Liquid clustering and capillary pressure in granular media

International audienceBy means of extensive lattice Boltzmann simulations, we investigate the process of growth and coalescence of liquid clusters in a granular material as the amount of liquid increases. A homogeneous grain-liquid mixture is obtained by means of capillary condensation, thus providing meaningful statistics on the liquid distribution inside the granular material. The tensile stress carried by the grains as a function of the amount of condensed liquid reveals four distinct states, with a peak stress occurring at the transition from a primary coalescence process, where the cohesive strength is carried mostly by the grains, to a secondary process governed by the increase of the liquid cluster volumes. We show that the evolution of capillary states is correctly captured by a simple model accounting for the competing effects of the Laplace pressure and grain-liquid interface

Investigating the effect of porosity on the soil water retention curve using the multiphase Lattice Boltzmann Method

The soil water retention curve (SWRC) is the most commonly used relationship in the study of unsaturated soil. In this paper, the effect of porosity on the SWRC is investigated by numerically modeling unsaturated soil using the Shan-Chen multiphase Lattice Boltzmann Method. The shape of simulated SWRCs are compared against that predicted by the van Genuchten model, demonstrating a good fit except at low degrees of saturation. The simulated SWRCs show an increase in the air-entry value as porosity decreases.Comment: 4 pages, 5 figures, submitted to Powder & Grains 2021, references adde

Study of capillary interaction between two grains: a new experimental device with suction control

International audienceWe investigated the behavior of a water liquid bridge formed between two grains. We mainly focused on tensile tests with suction control (capillary pressure). Theoretical and experimental studies are compared. A new experimental device involving suction control of the liquid bridge was developed specifically for this kind of test. Most of the liquid bridge variables and characteristics were measured by image analysis (gorge radius, volume, contact angles, filling angles). Capillary force was measured by differential weighting. Experimental conditions allows us to avoid viscous effects. Our experimental results were close to Young- Laplace equation solutions. The "gorge method", commonly used for calculating the capillary force, was also validated by our experiments. Liquid bridge rupture was studied and a new rupture criterion is proposed. This criterion depends on the grain radius, contact angle, surface tension and suction and was in agreement with the experimental results

Simulation numérique de la fissuration d'un matériau granulaire cimenté par une approche peridynamique

De nombreux matériaux naturels, aliments, bio-composites, présentent une grande variabilité de composition en termes de structure et de propriétés mécaniques des phases et interfaces qui les constituent. Dans ces matériaux le comportement à la rupture et la propagation des fissures ont fait l’objet de différentes études numériques. Ces études s'appuient sur une description plus ou moins fine du maillage et du comportement rhéologique associé aux phases et aux interfaces (éléments fusibles, méthodes sur réseaux réformables, éléments finis, zones cohésives...). Le choix d'une méthode numérique reste néanmoins complexe et conditionne les possibilités en termes de prise en compte de structures microscopiques riches. Récemment, une méthode appelée peridynamique a été appliquée par différents auteurs à la simulation de la propagation de fissures dans des milieux hétérogènes. Cette méthode semble présenter des qualités particulièrement intéressantes en termes d'efficacité numérique, mais reste peu explorée. Elle s'appuie sur une description alternative de la mécanique des milieux continus reposant sur l'intégration de forces à distance s’exerçant entre points matériels. Un avantage majeur de cette approche, qui s'apparente à la dynamique moléculaire 'gros grain', est de limiter la dépendance des directions de propagation de fissures à la discrétisation du milieu. Dans cet exposé on s'intéresse à une approche peridynamique simple (dite « bond-based ») pour laquelle on s'attachera à caractériser les effets de taille de mailles et l'impact d'une distribution de phases complexes, de type granulaire cimenté, sur le comportement mécanique et les régimes de ruptures

Changes in the starch-protein interface depending on common wheat grain hardness revealed using atomic force microscopy

The Atomic Force Microscope tip was used to progressively abrade the surface of non-cutted starch granules embedded in the endosperm protein matrix in grain sections from wheat near-isogenic lines differing in the puroindoline b gene and thus hardness. In the hard near-isogenic wheat lines, starch granules exhibited two distinct profiles corresponding either to abrasion in the surrounding protein layer or the starch granule. An additional profile, only identified in soft lines, revealed a marked stop in the abrasion at the protein-starch transition similar to a lipid interface playing a lubricant role. It was related to the presence of both wild-type puroindolines, already suggested to act at the starch-protein interface through their association with polar lipids.This study revealed, for the first time, in situ differences in the nano-mechanical properties at the starch-protein interface in the endosperm of wheat grains depending on the puroindoline allelic status

Fluidization of irregular particles - Part I: A discrete element method to model collisions between non-convex particles

The flow dynamics of a fluidized bed can be very complicated. As the solid volume fraction is generally high, particle-particle collisions cannot be ignored. Many studies in the literature deal with perfectly spherical particles while very few deal with non-spherical ones and even less with angular or non-convex particles. However, these irregularly shaped particles are not uncommon in chemical engineering. Among others, Escudié et al (1) showed that the particle shape influences markedly the dynamics of such a system. We suggest an accurate and efficient way to model collisions between particles of (almost) arbitrary shape, that can be integrated into a comprehensive modeling of a fluidized bed. For that purpose, we develop a Discrete Element Method (DEM) combined with a soft particle contact model that treats the contact between bodies of various shape and size (2). In particular, for non-convex bodies, our strategy is based on decomposing a non-convex body into a set of convex ones (3). Therefore, our novel method can be called “glued convex method”, as an extension of the popular “glued-spheres” method (4). It hence uses all the features involved in DEM simulations of convex bodies, such as the contact detection strategy based on a Gilbert-Johnson-Keerthi algorithm (5) and the linked-cell spatial sorting which accelerates the contact resolution (6). The problem of multiple contact requires a particular attention (4,7). The method is implemented in our granular dynamics code Grains3D. As an illustration of the powerful modelling capabilities of Grains3D, we show results of simulation of settling non-convex catalytic pellets in a cylindrical chemical reactor. REFERENCES R. Escudié, N. Epstein, J.R. Grace, H.T. Bi, Effect of particle shape on liquid-fluidized beds of binary (and ternary) solids mixtures: segregation vs. mixing. Chemical Engineering Science, 61(5): 1528, 2006. A. Wachs, L. Girolami, G. Vinay, and G. Ferrer. Grains3D, a flexible DEM approach for particles of arbitrary convex shape - Part I: Numerical model and validations. Powder Technology, 224:374-389, 2012. A. D. Rakotonirina, A. Wachs, J.-Y. Delenne, F. Radjai. Grains3D, a flexible DEM approach for particles of arbitrary convex shape - Part III: extension to non convex particles, submitted to Powder Technology, 2015. D. Höhner, S. Wirtz, H. Kruggel-Emden, and V. Scherer. Comparison of the multi-sphere and polyhedral approach to simulate non-spherical particles within the discrete element method: Influence on temporal force evolution for multiple contacts. Powder Technology, 208(3):643-656, 2011. Elmer G. Gilbert, Daniel W. Johnson, and S. Sathiya Keerthi. A fast procedure for computing the distance between complex objects in three-dimensional space. Robotics and Automation, IEEE Journal of Robotics and Automation, 4(2):193-203, 1988. Gary S. Grest, Burkhard Dünweg, and Kurt Kremer. Vectorized link cell Fortran code for molecular dynamics simulations for a large number of particles. Computer Physics Communications, 55(3):269-285, 1989. H. Kruggel-Emden, S. Rickelt, S. Wirtz, and V. Scherer. A study on the validity of the multi-sphere discrete element method. Powder Technology, 188(2):153-165, 2008

Cohesive granular materials composed of nonconvex particles

International audienceThe macroscopic cohesion of granular materials made up of sticky particles depends on the particle shapes. We address this issue by performing contact dynamics simulations of 2D packings of nonconvex aggregates. We find that the macroscopic cohesion is strongly dependent on the strain and stress inhomogeneities developing inside the material. The largest cohesion is obtained for nearly homogeneous deformation at the beginning of unconfined axial compression and it evolves linearly with nonconvexity. Interestingly, the aggregates in a sheared packing tend to form more contacts with fewer neighboring aggregates as the degree of nonconvexity increases. We also find that shearing leads either to an isotropic distribution of tensile contacts or to the same privileged direction as that of compressive contacts

Two-dimensional numerical simulation of chimney fluidization in a granular medium using a combination of discrete element and lattice Boltzmann methods

We present here a numerical study dedicated to the fluidization of a submerged granular medium induced by a localized fluid injection. To this end, a two-dimensional (2D) model is used, coupling the lattice Boltzmann method (LBM) with the discrete element method (DEM) for a relevant description of fluid-grains interaction. An extensive investigation has been carried out to analyze the respective influences of the different parameters of our configuration, both geometrical (bed height, grain diameter, injection width) and physical (fluid viscosity, buoyancy). Compared to previous experimental works, the same qualitative features are recovered as regards the general phenomenology including transitory phase, stationary states, and hysteretic behavior. We also present quantitative findings about transient fluidization, for which several dimensionless quantities and scaling laws are proposed, and about the influence of the injection width, from localized to homogeneous fluidization. Finally, the impact of the present 2D geometry is discussed, by comparison to the real three-dimensional (3D) experiments, as well as the crucial role of the prevailing hydrodynamic regime within the expanding cavity, quantified through a cavity Reynolds number, that can presumably explain some substantial differences observed regarding upward expansion process of the fluidized zone when the fluid viscosity is changed

Cinétique de croissance d'une cheminée fluidisée par modélisation DEM-LBM

Au-delà d'un certain seuil, un écoulement fluide ascendant à débit constant, injecté à travers une section de petite dimension, parvient à générer une fluidisation sur toute la hauteur d'une couche de grain, le long d'une cheminée fluidisée. La fluidisation est amorcée à proximité directe de l'injection puis la zone fluidisée croît progressivement vers le haut jusqu'à la surface supérieure de l'empilement granulaire. Nous présentons ici des résultats numériques pour cette cinétique de croissance de cheminée fluidisée dans un lit granulaire immergé en s'appuyant sur une simulation bidimensionnelle couplant la Méthode aux Eléments Discrets et la Méthode Lattice Boltzmann (DEM-LBM).Une étude paramétrique a été menée et une analyse dimensionnelle nous a permis de proposer des lois d'échelle pour le seuil de fluidisation et pour la taux de croissance de la zone fluidisée

Rheology of three-dimensional packings of aggregates: Microstructure and effects of nonconvexity

International audienceWe use 3D contact dynamics simulations to analyze the rheological properties of granular materials composed of rigid aggregates. The aggregates are made from four overlapping spheres and described by a nonconvexity parameter depending on the relative positions of the spheres. The macroscopic and microstructural properties of several sheared packings are analyzed as a function of the degree of nonconvexity of the aggregates. We find that the internal angle of friction increases with nonconvexity. In contrast, the packing fraction increases first to a maximum value but declines as nonconvexity further increases. At high level of nonconvexity, the packings are looser but show a higher shear strength. At the microscopic scale, the fabric and force anisotropy, as well as friction mobilization are enhanced by multiple contacts between aggregates and interlocking, revealing thus the mechanical and geometrical origins of shear strength