United States

We use three-dimensional particle dynamics simulations to investigate the microstructure evolution of granular material subjected to isochoric cyclic shearing, driving the system to a liquefaction state. The cyclically sheared assembly presents a realistic macroscopic response as observed in physical experiments. By analyzing the contact network evolution in the post-liquefaction period, we show that the onset of liquefaction state is characterized by a sudden drop of coordination number and a fragile particle connectivity network. The simulation suggests a critical coordination number for exiting the liquefaction state. Evolution of fabric anisotropy combined with coordination number implies the isotropic and anisotropic gain or loss of contacts at certain durations of a post-liquefaction loading cycle.

Powders &amp; Grains 2021

Geometrical network of granular materials under isochoric cyclic shearing

[EPJ Web of Conferences, Volume 249 (2021)](https://www.epj-conferences.org/articles/epjconf/abs/2021/03/contents/contents.html)

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Powders & Grains 2021

Powders & Grains 2021 will be held in Buenos Aires (Argentina, VIRTUAL) July 5, 13, 21, 29 and Aug 6, 2021. The aim of the conference is to give an up-to-date picture of the broad research field of granular media, powders and grains. Contributions from experts around the world will focus on research topics that cover a wide range of open problems in the field.

poster

The question of how soft granular matter, or dense amorphous systems, re-arrange their micro-structure under isotropic compression and decompression, at different strain rates, will be answered by particle simulations of frictionless model systems in a periodic three-dimensional cuboid. Starting compression below jamming, the systems experience the well-known jamming transition, with characteristic evolutions of the state variables elastic energy, elastic stress, coordination number, and elastic moduli. For large strain rates, kinetic energy comes into play and the evolution is more dynamic. In contrast, at extremely slow deformation, the system relaxes to hyper-elastic states, with well-defined elastic moduli, in static equilibrium between irreversible (plastic) re-arrangement events, discrete in time. Small, finite strains explore those reversible (elastic) states, before larger strains push the system into new states, by irreversible, sudden re-arrangements of the microstructure.

How does static granular matter re-arrange for different isotropic strain rate?

We experimentally investigate the effect of the grain shape on the flow of granular material. The grain shape is modified to highlight the effect of grain circularity on granular flow in a 2D rotating drum. Using a laser cutter, we create particles with decreasing circularity. We observe that the effect of grain shape depends on the rotation speed of the drum. For high rotation speed, granular flow is influenced by the packing’s dilatancy whereas, at low rotation speed, packing fraction seems to influence flowing dynamics. We link these two measurements to grain shape in order to explain its effect on granular flow.

Effect of grain shape on the dynamics of granular materials in 2D rotating drum

The rhombic sphere packing can be used to model granular soils in a very simple way. According to the angle of assemblage, the packing is dilatant or contractive. Correspondingly, the overall stresses are transmitted as chains of forces or oblique forces of contact. The connection of the soil stress-strain behaviour and the packing void ratio is achieved by mapping both of the plots. The mapping shows that dense soils are dilatant and loose soils are contractive, separated by the critical state. It also shows that the bifurcation point and the peak strength are features of dense soils. The line of strain localization is analysed in the elastic regime, and its inclination is found maximizing the intensity of the mobilized stress ratio. The stresses within the shear band are obtained by assuming a partially coaxial packing rotated for reaching the full plastic state. The equilibrium of the overall stress at the line of discontinuity leads to the relationship between the peak friction angle and the coefficient of lateral pressure at rest. As long as these parameters are independently obtained from each other, they allow the validation of the theory.

A sphere packing model for shear bands in dense soils

The present study is on segregation of granular mixtures during heap formation in a quasi two-dimensional rectangular bin where binary mixture of a specified composition is poured intermittently into the auxiliary hopper and then allowed to pass through the gap (k = 10 mm) between the divider and the plate and finally settle on to the heap. The profiles of number fraction of big particles are plotted along the flow directions to study the segregation phenomena for surface profile. It shows that larger particles travel more distance and smaller particles settle near the pouring point for all cases studied.

Experimental investigation of segregation of granular mixtures during heap formation

A nonlinear programming problem was developed for the separation and classification of
Portland cement particles into different fractions through the optimal design of two cyclones classifier in
series. The equations and restrictions considered included the global mass balances, the equations for the
geometric design of the cyclones, the equations for the efficiency calculation, the operating limitations of
the process and the pressure drops of the equipment. The results show that an increase on the particles cut
size led to a greater dimensions cyclone, as expected. Moreover, fractional efficiencies obtained increase
with the reduction of the cut size. The solids load effect is also evaluated, reductions of the solids feed by
50% of its maximum value does not result in a remarkably decrease of efficiencies values. The proposed
model has provided useful information and constitutes a starting point for a better understanding of the
overall separation process of multi-component Portland cement mixtures. It is a valuable tool for the correct
design of this type of separators, given its versatility to optimize under different operating conditions and
with different materials.


Optimal design of cyclones in series for the separation of multi-component mixtures of Portland cement

Simple geometry and multiple applications such as drying, mixing, segregation, drying, granulation and heat conduction have inspired many researchers from the mechanical, chemical, pharmaceutical and mining engineering fields to study the dynamics of the particle flow inside rotating drums. Therefore, knowledge of the dynamics of particles in a drum is of high interest in engineering such as velocity profiles. Various experimental and numerical studies have been carried out to study the velocity profiles of the particles inside horizontal rotating drums. Little emphasis has been laid on inclined rotating drums, though these drums are extensively used in granular process industries. In this study Discrete Element Method (DEM) is used to analyse the effect of angle of inclination of the drum on the particles velocity profile.

Particle velocity profile in an inclined rotating drum

When a mechanical wave travels through a medium, its intensity diminishes with distance. The research focuses on the energy transfer with distance as well as across different wavenumbers, as the mechanical wave propagates. The diffusive characteristic of energy propagation has been discussed for one-dimensional chains composed of random, pre-stressed soft and stiff particles interacting through Hertzian repulsive forces, which can be solved analytically after linearization. The effect of soft-stiff ratio (disorder in property) on energy transfer across wavenumbers is examined using a standing sinusoidal wave initial condition (with a specific wavenumber). From the total energy signals in wavenumber space, as a function of time, it is observed that stronger disorder leads to a more rapid loss of energy of the signal and faster transfer of energy to other wavenumbers.

Energy propagation in 1D granular soft-stiff chain

This paper experimentally analyzes the collapse of conical granular columns made out of non-cohesive granular materials. Dimensionless run-out lengths were found, as a function of the aspect ratio a, similar to those occurring in the collapse of cylindrical columns, although in the case of conical columns of high aspect ration, at the end of the collapse, a layer of grains that rotates around the cone axis appears and it affects the final profiles of the mounds.

Collapse of conical granular columns

Acoustic emission (AE) or vibration signal has been applied in detecting operations of grinding mills in many industries. This paper proposes an approach to generate AE signals based on the particle-wall impacts. Through a combination of multi-mode vibrations and the calibration of the key parameters, the model was able to reproduce experimental data. The AE model was then implemented into a discrete element method (DEM) modelling of particle flow in a rotating mill. The AE signals of the mill under different filling levels and rotation speeds were generated and analysed, mainly focusing on the frequency and magnitude of each vibration mode. The link between the AE signals and the particle-wall impact energy was explored

Acoustic signals of rotating drums generated based on DEM simulations

The soil water retention curve (SWRC) is the most commonly used relationship in the study of
unsaturated soil. In this paper, the eect 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.

Geometrical network of granular materials under isochoric cyclic shearing

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