link

   This paper presents a lattice Boltzmann framework for accurate and efficient simulation of free-surface granular flows. The granular assembly is treated as a viscoplastic fluid, whose apparent viscosity varies locally with the shear rate and pressure according to a regularized μ(I)-rheology. A single-phase free-surface model is employed to track the evolution of the particle-air interface. The lattice Boltzmann implementation is first validated by simulating a steady-state granular flow on a rough inclined plane and a good agreement with the analytical solution is achieved. The validated model is then applied to simulate a transient granular column collapse problem. Compared to a companion discrete element simulation, the lattice Boltzmann model with the μ(I)-rheology is able to capture the overall dynamic behaviors of granular column collapse. However, a different behavior is observed when a similar Bingham viscoplastic model with a fixed yield stress is applied, highlighting the pressure dependent nature of granular flows. The proposed lattice Boltzmann formulation is highly efficient compared to the conventional computational fluid dynamics, and has the potential to conduct three-dimensional continuum simulation of large-scale geophysical flows with microscopic granular physics.

2. Particle size segregation and diffusion in saturated granular flows: Implications for grain sorting in debris flows Cui, Kahlil Fredrick; Zhou, Gordon G.D.; Jing, Lu; Xie, Yunxu; and Lu, Xueqiang In E3S Web of Conferences 2023 abstract ▾ | link

   Sorting of rocks, boulders, and silt/sand-sized particles according to their size is a characteristic feature of debris flow deposits and is an active process during flow which significantly affects the mobility. The degree at which size sorting occurs in debris flows depends on the relative magnitudes of granular processes such as particle size segregation and diffusion. Since debris flows are fluid-saturated phenomena, accurate modelling of size sorting requires the understanding of the influence of fluids on these processes, which have not been systematically studied. Here, we present simulation results and the associated empirical expressions for particle size segregation and diffusion which take into account fluid effects due to buoyant and drag forces. These expressions are developed through scaling analysis of data obtained from coupled granular-fluid simulations of saturated bi-disperse mixtures under simple shear. We further show that using these scaling relationships, an existing segregation-diffusion continuum equation can be extended to model particle sorting in debris flows with various types of fluids.

2022

1. Drag force in granular shear flows: regimes, scaling laws and implications for segregation Jing, Lu; Ottino, Julio M.; Umbanhowar, Paul B.; and Lueptow, Richard M. Journal of Fluid Mechanics 2022 abstract ▾ | link

   The drag force on a spherical intruder in dense granular shear flows is studied using discrete element method simulations. Three regimes of the intruder dynamics are observed depending on the magnitude of the drag force (or the corresponding intruder velocity) and the flow inertial number: a fluctuation-dominated regime for small drag forces; a viscous regime for intermediate drag forces; and an inertial (cavity formation) regime for large drag forces. The transition from the viscous regime (linear force-velocity relation) to the inertial regime (quadratic force-velocity relation) depends further on the inertial number. Despite these distinct intruder dynamics, we find a quantitative similarity between the intruder drag in granular shear flows and the Stokesian drag on a sphere in a viscous fluid for intruder Reynolds numbers spanning five orders of magnitude. Beyond this first-order description, a modified Stokes drag model is developed that accounts for the secondary dependence of the drag coefficient on the inertial number and the intruder size and density ratios. When the drag model is coupled with a segregation force model for intruders in dense granular flows, it is possible to predict the velocity of gravity-driven segregation of an intruder particle in shear flow simulations.

2. Particle segregation and diffusion in fluid-saturated granular shear flows Cui, Kahlil F. E.; Zhou, Gordon G. D.; and Jing, Lu Physical Review Fluids 2022 abstract ▾ | link

   Differently sized particles in sheared granular mixtures undergo size segregation and diffusive remixing, the relative magnitude of which controls the degree to which distinct particle layers form. The presence of viscous interstitial fluids affects individual particle dynamics, resulting in complex segregation and diffusion behaviors. In this study, the effects of different types of fluids (characterized by fluid density and viscosity) on these two processes are investigated, for various flow conditions and material parameters, via coupled numerical simulations of immersed granular shear flows. We observe that both the segregation velocity and the diffusion strength decrease with the fluid viscosity, but these effects occur only when the viscosity exceeds certain threshold values, indicating a transition from viscous to inertial regimes. In the low-viscosity limit where fluid and grain inertia dominate, both segregation and diffusion processes depend on flow conditions and material properties in a manner that is similar to those in dry inertial granular flows. On the other hand, decreasing the relative density between the particles and the fluid slows down segregation but does not significantly affect diffusion. Based on scaling analysis of the simulation data, empirical relationships for the segregation velocity and diffusion coefficient are developed as functions of a modified Stokes number, and are then used to extend an existing segregation-diffusion continuum equation for granular mixtures immersed in different types of fluids.

3. Segregation forces in dense granular flows: Closing the gap between single intruders and mixtures Duan, Yifei; Jing, Lu; Umbanhowar, Paul B; Ottino, Julio M; and Lueptow, Richard M Journal of Fluid Mechanics 2022 abstract ▾ | link

   Using simulations and a virtual-spring-based approach, we measure the segregation force, F_seg, in size-bidisperse sphere mixtures over a range of concentrations, particle-size ratios and shear rates to develop a semiempirical model for F_seg that extends its applicability from the well-studied non-interacting intruders regime to finite-concentration mixtures where cooperative phenomena occur. The model predicts the concentration below which the single-intruder assumption applies and provides an accurate description of the pressure partitioning between species.

2021

1. Size effects in underwater granular collapses: Experiments and coupled lattice Boltzmann and discrete element method simulations Yang, G. C.; Jing, L.; Kwok, C. Y.; and Sobral, Y. D. Physical Review Fluids 2021 abstract ▾ | link

   Immersed granular collapse is a common model case for the study of transient geo- physical flows. This paper examines the effects of column size on granular collapses in water, with an emphasis on the granular flow mobility. Laboratory-scale experiments of underwater granular collapses with three different column sizes are carried out, together with their numerical simulations using the coupled lattice Boltzmann and discrete element method. Both experimental and numerical data show that, for an identical aspect ratio, a larger underwater granular collapse results in higher flow mobility and a longer runout distance normalized by the initial column length Li (increased by 18% on average as Li increases from 3 to 10 cm). Simulations show that as the column size increases, there is more potential energy being transferred into the kinetic energies of the fluid and the particles, and there is a positive relationship between the column size and the efficiency of energy conversion of the particle kinetic energies from vertical to horizontal directions, which contributes to a higher underwater granular flow mobility in larger cases. The reason is twofold. First, the fluid inertia scales disproportionately with the column size. A stronger eddy with high inertia is induced in the large case, which penetrates through the flowing layer of the granular phase and pushes the particles forward to reach a longer runout dis- tance. Second, large underwater granular collapses are accompanied with more significant contact lubrication, which promotes basal slip and dissipates less energy during horizontal spreading.

2. A unified description of gravity- and kinematics-induced segregation forces in dense granular flows Jing, Lu; Ottino, Julio M.; Lueptow, Richard M.; and Umbanhowar, Paul B. Journal of Fluid Mechanics 2021 abstract ▾ | link

   Particle segregation is common in natural and industrial processes involving flowing granular materials. Complex, and seemingly contradictory, segregation phenomena have been observed for different boundary conditions and forcing. Using discrete element method simulations, we show that segregation of a single particle intruder can be described in a unified manner across different flow configurations. A scaling relation for the net segregation force is obtained by measuring forces on an intruder particle in controlled-velocity flows where gravity and flow kinematics are varied independently. The scaling law consists of two additive terms: a buoyancy-like gravity-induced pressure gradient term and a shear rate gradient term, both of which depend on the particle size ratio. The shear rate gradient term reflects a kinematics-driven mechanism whereby larger (smaller) intruders are pushed toward higher (lower) shear rate regions. The scaling is validated, without refitting, in wall-driven flows, inclined wall-driven flows, vertical silo flows, and free-surface flows down inclines. Comparing the segregation force with the intruder weight results in predictions of the segregation direction that match experimental and computational results for various flow configurations.

3. Viscous effects on the particle size segregation in geophysical mass flows: Insights from immersed granular shear flow simulations Cui, Kahlil F. E.; Zhou, Gordon G. D.; and Jing, Lu Journal of Geophysical Research: Solid Earth 2021 abstract ▾ | link

   Size segregation is a common feature in geophysical mass flow deposits and is an active process during motion. The presence of interstitial fluids in such flows affect the motion of constituent particles and result in complex segregation behaviors. Effects of the viscosity and density of various interstitial fluids on the rate of particle size segregation are investigated through coupled granular-fluid simulations of immersed plane-sheared flows. The segregation rate decreases as the fluid viscosity increases, but remains constant when the viscosity is below certain threshold values. In the low viscosity limit, segregation is affected only by the relative density between the particles and the fluid, and by flow inertial conditions. Analysis of segregation forcing terms based on the mixture theory of segregation reveals that the decrease of segregation rates with the viscosity is due to the increase of fluid drag forces, which effectively weaken the contact stress gradients responsible for driving the large particles to migrate upward. Viscous damping also diminishes velocity fluctuations and thereby inhibits the migration of particles throughout the mixture. The transition in the viscosity dependence is shown to correspond to the transition between granular-fluid flow regimes. An empirical scaling formula is developed which accounts for the effects of fluid viscosity and the relative density on size segregation immersed in different fluids.

4. Exploring shear-induced segregation in controlled-velocity granular flows Jing, Lu; Ottino, Julio M.; Lueptow, Richard M.; and Umbanhowar, Paul B. In EPJ Web of Conferences 2021 abstract ▾ | link

   Particle segregation in geophysical and industrial granular flows is typically driven by gravity and shear. While gravity-induced segregation is relatively well understood, shear-induced segregation is not. In particular, what controls segregation in the absence of gravity and the interplay between shear- and gravity-driven segregation remain unclear. Here, we explore the shear-induced segregation force on an intruder particle in controlled-velocity granular flows where the shear profile is systematically varied. The shear-induced segregation force is found to be proportional to the shear rate gradient, which effectively pushes the large intruder from lower to higher shear rate regions. A scaling law is developed for the segregation force that is accurate over a wide range of overburden pressures and shear rates, and hence inertial numbers.

2020

1. Generalized friction and dilatancy laws for immersed granular flows consisting of large and small particles Cui, Kahlil F. E.; Zhou, Gordon G. D.; Jing, Lu; Chen, Xiaoqing; and Song, Dongri Physics of Fluids 2020 abstract ▾ | link

   The motion of fully immersed granular materials, composed of two distinct particle sizes, flowing down rough inclined planes is studied through fluid–particle numerical simulations. We focus on the effect of ambient fluids, as well as their interplay with particle size segregation, on the steady-state kinematic and rheological profiles of the granular-fluid mixture flow. Simulation results are analyzed in the framework of a visco-inertial rheological model, which is first validated in monodisperse flows with a wide range of the ambient fluid viscosity (i.e., from air to water and slurry) and then generalized for size-bidisperse mixtures. It is found that the local effective friction and volume fraction of mixtures with different particle sizes can be approximated from the rheology of single-component flows. While the presence of viscous ambient fluids slows down size segregation (perpendicular to the flow) depending on the mixture composition and flow viscosity, the effective bulk friction is shown to be independent of the state and progress of segregation.

2. Particle size segregation in granular mass flows with different ambient fluids Zhou, Gordon G. D.; Cui, Kahlil F. E.; Jing, Lu; Zhao, Tao; Song, Dongri; and Huang, Yu Journal of Geophysical Research: Solid Earth 2020 abstract ▾ | link

   Size segregation, which is a robust feature of sheared granular mixtures and geophysical mass flow deposits, is found to diminish in the presence of a viscous fluid. We study this inhibitive effect through coupled fluid‐particle simulations of granular flows fully immersed in different ambient fluids. Granular‐fluid mixture flows are modeled according to three distinct flow regimes—free fall, fluid inertial, and viscous—at different angles of inclination. Each flow regime corresponds to distinct flow dynamics and segregation behaviors. We find that segregation is indeed weaker and slower in the presence of an ambient fluid, which is more so as the flow becomes more viscous. The ambient fluid affects segregation in two major ways. First, buoyancy reduces the contact pressure gradients which are needed to drive large particles up, while at the same time reduces the particles’ apparent weight. On the other hand, the streamwise drag force substantially changes the flow rheology, specifically the shear rate profile, thereby modifying the segregation behavior in the normal direction. Surprisingly, the fluid drag in the normal direction is negligible regardless of the fluid viscosity and does not affect segregation in a direct manner.

3. Rising and sinking intruders in dense granular flows Jing, Lu; Ottino, Julio M.; Lueptow, Richard M.; and Umbanhowar, Paul B. Physical Review Research 2020 abstract ▾ | link

   We computationally determine the net bed force on single spherical intruder particles in dense granular flows as a function of particle size, particle density, shear rate, overburden pressure, and gravity. A simple buoyancy-like scaling law is recovered (analogous to that in fluids), but with a scale factor that depends on the particle size ratio due to discrete contacts. Comparing the bed force with the intruder weight results in predictions of whether an intruder rises or sinks that agree with data from various independent experiments of free surface granular flows.

4. Fast, flexible particle simulations – An introduction to MercuryDPM Weinhart, Thomas; Orefice, Luca; Post, Mitchel; Schrojenstein Lantman, Marnix P.; Denissen, Irana F. C.; Tunuguntla, Deepak R.; Tsang, J. M. F.; Cheng, Hongyang; Shaheen, Mohamad Yousef; Shi, Hao; Rapino, Paolo; Grannonio, Elena; Losacco, Nunzio; Barbosa, Joao; Jing, Lu; Alvarez Naranjo, Juan E.; Roy, Sudeshna; Otter, Wouter K.; and Thornton, Anthony R. Computer Physics Communications 2020 abstract ▾ | link

   We introduce the open-source package MercuryDPM, which we have been developing over the last few years. MercuryDPM is a code for discrete particle simulations. It simulates the motion of particles by applying forces and torques that stem either from external body forces, (gravity, magnetic fields, etc.) or particle interactions. The code has been developed extensively for granular applications, and in this case these are typically (elastic, plastic, viscous, frictional) contact forces or (adhesive) short-range forces. However, it could be adapted to include long-range (molecular, self-gravity) interactions as well. MercuryDPM is an object-oriented algorithm with an easy-to-use user interface and a flexible core, allowing developers to quickly add new features. It is parallelised using MPI and released under the BSD 3-clause licence. Its open-source developers’ community has developed many features, including moving and curved walls; state-of-the-art granular contact models; specialised classes for common geometries; non-spherical particles; general interfaces; restarting; visualisation; a large self-test suite; extensive documentation; and numerous tutorials and demos. In addition, MercuryDPM has three major components that were originally invented and developed by its team: an advanced contact detection method, which allows for the first time large simulations with wide size distributions; curved (non-triangulated) walls; and multicomponent, spatial and temporal coarse-graining, a novel way to extract continuum fields from discrete particle systems. We illustrate these tools and a selection of other MercuryDPM features via various applications, including size-driven segregation down inclined planes, rotating drums, and dosing silos.

5. Pore-scale simulation of immersed granular collapse: Implications to submarine landslides Yang, G.C.; Jing, L.; Kwok, C.Y.; and Sobral, Y.D. Journal of Geophysical Research: Earth Surface 2020 abstract ▾ | link

   The collapse of granular columns in a viscous fluid is a common model case for submarine geophysical flows. In immersed granular collapses, dense packings result in slow dynamics and short runout distances, while loose packings are associated with fast dynamics and long runout distances. However, the underlying mechanisms of the collapse initiation and runout, particularly regarding the complex fluid-particle interactions at the pore scale, are yet to be fully understood. In this study, a three-dimensional approach coupling the lattice Boltzmann method and the discrete element method is adopted to investigate the influence of packing density on the collapsing dynamics. As a supplement to previous experimental measurements, the direct numerical simulation of fluid-particle interactions explicitly provides micromechanical evidence of the pore pressure feedback mechanism. In dense cases, a strong arborescent contact force network can form to prevent particles from sliding, resulting in a creeping failure behavior. In contrast, the granular phase is liquefied substantially in loose cases, leading to a rapid and catastrophic failure. This opposing dilative/contractive behavior linked to the initial packing is robust and does not depend on the column size. Furthermore, hydroplaning can take place in large enough loose cases due to the fast-moving surge front, which reduces the frictional resistance dramatically and thereby results in a long runout distance. More quantitatively, we are able to linearly correlate the normalized runout distance and the densimetric Froude number across a wide range of length scales, including small-scale numerical/experimental data and large-scale field data.

2019

1. A comprehensive parametric study of LBM-DEM for immersed granular flows Yang, G.C.; Jing, L.; Kwok, C.Y.; and Sobral, Y.D. Computers and Geotechnics 2019 abstract ▾ | link

   This paper presents a parametric study of a fluid-particle model which couples Lattice Boltzmann Method (LBM) and Discrete Element Method (DEM) using an immersed moving boundary technique. Benchmark cases with increasing complexity are simulated to understand the numerical accuracy, stability and efficiency of the algorithm. A guideline for a high-quality LBM-DEM model is proposed and applied to a test case of granular collapse in water. The simulation result shows excellent agreement with a companion experiment, which demonstrates the capability of LBM-DEM to describe the dynamics of densely packed and friction dominant immersed granular flows, highlighting its potential to study geophysical mass movements.

2. Flow regimes and dynamic similarity of immersed granular collapse: A CFD-DEM investigation Jing, L.; Yang, G.C.; Kwok, C.Y.; and Sobral, Y.D. Powder Technology 2019 abstract ▾ | link

   Immersed granular collapses may encounter different flow regimes, such as free-fall (dry), fluid-inertial, and viscous regimes, depending on column geometry, particle size, particle density, fluid viscosity, and many other parameters. Understanding the controlling parameters of these regimes is important for both industrial and geological applications where grains and fluids coexist. It is also important to combine these parameters into dimensionless groups to guide down-scaled experiments and numerical simulations. In this work, we derive a set of dimensionless numbers (i.e., Stokes number, density ratio, and Reynolds number) based on typical time scales in the sedimentation of a sphere, and successfully verify the relevance of these numbers in determining flow regimes and maintaining dynamic similitude across length scales. The numerical method we use couples the computational fluid dynamics and discrete element method (CFD-DEM), which allows a wide variety of particle size and fluid viscosity to be chosen, keeping constant the Stokes number and density ratio. Quantitative data of front propagation and energy evolution are presented to characterize flow dynamics in different flow regimes. The collapse exhibits a transition from sliding-dominant to suspension-dominant behaviors as the Stokes number decreases, which gives rise to distinct deposit morphology in different regimes. Our findings enhance the understanding of inertial and viscous behaviors of immersed granular flows. The verified scaling rules and dimensionless parameters are of potential use in small-scale experiments and simulations where appropriate scaling is essential.

2018

1. Dynamics and scaling laws of underwater granular collapse with varying aspect ratios Jing, L.; Yang, G. C.; Kwok, C. Y.; and Sobral, Y. D. Physical Review E 2018 abstract ▾ | link

   We perform coupled fluid-particle simulations to understand the granular collapse in an ambient fluid (in particular, water) with a wide range of initial aspect ratios. We observe both similar and distinct features in underwater collapses compared to their dry counterparts. As aspect ratio a increases, the normalized runout distance follows a piecewise power-law growth, transitioning at a=2.5. We associate this transition with the different growth rates of kinetic energy (with a) in vertical and horizontal directions. The ability of utilizing available energy for horizontal motion becomes limited when a\textgreater2.5. Moreover, the front propagation during underwater collapses can be well scaled by using the initial column height as length scale and considering a reduced gravity (due to buoyancy) in timescale. Under the reduced gravity, the initial fall of tall columns is found to be ballistic, consistent with dry collapses. On the other hand, underwater collapses (especially for large a) exhibit unique dynamics due to the presence of water. The eddies generated in water, which may carry considerable fluid inertia, tend to erode the surface of the granular layer, thus modifying the deposit morphology. The energy conversion is also affected by the ambient fluid. While water obviously consumes energy from the granular phase through fluid-particle interactions, it actually increases the efficiency of energy conversion from vertical to horizontal directions. The latter effect compensates the difference of runout distance between underwater and dry collapses.

2. Breaking size-segregation waves and mobility feedback in dense granular avalanches Vaart, K.; Thornton, A. R.; Johnson, C. G.; Weinhart, T.; Jing, L.; Gajjar, P.; Gray, J. M. N. T.; and Ancey, C. Granular Matter 2018 abstract ▾ | link

   Through experiments and discrete particle method (DPM) simulations we present evidence for the existence of a recirculating structure, that exists near the front of dense granular avalanches, and is known as a breaking size-segregation (BSS) wave. This is achieved through the study of three-dimensional bidisperse granular flows in a moving-bed channel. Particle-size segregation gives rise to the formation of a large-particle-rich front and a small-particle-rich tail with a BSS wave positioned between the tail and front. We experimentally resolve the structure of the BSS wave using refractive-index matched scanning and find that it is qualitatively similar to the structure observed in DPM simulations. Our analysis demonstrates a relation between the concentration of small particles in the flow and the amount of basal slip, in which the structure of the BSS wave plays a key role. This leads to a feedback between the mean bulk flow velocity and the process of particle-size segregation. Ultimately, these findings shed new light on the recirculation of large and small grains near avalanche fronts and the effects of this behaviour on the mobility of the bulk flow.

3. Runout scaling and deposit morphology of rapid mudflows Jing, L.; Kwok, C. Y.; Leung, Y. F.; Zhang, Z.; and Dai, L. Journal of Geophysical Research: Earth Surface 2018 abstract ▾ | link

   Prediction of runout distance and deposit morphology is of great importance in hazard mitigation of geophysical flows, including viscoplastic mudflows. The major rheological parameters of mudflows, namely, yield stress and viscosity, are crucial factors in controlling the runout and deposition processes. However, the roles of the two parameters, especially in mudflows with high inertia, remain poorly understood and are not accounted for in runout scaling relations with source volume. Here we investigate the effects of flow rheology on runout scaling and deposit morphology using small-scale laboratory experiments and three-dimensional numerical simulations. We find that yield stress and viscosity both influence flow velocity gained during downslope propagation of mudflows, which is strongly correlated with the runout distance; the role of yield stress is more significant than viscosity. High yield stress and low viscosity lead to an elongated deposit, where longitudinal propagation is more significant than lateral spreading. In contrast, high viscosity promotes the dominance of lateral spreading of the deposit, while low yield stress and moderate viscosity produce an initial elongate deposit, followed by a secondary surge that spreads laterally near the head of the deposit. Following appropriate scaling relations for viscosity and yield stress, a general scaling function is proposed to incorporate flow properties in the well-known correlation of runout distance and source volume. Our findings regarding the inertia effects and the roles of yield stress and viscosity enhance our understanding of mudflows, muddy debris flows, and other viscoplastic geophysical flows.

4. Effect of coefficient of friction on arch network in shearing process under low confinement Meng, Yue; Zhu, Hejian; Kwok, Chung Yee; Kuo, Matthew; Jing, Lu; and Huang, Xin Powder Technology 2018 abstract ▾ | link

   Subsea pipelines, trenching picks and ploughing blades are examples where soil-structure interaction occurs at very low vertical effective stress. Arches are multi-particle structures encountered during the interface shearing process, the existence of which dictates the properties of the overall granular assembly. In order to understand the behavior of dense granular materials in response to shearing under low confining pressure, physical modeling together with discrete element method (DEM) modeling of granular layers has been performed. Since friction can control the behaviors of granular flow, this study focuses on the effect of μ on the arch network as a parametric study. Grains have been subjected to horizontal shearing by a triangle wedge at an enlarged scale so that observations of the shearing process and behavior of particles can be made. The model was validated against physical modeling by comparing the rearrangement of selected particles, velocity vector field and arch network. During the shearing process, the arch network changed with the formation and collapse of arches, which was quantitatively analyzed in terms of arch size distribution and duration. The micromechanical responses from the granular assembly demonstrated that when μ increased, particle rearrangement shifted from sliding to rotation, resulting in longer and more durable arches. The investigation on contact forces showed that in-arch grains withstood larger forces and had a higher probability to participate in force chain than out-of-arch grains. Taking the effect of μ into consideration, the study explores the relationship among particle rearrangement, arch network, and force chains.

5. DEM modeling of hydraulic fracturing in permeable rock: influence of viscosity, injection rate and in situ states Duan, Kang; Kwok, Chung Yee; Wu, Wei; and Jing, Lu Acta Geotechnica 2018 abstract ▾ | link

   Hydraulic fracturing in permeable rock is a complicated process which might be influenced by various factors including the operational parameters (e.g., fluid viscosity, injection rate and borehole diameter) and the in situ conditions (e.g., in situ stress states and initial pore pressure level). To elucidate the effects of these variables, simulations are performed on hollow-squared samples at laboratory scale using fully coupled discrete element method. The model is first validated by comparing the stress around the borehole wall measured numerically with that calculated theoretically. Systematic parametric studies are then conducted. Modeling results reveal that the breakdown pressure and time to fracture stay constant when the viscosity is lower than 0.002 Pa-s or higher than 0.2 Pa-s but increases significantly when it is between 0.002 and 0.2 Pa-s. Raising the injection rate can shorten the time to fracture but dramatically increase the breakdown pressure. Larger borehole diameter leads to the increase in the time to fracture and the reduction in the breakdown pressure. Higher in situ stress requires a longer injection time and higher breakdown pressure. The initial pore pressure, on the other hand, reduces the breakdown pressure as well as the time to fracture. The increase in breakdown pressure with viscosity or injection rate can be attributed to the size effect of greater tensile strength of samples with smaller infiltrated regions.

6. Coupled fluid-particle modeling of submerged granular collapse Jing, L.; Yang, G. C.; Kwok, C. Y.; and Sobral, Y. D. In Micro to MACRO Mathematical Modelling in Soil Mechanics 2018 abstract ▾ |

   We perform coupled fluid-particle modeling to understand the collapse of underwater granular columns in comparison with dry cases, with a variety of initial aspect ratios. Our results show that the submerged collapse leads to a shorter runout and thicker front due to the resistance provided by the ambient fluid. An interesting process of vortex formation is observed in the fluid as particles turn into a shear flow. At high aspect ratios, the vortex in water can significantly modify the surface morphology of the final deposit due to the fluid inertia developed on the surface of the granular layer.

7. Effect of particle size segregation in debris flow deposition: A preliminary study Jing, Lu; Kwok, Fiona C. Y.; Zhao, Tao; and Zhou, Jiawen In Proceedings of GeoShanghai 2018 International Conference: Geoenvironment and Geohazard 2018 abstract ▾ | link

   To understand the effect of size segregation in the depositional process of debris flows, both flume experiments at the laboratory scale and numerical simulations using the discrete element method (DEM) are performed. A variety of particle size distributions with coarse and fine particles are adopted. It is found that larger particles tend to reach the front of the final deposits, while small particles are accumulated at the tail of the flows. Quantitative agreement is achieved in the DEM simulations, where rolling resistance and geometric roughness at boundaries are adopted to account for the effect of particle shape. With the DEM results, the effect of segregation on the runout distance is studied from the perspective of energy dissipation. The progress of segregation is analyzed in detail, which revealed that segregation occur slowly while the flow is propagating rapidly over the slopes; it becomes significant during the deposition stage, where more large particles are found near the surface. The effect of segregation in debris flow deposition can help better predict the runout distance and impact pressure, which is crucial in the assessment and mitigation of debris flow-related natural hazards.

2017

1. Micromechanical origin of particle size segregation Jing, L.; Kwok, C. Y.; and Leung, Y. F. Physical Review Letters 2017 abstract ▾ | link

   We computationally study the micromechanics of shear-induced size segregation and propose distinct migration mechanisms for individual large and small particles. While small particles percolate through voids without enduring contacts, large particles climb under shear through their crowded neighborhoods with anisotropic contact network. Particle rotation associated with shear is necessary for the upward migration of large particles. Segregation of large particles can be suppressed with inadequate friction, or with no rotation; increasing interparticle friction promotes the migration of large particles, but has little effect on the percolation of small particles.

2. Effect of geometric base roughness on size segregation Jing, L.; Kwok, C. Y.; Leung, Y. F.; and Sobral, Y. D. In EPJ Web of Conferences 2017 abstract ▾ | link

   The geometric roughness at boundaries has a profound impact on the dynamics of granular flows. For a bumpy base made of fixed particles, two major factors have been separately studied in the literature, namely, the size and spatial distribution of base particles. A recent work (Jing et al. 2016) has proposed a roughness indicator R_a, which combines both factors for any arbitrary bumpy base comprising equally-sized spheres. It is shown in mono-disperse flows that as R_a increases, a transition occurs from slip (R_a < 0.51) to non-slip (R_a > 0.62) conditions. This work focuses on such a phase transition in bi-disperse flows, in which R_a can be a function of time. As size segregation takes place, large particles migrate away from the bottom, leading to a variation of size ratio between flow- and base-particles. As a result, base roughness R_a evolves with the progress of segregation. Consistent with the slip/non-slip transition in mono-disperse flows, basal sliding arises at low values of R_a and the development of segregation might be affected; when R_a increases to a certain level (R_a > 0.62), non-slip condition is respected. This work extends the validity of R_a to bi-disperse flows, which can be used to understand the geometric boundary effect during segregation.

3. Basal effect in mono- and bi-disperse chute flows Jing, Lu; Kwok, C. Y.; Leung, Y. F.; and Sobral, Yuri D. In Proceedings of the 7th International Conference on Discrete Element Methods 2017 abstract ▾ | link

   The basal condition for granular chute flows is assumed rough and non-slip in experimental and numerical studies. In mono-disperse flows, a rough base is usually constructed by gluing/fixing a layer of particles that is similar or identical to the flowing particles. However, the base condition is not so clear in bi-disperse flows, where size segregation changes the relative basal roughness. In this paper, basal effect is studied in both mono- and bi-disperse chute flows. Different size ratios are adopted to understand how size segregation affects the basal condition and velocity profile in the steady, fully developed state. It is found that considerable sliding may arise as large particles stay in contact with the base, and it may affect the development of segregation. The velocity profile at the steady state is different due to the occurrence of segregation. In addition, sensitivity analyses show that the basal effect is independent of the sample size and contact parameters.

2016

1. Characterization of base roughness for granular chute flows Jing, L.; Kwok, C. Y.; Leung, Y. F.; and Sobral, Y. D. Physical Review E 2016 abstract ▾ | link

   Base roughness plays an important role in the dynamics of granular flows but is still poorly understood due to the difficulty of its quantification. For a bumpy base made of spheres, at least two factors should be considered in order to characterize its geometric roughness, namely, the size ratio of flow to base particles and the packing arrangement of base particles. In this paper, we propose an alternative definition of base roughness, R_a, as a function of both the size ratio and the distribution of base particles. This definition is generalized for random and regular packings of multilayered spheres. The range of possible values of R_a is presented, and optimal arrangements for maximizing base roughness are studied. Our definition is applied to granular chute flows in both two- and three-dimensional configurations, and is shown to successfully predict whether slip occurs at the base. A transition is observed from slip to nonslip conditions as R_a increases. Critical values of R_a are identified for the construction of a nonslip base at various angles of inclination.

2. Extended CFD-DEM for free-surface flow with multi-size granules Jing, L.; Kwok, C. Y.; Leung, Y. F.; and Sobral, Y. D. International Journal for Numerical and Analytical Methods in Geomechanics 2016 abstract ▾ | link

   Computational fluid dynamics and discrete element method (CFD–DEM) is extended with the volume of fluid (VOF) method to model free-surface flows. The fluid is described on coarse CFD grids by solving locally averaged Navier–Stokes equations, and particles are modelled individually in DEM. Fluid–particle interactions are achieved by exchanging information between DEM and CFD. An advection equation is ap- plied to solve the phase fraction of liquid, in the spirit of VOF, to capture the dynamics of free fluid surface. It also allows inter-phase volume replacements between the fluid and solid particles. Further, as the size ratio (SR) of fluid cell to particle diameter is limited (i.e. no less than 4) in coarse-grid CFD–DEM, a porous sphere method is adopted to permit a wider range of particle size without sacrificing the resolution of fluid grids. It makes use of more fluid cells to calculate local porosities. The developed solver (cfdemSolverVOF) is validated in different cases. A dam break case validates the CFD-component and VOF-component. Particle sedimentation tests validate the CFD–DEM interaction at various Reynolds numbers. Water-level rising tests validate the volume exchange among phases. The porous sphere model is validated in both static and dynamic situations. Sensitivity analyses show that the SR can be reduced to 1 using the porous sphere approach, with the accuracy of analyses maintained. This allows more details of the fluid phase to be revealed in the analyses and enhances the applicability of the proposed model to geotechnical problems, where a highly dynamic fluid velocity and a wide range of particle sizes are encountered.

3. Experimental and numerical study of depositional mechanism of mudflow Jing, L.; Kwok, C. Y.; Leung, Y. F.; Zhang, Z.; and Dai, L. In Proceedings of International Geotechnics Symposium cum International Meeting of CSRME 14th Biennial National Congress 2016 abstract ▾ |

   Mudflows tend to deposit after a rapid evolution on the steep channel down hillside. The large runout distance and deposit area are crucial concerns of geotechnical and geological engineers. In the rheological point of view, mudflows are non-Newtonian viscoplastic fluids which behave like a solid until submitted to a higher stress than the yield strength. Upon yielding, flow takes place and the flow behavior is described by viscosity. How viscosity and yield stress control the deposition of mudflows remains yet unclear due to the difficulties of performing large-scale experiments and scaling over a wide range of length scales.

   In this work, scaling of rheological parameters are proposed in a dimensional analysis. It is successfully used in numerical simulations to bridge the depositional behaviors of mudflows at different scales. The numerical modeling with Bingham and Herschel-Bulkley models are validated against small-scale flume experiments. The depositional mechanism of mudflows is associated with yield stress and viscosity. It is found that a deposition of slurry relevant to natural mudflows has the following features: (i) fast runout and stoppage due to relatively low viscosity; (ii) elongated shape due to fast runout (more stream-wise spreading than lateral spreading); (iii) remains stuck on the channel and steep edges due to high yield stress; (iv) flat deposit surface due to the effect of yield stress.

2015

1. Discrete element modelling of grain size segregation in bi-disperse granular flows down chute Jing, L.; Kwok, C. Y.; and Leung, Y. F. In International Conference on Particle-Based Methods, Fundamentals and Applications 2015 abstract ▾ | link

   Three-dimensional DEM simulations of size segregation in granular flows down chute are presented. Different cubic bi-disperse samples are generated by pluviation, on the rough base formed by randomly placed particles. Periodic boundaries are applied to the flow direction and the two sides. Parametrical studies involving slope angle, width, volume fraction, and coefficient of friction are systemically performed. In all presented cases, steady, fully developed (SFD) state is achieved, where the kinetic energy and fractional volume distribution remain constant. From the macroscopic view, segregations are completed in the SFD state with slightly different extents and a thick layer of pure coarse grains appears on the top of the flow. The profiles of volume fractions are calculated and presented by shear layers. In addition, the trajectories of individual particles are tracked and analysed, showing clearly the contact conditions and shear history experienced by individual particles. It is found that the connectivity of small particles is generally at a lower level than that of the large ones, indicating a high probability of small particles dropping into voids under gravity is higher. On the other hand, the large particles experience a significant increase of connectivity as they migrate through the layer of small particles.

2014

1. A coupled CFD-DEM model for fluid-particle flows with free surface: formulation and validation Jing, Lu; Kwok, C. Y.; and Leung, Y. F. In Geomechanics from micro to macro 2014 abstract ▾ |

   A coupled Computational Fluid Dynamics and Discrete Element Method (CFD-DEM) model is presented for fluid-particle flows with free fluid surface. The fluid flow is modeled by solving the locally averaged Navier-Stokes equations, and the particle motions are captured separately in the DEM. Fluid-particle interactions are taken into consideration by exchanging necessary information between the CFD and the DEM. A numerical solver, referred to as cfdemSolverVOF in this study, is developed to capture the dynamics of the free fluid surface within the CFD-DEM framework. This is achieved by applying an advection equation to solve the volume fraction of the liquid in each fluid cell, in the spirit of the Volume of Fluid (VOF) method. Different components of the developed numerical solver are verified and validated in the dam break case and sedimentation of particle tests. The numerical predictions agree well with the analytical/empirical solutions.

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