Discrete modelling of capillary mechanisms in multi-phase granular media

A numerical study of multi-phase granular materials based upon micro-mechanical modelling is proposed. Discrete element simulations are used to investigate capillary induced effects on the friction properties of a granular assembly in the pendular regime. Capillary forces are described at the local scale through the Young-Laplace equation and are superimposed to the standard dry particle interaction usually well simulated through an elastic-plastic relationship. Both effects of the pressure difference between liquid and gas phases and of the surface tension at the interface are integrated into the interaction model. Hydraulic hysteresis is accounted for based on the possible mechanism of formation and breakage of capillary menisci at contacts. In order to upscale the interparticular model, triaxial loading paths are simulated on a granular assembly and the results interpreted through the Mohr-Coulomb criterion. The micro-mechanical approach is validated with a capillary cohesion induced at the macroscopic scale. It is shown that interparticular menisci contribute to the soil resistance by increasing normal forces at contacts. In addition, more than the capillary pressure level or the degree of saturation, our findings highlight the importance of the density number of liquid bonds on the overall behaviour of the material

Localized fluidization in granular materials: Theoretical and numerical study

We present analytical and numerical results on localized fluidization within a granular layer subjected to a local injection of fluid. As the injection rate increases the three different regimes previously reported in the literature are recovered: homogeneous expansion of the bed, fluidized cavity in which fluidization starts developing above the injection area, and finally the chimney of fluidized grains when the fluidization zone reaches the free surface. The analytical approach is at the continuum scale, based on Darcy's law and Therzaghi's effective stress principle. It provides a good description of the phenomenon as long as the porosity of the granular assembly remains relatively homogeneous, i.e. for small injection rates. The numerical approach is at the particle scale based on the coupled DEM-PFV method. It tackles the more heterogeneous situations which occur at larger injection rates. The results from both methods are in qualitative agreement with data published independently. A more quantitative agreement is achieved by the numerical model. A direct link is evidenced between the occurrence of the different regimes of fluidization and the injection aperture. While narrow apertures let the three different regimes be distinguished clearly, larger apertures tend to produce a single homogeneous fluidization regime. In the former case, it is found that the transition between the cavity regime and the chimney regime for an increasing injection rate coincides with a peak in the evolution of inlet pressure. Finally, the occurrence of the different regimes is defined in terms of the normalized flux and aperture

Numerical simulations of dense suspensions rheology using a dem-fluid coupled model

The understanding of dense suspensions rheology is of great practical interest for both industrial and geophysical applications and has led to a large amount of publications over the past decades. This problem is especially difficult as it is a two-phase media in which particle-particle interactions as well as fluid-particle interactions are significant. In this contribution, the plane shear flow of a dense fluid-grain mixture is studied using the DEM-PFV coupled model. We further improve the original model: including the deviatoric part of the stress tensor on the basis of the lubrication theory, and extending the solver to periodic boundary conditions. Simulations of a granular media saturated by an incompressible fluid and subjected to a plane shear at imposed vertical stress are presented. The shear stress is decomposed in different contributions which can be examined separately: contact forces, lubrication forces and drag forces associated to the poromechanical couplings

What is wrong in love-weber stress for unsaturated granular materials?

This paper presents the micromechanical model for unsaturated soil in pendular regime, taking into account the roughness of the grains and the interfaces that separate the different phases present in the medium. It supplements the oral presentation with more technical content. Laplace equation is solved for two grains configuration to calculate the capillary force and all the geometric properties of the meniscus connecting the grains. Many configurations are solved and the look up table method is then used during the simulations. Results for grains moving at constant suction and constant vol- ume are presented. It is also shown that the roughness has an important impact on the value of capillary force and it is evolution with the change of suction

A pore-scale approach of two-phase flow in granular porous media

A pore-scale model is presented for simulating two-phase flow in granular materials. The solid phase is idealized as dense random packings of polydisperse spheres, generated with the discrete element method (DEM). The pore space is conceptualized as a network of pores connected by throats, which is obtained by using regular triangulation. Theoretical formulas for calculating geometrical properties and entry capillary pressure for given pores are developed by extending the Mayer and Stowe-Princen (MS-P) theory of drainage. Such relationships are employed in the network for defining as local invasion criteria, so that the drainage can be represented by the replacement of W-phase when the threshold value is reached. The events of W-phase entrapment are considered during the coupling procedures. This pore-scale model is verified by comparing simulation results with experimental data of quasi-static drainage experiments in a synthetic porous medium. The simulated Pc −Sw curve in primary drainage is in agreement with the experimental one

К вопросу применения комплексных систем контроля производственного процесса на урановых шахтах

Викладено технічні, технологічні й соціальні передумови застосування комплексних систем контролю виробничого процесу на уранових шахтах.Sets out technical, technological and social conditions of application of complex process control systems in the uranium mines

A pore-scale hydro-mechanical coupled model for geomaterials

We present a model for fluid-saturated granular media coupled flow and mechanical deformation. The fluid is assumed to be incompressible and the solid part is assumed to be a cohesive granular material. Forces exerted by the fluid in motion are determinated and applied to solid particles. We derive a finite volumes formulation of the flow problem and we couple it to a discrete element method (DEM) formulation of the solid deformation. The ability of the algorithm to solve transient problems is tested by simulating an oedometer test on a soil sample. The numerical solution of our model is in good agreement with Terzaghi’s analytical solution

Investigation of internal erosion processes using a coupled DEM-fluid method

The evolution of granular beds subjected to upward seepage flow is investigated using a coupled DEM-fluid model implemented by Catalano et al. in the open-source software Yade-DEM. Firstly, filtration properties of a coarse narrowly graded material are analyzed by simulating the transport of smaller particles from a base layer through the coarse filter by gravitational loading or downward flow with uniform pressure gradient. The results are analysed on the basis of the constriction size distribution (CSD) of the filter which describes statistically the sizes of throats between pores in the material. Secondly, we examine the results obtained when, instead of two different layers, the coarse and fine materials are initially mixed in one unique layer and subjected to gravity. Thirdly, this mixture of coarse and fine particles is subjected to both gravity and a non-uniform pressure gradient, by injecting the fluid in one point below the layer, as inspired by previous experiments. Similar channeling patterns are obtained in both experiments and simulations when the boundary condition at the injection point is an imposed flux. This boundary condition results in a recirculation mechanism that remains confined in a finite zone around the injection point as long as the flux is below a threshold value. By simulating an imposed pressure condition, we finally show that instabilities can be triggered by the transport of small particles away from the injection point. This segregation process results in a lower porosity and an increased pressure gradient above the eroded zone, so that the instability-triggering pressure gradient in bi-dispersed mixtures is lower than in mono-dispersed mixtures

Numerical modeling of particle migration in granular soils

Suffusion is the process of internal erosion where fine particles migrate under water seepage through a coarser soil matrix. Relevant models of suffusive phenomena must reproduce the poromechanical effects that result from the two-way coupling between the deformation of the solid matrix, the fluid pressure and the flow. In this work, an advanced computational method is used to study the particle migration in granular soils. The so called coupled Discrete Element Method - Pore scale Finite Volume (DEM-PFV) is based on a microscopic hydromechanical approach. It couples the discrete element method that solves the equations of motion for the solid fraction, with a PFV method that solves the fluid flow equations. We use this method to study particle transport through coarser granular assemblies that do not evolve with time. These simulations allow us to obtain, for different cases, the parameters to include in a general advection-dispersion equation (ADE). We paid particular attention to the role played by the intermittent formation of blockages of transported particles in the constrictions of the granular assembly. These temporary and collective trapping events change local fluid flows and affect the particle transport on short time or length scales. As the transport time between consecutive blockages and the duration of blockages have exponential decays, sink and source terms can be added to the ADE

A discrete numerical description of the mechanical response of soils subjected to degradation by suffusion

Internal erosion is a major cause of the failure of hydraulic earthen structures. A particular case of such an erosion process is suffusion which constitutes a strongly coupled fluid-solid interaction problem. It is a selective erosion of fine particles from an unstable soil structure leaving behind the granular skeleton which possibly leads to deformations. Such a process may cause modification in the mechanical behavior of the soil. To study this problem numerically, a model is established based on the discrete element method implemented in Yade software (Smilauer et al. 2015). Periodic boundary conditions are adopted and the soil is represented by a 3D assembly of spherical discrete elements. Such an oversimplified particle’s shape leads to excessive rolling. To overcome this obstacle, rolling resistance was taken into consideration in the inter-particle contact law. Bearing in mind that numerical modeling of suffusion can constitute a difficult task requiring important computational resources due to the direct description of interactions between solid and liquid phases, a one-way coupling with a fluid phase is considered here. However, effects on the soil due to the loss of a fraction of fine particles is investigated either by modeling soils with different grains size distribution and different initial fines content to characterize its influence on the soil microstructure, or by mimicking the suffusion process by defining an extraction criterion of potentially erodible particles. This extraction criterion is based on the size of the particles, constriction sizes, and the velocity of particles under the effect of fluid forces. From these two approaches, we were able to specify the fines content from which their erosion may have a significant influence on the microstructure. Moreover, the defined extraction criterion was able to describe the effect of erosion on the stability of the soil structure