Geophysical mass flows involve granular earth materials surging down natural slopes, one of the m... more Geophysical mass flows involve granular earth materials surging down natural slopes, one of the major threats to mountainous regions worldwide. Accurate modeling of geophysical mass flows requires closure relations both within the flow (rheology) and at the flow-substrate interface (boundary conditions). However, although recent years have seen significant advances in the modeling of granular flow rheology, our understanding of how flowing granular materials interact with the substrate remains largely elusive. Here, we focus on micro-topography, i.e., geometric base roughness that is about the same size as the grain size, and investigate its effects on the granular flow dynamics as well as the associated closure relations. To systematically vary the base roughness from smooth to rough, we generate the base using immobile particles with varying particle size and spatial arrangement in laboratory experiments (with particle image velocimetry for flow kinematics extraction) and discrete element method simulations. Two granular flow scenarios are considered, including steady-state flow down inclines and granular column collapse. In the first scenario, it is found that basal slip occurs when the base roughness is below a range of intermediate values and a general slip law connecting the slip velocity, the mean flow velocity, and the base roughness is developed. In the second, transient flow scenario, basal slip inevitably occurs even for very rough bases due to inertial effects and a transient basal slip law is proposed to correlate the slip velocity with local flow properties based on kinetic theory arguments. The basal slip laws developed in this work can be readily incorporated as a dynamic boundary condition in continuum modeling of granular flows. In future work, grain-scale mechanisms relevant to more realistic geophysical flows will be investigated, including the feedback effects of pore fluid pressure on the flow mobility during basal sliding and the role of irregular particle shapes in hydro-mechanical modeling of geophysical mass flows.
To understand the effect of size segregation in the depositional process of debris flows, both fl... more 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...
Submitted for the DFD19 Meeting of The American Physical Society DEM Modeling of Coupled Multipha... more Submitted for the DFD19 Meeting of The American Physical Society DEM Modeling of Coupled Multiphase Flow and Granular Mechanics: Wettability Control on Fracture Patterns YUE MENG, BAUYRZHAN PRIMKULOV, Massachusetts Institute of Technology, ZHIBING YANG, State Key Laboratory of Water Resources and Hydropower Engineering, Wuhan University, FIONA KWOK, University of Hong Kong, RUBEN JUANES, Massachusetts Institute of Technology — The interplay between multiphase flow in a granular medium and the displacement of the grain particles generates a wide range of patterns. The balance between frictional, viscous, and capillary forces has been studied in experiments and simulations, and has helped understanding the underlying mechanisms for a wide range of phenomena, such as methane migration in lake sediments. Here we study fluid-induced fracturing of granular media by hydromechanically coupling a moving capacitor dynamic network model with discrete element modeling. We inject a less viscous ...
The basal condition for granular chute flows is assumed rough and non-slip in experimental and nu... more 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.
Grain size segregation in dense granular chute flows is generally explained by the mechanisms of ... more Grain size segregation in dense granular chute flows is generally explained by the mechanisms of kinetic sieving and squeeze expulsion (after [1]). The basic idea is that small grains preferentially fall through the underneath local voids, which are randomly opened by the shearing developed with the grains flowing down chute. The imbalance in contact force and the mass conservation normal to the base lead the larger grains to drift towards the top of the flow. In addition to extensive experimental studies, some theories have been proposed to describe the size segregation in bi-disperse flows [2]. Recently, discrete element (particle) modelling have been reported, which allows a more detailed description of particle velocities, volume fractions and contact forces. The results from this sort of modelling are also interpreted in the macroscopic perspectives, e.g. the rheology of the flow [3]. In this paper, three-dimensional DEM simulations are presented. Different cubic bi-disperse sa...
It is well established that the shear deformation response of granular materials depends on both ... more It is well established that the shear deformation response of granular materials depends on both the packing density and the stress level. One way to quantify the combined effect of these two parameters is the state parameter, ψ, that was proposed by [1]. The state parameter has formed the basis for a number of constitutive models to predict sand behaviour in continuum analysis. This contribution demonstrates that DEM simulations can quantitatively capture many of the correlations between the state parameter and features of soil strength and dilation that are experimentally documented in [2]. For example Figure 1(a) illustrates that DEM simulations of triaxial and true triaxial tests, using spherical particles, can capture the correlation between φ′p − φ′cs and the initial state parameter (ψ0), where φ′p and φ′cs are the peak and critical state angles of shearing resistance (friction angles), respectively. As demonstrated in [3] particular advantage of DEM over physical experiments ...
Base roughness plays an important role in the dynamics of granular flows but is still poorly unde... more 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, Ra , 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 Ra 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 Ra increases. Critical values of Ra are identified for the construction of a nonslip base at various angles of inclination.
Computational fluid dynamics and discrete element method (CFD–DEM) is extended with the volume of... more 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.
Three-dimensional DEM simulations of size segregation in granular flows down chute are presented.... more 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.
Geophysical mass flows involve granular earth materials surging down natural slopes, one of the m... more Geophysical mass flows involve granular earth materials surging down natural slopes, one of the major threats to mountainous regions worldwide. Accurate modeling of geophysical mass flows requires closure relations both within the flow (rheology) and at the flow-substrate interface (boundary conditions). However, although recent years have seen significant advances in the modeling of granular flow rheology, our understanding of how flowing granular materials interact with the substrate remains largely elusive. Here, we focus on micro-topography, i.e., geometric base roughness that is about the same size as the grain size, and investigate its effects on the granular flow dynamics as well as the associated closure relations. To systematically vary the base roughness from smooth to rough, we generate the base using immobile particles with varying particle size and spatial arrangement in laboratory experiments (with particle image velocimetry for flow kinematics extraction) and discrete element method simulations. Two granular flow scenarios are considered, including steady-state flow down inclines and granular column collapse. In the first scenario, it is found that basal slip occurs when the base roughness is below a range of intermediate values and a general slip law connecting the slip velocity, the mean flow velocity, and the base roughness is developed. In the second, transient flow scenario, basal slip inevitably occurs even for very rough bases due to inertial effects and a transient basal slip law is proposed to correlate the slip velocity with local flow properties based on kinetic theory arguments. The basal slip laws developed in this work can be readily incorporated as a dynamic boundary condition in continuum modeling of granular flows. In future work, grain-scale mechanisms relevant to more realistic geophysical flows will be investigated, including the feedback effects of pore fluid pressure on the flow mobility during basal sliding and the role of irregular particle shapes in hydro-mechanical modeling of geophysical mass flows.
To understand the effect of size segregation in the depositional process of debris flows, both fl... more 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...
Submitted for the DFD19 Meeting of The American Physical Society DEM Modeling of Coupled Multipha... more Submitted for the DFD19 Meeting of The American Physical Society DEM Modeling of Coupled Multiphase Flow and Granular Mechanics: Wettability Control on Fracture Patterns YUE MENG, BAUYRZHAN PRIMKULOV, Massachusetts Institute of Technology, ZHIBING YANG, State Key Laboratory of Water Resources and Hydropower Engineering, Wuhan University, FIONA KWOK, University of Hong Kong, RUBEN JUANES, Massachusetts Institute of Technology — The interplay between multiphase flow in a granular medium and the displacement of the grain particles generates a wide range of patterns. The balance between frictional, viscous, and capillary forces has been studied in experiments and simulations, and has helped understanding the underlying mechanisms for a wide range of phenomena, such as methane migration in lake sediments. Here we study fluid-induced fracturing of granular media by hydromechanically coupling a moving capacitor dynamic network model with discrete element modeling. We inject a less viscous ...
The basal condition for granular chute flows is assumed rough and non-slip in experimental and nu... more 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.
Grain size segregation in dense granular chute flows is generally explained by the mechanisms of ... more Grain size segregation in dense granular chute flows is generally explained by the mechanisms of kinetic sieving and squeeze expulsion (after [1]). The basic idea is that small grains preferentially fall through the underneath local voids, which are randomly opened by the shearing developed with the grains flowing down chute. The imbalance in contact force and the mass conservation normal to the base lead the larger grains to drift towards the top of the flow. In addition to extensive experimental studies, some theories have been proposed to describe the size segregation in bi-disperse flows [2]. Recently, discrete element (particle) modelling have been reported, which allows a more detailed description of particle velocities, volume fractions and contact forces. The results from this sort of modelling are also interpreted in the macroscopic perspectives, e.g. the rheology of the flow [3]. In this paper, three-dimensional DEM simulations are presented. Different cubic bi-disperse sa...
It is well established that the shear deformation response of granular materials depends on both ... more It is well established that the shear deformation response of granular materials depends on both the packing density and the stress level. One way to quantify the combined effect of these two parameters is the state parameter, ψ, that was proposed by [1]. The state parameter has formed the basis for a number of constitutive models to predict sand behaviour in continuum analysis. This contribution demonstrates that DEM simulations can quantitatively capture many of the correlations between the state parameter and features of soil strength and dilation that are experimentally documented in [2]. For example Figure 1(a) illustrates that DEM simulations of triaxial and true triaxial tests, using spherical particles, can capture the correlation between φ′p − φ′cs and the initial state parameter (ψ0), where φ′p and φ′cs are the peak and critical state angles of shearing resistance (friction angles), respectively. As demonstrated in [3] particular advantage of DEM over physical experiments ...
Base roughness plays an important role in the dynamics of granular flows but is still poorly unde... more 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, Ra , 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 Ra 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 Ra increases. Critical values of Ra are identified for the construction of a nonslip base at various angles of inclination.
Computational fluid dynamics and discrete element method (CFD–DEM) is extended with the volume of... more 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.
Three-dimensional DEM simulations of size segregation in granular flows down chute are presented.... more 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.
Uploads