Structure of Sheared Cohesive Granular Bulk (original) (raw)

Shear flow of assemblies of cohesive and non-cohesive granular materials

Powder Technology, 2006

We have studied plane shear flow of nearly homogeneous assemblies of uniformly sized, spherical particles in periodic domains. Our focus has been on the effect of interparticle attractive forces on the flow behavior manifested by dense assemblies. As a model problem, the cohesion resulting from van der Waals force acting between particles is considered. Simulations were performed for different strengths of cohesion, shear rates, particle stiffnesses, particle volume fractions and coefficients of friction. From each simulation, the average normal and shear stresses and the average coordination number have been extracted. Not surprisingly, the regimes of flow reported by Campbell [C.S. Campbell, Granular Shear Flows at the Elastic Limit, J. Fluid Mech. 465 (2002) 261-291] for the case of cohesionless particlesnamely, inertial, elastic-inertial and elasticquasistatic regimespersist when cohesion is included. The elastic-quasistatic regime was found to correspond with the coordination number decreasing with increasing shear rate, while in the inertial regime the coordination number increased with shear rate. A striking result observed in the simulations is that the influence of cohesion on stress becomes more pronounced with decreasing particle volume fraction. Furthermore, the window in the shear rate-particle volume fraction space over which the elastic-quasistatic regime is obtained was found to expand as the strength of cohesion was increased. When the particle volume fraction is so high that even a cohesionless system would be in the elastic-quasistatic regime, the addition of cohesion had minimal effect on the stresses. At lower particle volume fractions where a cohesionless assembly would have been in the inertial regime, we present a new scaling which permits collapse of all the data at various strengths of cohesion and shear rates into a single master curve for each particle volume fraction and coefficient of friction. The regimes of flow in this master curve are discussed and the scaling for the normal stress in elastic-quasistatic flow is identified.

Shear flow characteristics of densely packed granular material subjected to slow deformations

Journal of Nepal Geological Society

The densely packed assembly of granular materials subjected to slow deformations is studied experimentally in the 2D shear fl ow apparatus. High speed video camera and subsequent image processing techniques help to document the positions of the particles in the fl ow. Effective algorithms are formulated to determine the particle rotation, group size and local particle concentrations. Experimental results depict that the consecutive cycles of solid like (jammed) and fl uid like (un-jammed) states characterize the fl ow. The jammed state is represented by negligible mobilization of particles, whereas the un-jammed state is represented by considerable mobilization of particles. The rotational and translational kinetic energy shares their dominancy in the jammed and un-jammed states respectively. Nevertheless, rotational counterpart also increases quite high in un-jammed state. There exists clearly a gradient of translational and rotational velocity across the shear cell especially in the un-jammed state indicating the phenomenon of strain localization. The un-jammed state originates because of the breaking and buckling of few columns near to the inner moving wall as noticed by previous researchers, and the jammed state regenerates once the broken and buckled columns regrouped into new columns. The dilatation phenomenon is found to be associated with the un-jamming states indicated by the drop in the local particle concentrations.

Shear flow of assemblies of cohesive granular materials under constant applied normal stress

Powder Technology, 2008

We have studied plane shear flow of nearly homogeneous assemblies of uniformly sized, spherical, cohesive particles in periodic domains under constant applied normal stress. Our focus has been on (a) exploration of the effect of inter-particle attractive forces on the flow behavior manifested by dense assemblies under constant applied normal stress, and (b) comparison of the rheological characteristics observed under constant-applied normal stress and constant-volume conditions. As a model problem, the cohesion resulting from van der Waals force acting between particles is considered. Simulations were performed for different strengths of cohesion, shear rates, and applied stresses. From each simulation, the volume fraction, shear stress and the average coordination number have been extracted. We find that cohesive assemblies sheared under constant applied normal stress shear differently from those sheared at constant volume only in the dynamic sense, while the time-averaged rheological characteristics are essentially indistinguishable. At constant volume, the fluctuations in shear stress are larger than, but have the same dependence on cohesion as under constant applied normal stress. This study has also exposed a pronounced dependence of the apparent coefficient of friction on particle volume fraction in the quasi-static flow regime.

Rheology of cohesive granular materials across multiple dense-flow regimes

Physical Review E

We investigate the dense-flow rheology of cohesive granular materials through discrete element simulations of homogeneous, simple shear flows of frictional, cohesive, spherical particles. Dense shear flows of noncohesive granular materials exhibit three regimes: quasistatic, inertial, and intermediate, which persist for cohesive materials as well. It is found that cohesion results in bifurcation of the inertial regime into two regimes: (a) a new rate-independent regime and (b) an inertial regime. Transition from rate-independent cohesive regime to inertial regime occurs when the kinetic energy supplied by shearing is sufficient to overcome the cohesive energy. Simulations reveal that inhomogeneous shear band forms in the vicinity of this transition, which is more pronounced at lower particle volume fractions. We propose a rheological model for cohesive systems that captures the simulation results across all four regimes.

Measurement of Particle Dynamics in Rapid Granular Shear Flows

Journal of Engineering Mechanics, 2009

Micromechanics of rapid granular flows is studied in a two-dimensional planar granular Couette flow apparatus. The device is capable of generating particulate flows at different shearing rates and solid fractions. Mono size plastic disks are sheared across an annular test-section for several shear rates. Motion of particles is recorded through high speed digital camera and analyzed by image processing techniques. The average and fluctuation velocity profiles are obtained and granular temperature relations with shear rate are investigated. Average streaming velocity across the shear cell decays slightly faster than exponential, and is rather Gaussian when not too close to the wall. Fluctuation velocities and granular temperature across the shear cell are related to effective shear rate. Interparticle collisions are estimated from the particle trajectories and probability distribution of collision angles obtained from particle collision data. In dense flows, three peaks of collision angles are observed which signal the onset of triangular structure formulation and cause crystallization. It is found that the distribution of collision angles is anisotropic. CE Database subject headings: Couette flows; Flow properties; Granular temperature; Interparticle collisions

Steady state rheology of homogeneous and inhomogeneous cohesive granular materials

Granular Matter

This paper aims to understand the effect of different particle/contact properties like friction, softness and cohesion on the compression/dilation of sheared granular materials. We focus on the local volume fraction in steady state of various non-cohesive, dry cohesive and moderate to strong wet cohesive, frictionless-to-frictional soft granular materials. The results from (1) an inhomogeneous, slowly sheared split-bottom ring shear cell and (2) a homogeneous, stress-controlled simple shear box with periodic boundaries are compared. The steady state volume fractions agree between the two geometries for a wide range of particle properties. While increasing inter-particle friction systematically leads to decreasing volume fractions, the inter-particle cohesion causes two opposing effects. With increasing strength of cohesion, we report an enhancement of the effect of contact friction i.e. even smaller volume fraction. However, for soft granular materials, strong cohesion causes an inc...

Unsteady Shear of Dense Assemblies of Cohesive Granular Materials under Constant Volume Conditions

Industrial & Engineering Chemistry Research, 2010

The response characteristics of dense assemblies of cohesive granular materials to unsteady simple shear in the quasi-static regime are investigated through discrete element method (DEM) simulations of monodisperse spherical and frictional particles in periodic domains at constant volume. The dynamics of the volume-averaged normal and shear stresses in materials, undergoing stop-and-go shearing and oscillatory shear, are studied in detail. Furthermore, the evolution of microstructure anisotropy has been quantified through a fabric tensor. The stresses and the microstructure anisotropy depend on the strain extent but not on the shear rate. They both undergo a transition following reversal of shear direction, which requires a shear strain of order unity to fully adapt. The results reveal a correlation between the stress evolution and the microstructure anisotropy development.

Density behavior of cohesive granular materials

Powder Technology, 2011

A new experimental methodology for the characterization of density in a powder bed utilizing X-ray microcomputerized tomography (micro-CT) was developed to quantify the density fluctuations in three common pharmaceutical powders (α-Lactose Monohydrate, Lactose 310, and Avicel 102). The method begins by filling an acrylic cylinder with powder and subsequently subjecting the system to vibrations using a mechanical shaker while monitoring the density at predetermined bed heights. Three key parameters were isolated including frequency, amplitude, and number of strokes. It was found that the three powders exhibited different packing rates and final states. It was also found that the density increased in the powder bed as a function of the number of taps, frequency, and amplitude. Additionally, a more uniform density profile was achieved by utilizing higher amplitudes. The cohesive properties of the three powders were investigated using the FT4 powder rheometer and correlated with the results found with the micro-CT scanner. It was found that changes in density were more significant in less cohesive powders, such as Avicel. As powders increase in cohesion, it was found that more mechanical energy was required to alter the agglomerated powder bed. Additionally, the density at the top of the powder bed was significantly more dense than that at the bottom for Avicel, however, the results were directly opposite for the other more cohesive powders. The results have indicated that micro-CT may be used as a more comprehensive and higher resolution technique for analyzing the density of powders and provide a unique insight to packing at different powder bed heights. Published by Elsevier B.V.

Concentration non-uniformity in simple shear flow of cohesive powders

Powder Technology, 2000

A method is developed to quantify the particle concentration non-uniformity of cohesive powders in a simple shear flow for both dilute and dense conditions. The ratio of the variance of the particle number concentration, n, to the square of mean particle number ² X 2 : ² : 2 concentration, r s n r n , is first obtained using a series of sub-cells of different volumes based on a discrete element simulation of particle dynamics in a simple shear flow. The deviation, D r, of r from the rapid shear rate limit at the same bulk concentration is taken as the measure of the particle concentration non-uniformity. The effects of shear rates on this concentration non-uniformity are quantitatively examined. The quantitative value of this non-uniformity is used to identify a change in the micro-scale cluster structure of dense powder flows under various shear rates and to help in understanding the mechanism for the observed non-monotonic stress-strain rate behavior. q 2000 Elsevier Science S.A. All rights reserved.

Numerical study of a thin layer of cohesive particles under plane shearing

Powder Technology, 2005

This article presents the development of 3D computer simulations using the discrete element method to study solid third body flows (particles trapped in a contact between two bodies). The configuration chosen here is plane shearing in which a thin layer of particles is sheared between two walls. A confining pressure is imposed on the upper wall and a horizontal velocity is imposed on the lower wall, thereby shearing the granular media between the two walls. The rheology of this granular media is studied and a transformation from a quasi-fluid behaviour to quasi-solid behaviour is highlighted. This rheology considerably affects the macroscopic friction coefficient, which increases with adhesion and then decreases. These variations are linked to the different states of the granular media. This work emphasises the complexity of the behaviour of the interfacial particle layer. Consequently, the friction coefficient measured when two pieces are in contact may greatly depend on the behaviour of this particle layer.