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Flowing grains can clog an orifice by developing arches, an undesirable event in many cases. Several strategies have been put forward to avoid this. One of them is to vibrate the system in order to undo the clogging. Nevertheless, the time taken to break an arch under a constant vibration has a distribution displaying a heavy tail. This can lead to a situation where the average breaking time is not well defined. Moreover, it has been observed in some experiments that these tails tend to flatten for very long times, exacerbating the problem. Here we will review two conceptual frameworks that have been proposed to understand the phenomenon and discuss their physical implications.

Powders &amp; Grains 2021

On the broad tails in breaking time distributions of vibrated clogging arches

[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

We report numerical simulations of surface granular flows confined between two sidewalls. These systems exhibit both very slow and very energetic flows.
Zhu et al. [Zhu et al. granular Matter, 22, 82, 2020] have shown that in energetic confined systems, the Froude number at sidewalls and the sidewall effective friction coefficient are linked through a unique relation.
We show that this relation is also valid for creep flows.
It is independent of the angle of the flow but depends on the sidewall-grain friction coefficient. Our results shed light on boundary conditions that have to be used at sidewalls in continuum theories aiming to capture the behavior of granular systems from creeping to energetic flows.


Sidewall friction in confined surface flows of granular materials

Mixing granular particles of different sizes is a common way of increasing the packing fraction. Recently, a model predicting the packing fraction, taking into account the inhomogeneity of the mixed small and large particles, has been proposed by S. Pillitteri et al. Under certain conditions, this model can be simplified and analytical solutions can be found. We present here these solutions, compared to experimental data, and the physical interpretation they can bring.

Forced segregation in binary granular mixtures

The aim of this work is to explore the capability of the μ(I)-rheology model and its numerical implementation in addressing a silo discharge problem by computational simulation. In order to do so, the model was implemented in the general structure of an Eulerian multiphase solver based on the Volume-Of-Fluid (VOF) method of the
OpenFOAM(R) suite. First, the implementation is validated against the results of another Lagrangian and Eulerian codes in a two-dimensional discharge problem. After that, the model is tested against the experimental results of a lab-scale and industrial-scale discharge problem. While the results of the first one were satisfactory in terms of discharge rate, for the latter one, the model exhibits disagreements in the flow patterns inside the silo. The study shows the limits of applicability of the standard formulation of the model for real scale silos and sets the ground for further discussion and improvements.

The performance of the μ(I)-rheology model on flat bottom silos discharge

As a follow-up of an earlier work on the numerically exact Coulomb friction in two-dimensional simulations, we present here the relations and implementation for three-dimensional discrete element particles.

Implementation of numerically exact Coulomb friction for discrete element simulations in three dimensions

A discrete element study of a reinforced footing by rigid inclusions, submitted to cyclic loadings is presented. The discrete element method (based on molecular dynamics method) enabled to understand the load transfer mechanisms into the granular layer (just above the inclusions) with cyclic loadings. A multiscale study showed that the soil above the rigid inclusions retrieves the larger forces, illustrating the role of the granular layer as a load transfer layer, while the settlement of both the granular layer and loading slab increase with loading. Efficiency to transfer the load on piles and ability (capacity of the granular material to postpone the overloads to the piles) decreases with cycling, but keeping high values at the end of cycles. The transfer of forces in the granular layer is achieved by two mechanisms, interacting together (inverted pyramid above rigid inclusions and arching), confirming results found in the literature.

Discrete element simulations of load transfer mechanisms for a reinforced granular embankment submitted to loading cycles

Granular dampers are passive devices used to attenuate mechanical vibrations. The most common configuration consists in an enclosure, partially filled with particles, attached to the vibrating structure that needs to be damped. The energy is dissipated due to inelastic collisions and friction between the grains and between the grains and the inner walls of the container as the structure vibrates. As a result of the collisions, the mechanical response of the system often results in chaotic motion even if the driving is harmonic. Despite the vibration attenuation achieved, this chaotic response may render the granular damper unsuitable for a range of applications. In this work, we showcase two simple modifications of the enclosure design that are able to mitigate the chaotic response of the granular damper. To this end we use Discrete Element Method simulations of: (a) a granular damper with a conical base, and (b) a granular damper with obstacles fixed inside the enclosure. We compare results against a standard flat-base enclosure damper. The basic mechanical response of the dampers is characterized by measuring the apparent mass and the loss factor. The suppression of the chaotic response is assessed qualitatively via the phase space diagram.

Avoiding chaos in granular dampers

Wet granulation processes can be driven from low to high water content. In this study, we consider the model situation of the growth of a single wet aggregate rolling in a dry granular flow inside a rotating drum. We measure the time evolution of its diameter for dierent grains and liquids, as well as various rotation rates of the drum. Using X-ray tomography, we are able to characterize the internal structure of the granular aggregate at dierent times during the process. We show that the growth rate of the aggregate can be related to the transport of the liquid inside the granule and the capture of grains. We propose a model to rationalize the maximum size of the aggregate and its growth rate.

Wet rolling stones: growth of a granular aggregate under flow

This talk summarises the numerical and experimental studies of brittle, hollow, cylindrical particles, called shells. It addresses the influence of shell properties both at the particle and assembly scales. Extreme compressibility has been recorded in the oedometer tests. Due to the large internal porosity of the shell, the breakage phenomena lead to highly compressive deformations with a significant stress dissipation. Using the Discrete Element Method (DEM), we have investigated in depth the micro-mechanical phenomena governing this macroscopic response. By quantifying the breakage and separating the double-porosity of the material, the foundations of a future constitutive model have been established throughout a simple prediction model applicable to the engineering practice.

High compressibility caused by particle breakage: a DEM investigation

To understand the underlying thermal and physical phenomena in selective laser melting (SLM) process, discrete element method (DEM) is applied to generate the randomly packed powder, then computational fluid dynamics (CFD) is adopted to simulate the laser-powder interaction. The penetration and multiple reflection of laser rays is traced. The physics of melting and solidification is captured. The temperature profile indicates the laser travel path and the adsorption and transmission of laser rays with the powder. The wetting behaviour of the melt pool driven by the capillary forces leads to the formation of pores at the connection zone. It has been demonstrated that the developed model can capture the laser-powder interaction for further understanding of SLM process.

Investigation of laser-powder interaction in laser powder bed fusion process in additive manufacturing

In this work, we study bidispese mixtures of self-propelled magnetic particles of different shapes via discrete element method simulations. We show how this particles self-assemble into clusters and how these clusters depends on the ratio of the mixture, the magnetic interaction, and the shape of the grains. It is found that the mix ratio of the system controls the cluster size. Besides, the intensity of the magnetic dipoles and the shape of the grains in the mixture rule the average number of neighbors in contact and the shape of the clusters. By varying the intensity of the interactions, globular, linear and branched clusters were obtained.

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Sidewall friction in confined surface flows of granular materials

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