Analytical and DEM studies of thermal stress in granular materials

United States

The ability to predict thermal-induced stresses in granular materials is of practical importance across a range of disciplines ranging from process engineering to geotechnical engineering. This study presents an analytical formula to predict thermal-induced stress increments in mono-disperse granular materials subject to an initial isotropic stress state. A complementary series of DEM simulations were carried out to explore the applicability of the proposed analytical formula. The comparative analysis showed that the proposed expression can accurately predict stress changes in packings where there are negligible particle displacements as a consequence of the thermal loading (e.g. regular packings and medium/dense random packings); however large errors were observed in loose samples with a random packing.

Powders &amp; Grains 2021

[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.

technical paper

The frictional behaviour of a series of numerical 2D granular mass flows down a model topography is analysed. Effective friction coefficients estimated from final deposits are compared with data from documented natural geophysical flows, and show a consistent behaviour as far as run-out distances are concerned. The latter is used to estimate effective friction coefficients which capture well the frictional behaviour derived from the computation of micro-mechanical stress tensors near the gravity centre. Distinguishing between the different parts of the mass while spreading, we show that the downstream part of the flow exhibits a much larger friction than the core and the tail. A dependence between friction and flow volume is however observed 
in each region.


Mobilisation of friction in unstationary flows down a model topography

Currently, very little is known about reliable phenomelogical correlations to estimate the gravity-driven mass flow rate, of dry non-cohesive granular material, outflowing from thin thickness slots in vertical sidewalls of rectangular silos. Here, we validate a simple and general formula that fits pretty well data published elsewhere, including the cases of vertically and horizontally elongated slots.

Granular flow through vertical slots: analysis of a general formula

For homogeneous systems like classical fluid dynamics and structural mechanics, finite element method (FEM) grid generation has reached a mature state. On the other hand, for multi-physics-problems like fluids with a high density of immersed particles, many researchers may not even be aware of the types of instabilities which may be triggered by unsuitable meshes. We review common types of grid generation, point out previously unrecognised types of instabilities for particles in fluids as well as remedies to obtain particle-fluid simulations with higher stability and fewer redundant degrees of freedom.

Grid-algorithm improvements for dense suspensions of discrete element particles in finite element fluid simulations

The effect of boundary roughness on dense granular flows

Passages of people or cattle through narrow entrances or exits occur in manifold situations. They are difficult to study experimentally, because one has to carefully separate objective, physical parameters from subjective, individual motivations, manners and temperament. Mechanically excited physical model systems can help to discriminate some of these classes of parameters. We characterize active and passive particles of equal shape and mass on a vibrating plate and study their bottleneck passage dynamics. They show fundamentally different scaling behavior.

Mechanically driven active and passive grains as models for egress dynamics

Magnetic forces are used to heat up thousands of spherical particles under low-gravity. This longrange external excitation, combined with the induced particle-particle interactions, results in a homogeneousspatial distribution of the particles. Comparisons with predictions of kinetic theories can hence be carried out.Haff’s cooling law is verified qualitatively, while the measured cooling time scale is quantitatively differentfrom the prediction. The high velocity tail of the velocity distribution during homogeneous cooling state (HCS)is measured, while the expected cluster formation after HCS can not be verified by our experiment.

Magnetically heated granular gas in a low-gravity environment

The transport of a colouring solute, driven by the buoyant displacement of microscopic suspended particles, and in the absence of net flow, is studied experimentally in a Hele Shaw cell. Initially, a sharp interface between a transparent fluid without particles and an underlying coloured suspension is obtained. From this situation, the suspended particles rise, carrying the solute in the form of a fingering instability across the interface, where a light transmission technique is used to measure the local solute concentration. This one attains an asymptotic value that increases with the solid fraction ϕ of suspended particles, and decreases with the distance to the interface. The solute mass discharge also increases with ϕ, always being relatively small (< 3%). The onset and development of the instability as the mechanism driving the transport of the solute is discussed.

Solute transport by suspended buoyant particles

To prevent texture defects in powder-based processes, the sintering time needs to be adjusted such that a certain amount of coalescence is achieved. However, predicting the required sintering time is extremely challenging to assess in materials such as polymers because the kinetics exhibit both elastic and viscous characteristics when undergoing deformation. The present work introduces a computational approach to model the viscoelastic effect in the sintering of particles. The model contains three stages, three different mechanisms driven by adhesive inter-surface forces and surface tension, which describes the non-linear sintering behaviour. Experimental data from the binary coalescence of Polystyrene (PS), Polyamide (PA) 12 and PEEK 450PF particles are employed to calibrate the contact model, as implemented in MercuryDPM, an open-source software package. Using machine learning-based Bayesian calibration, good agreement is obtained between the experimental data and the numerical results. The findings will be used in future studies to predict densification rates in powder-based processes.

Neck growth kinetics during polymer sintering for powder-based processes

In this presentation a general concept of frictional heat and the associated rise in contact temperature is discussed, the theoretical/analytical models for predicting the rise in contact temperature are presented. Similarly, a finite element method model is discussed. Results for a maximum contact temperature for a friction slide between two spherical particles show that the TK model predictions are in good agreement with the Finite element analysis.

Friction-induced heat generation between two particles

The objective of this work was to measure the coefficient of restitution of fine granite sand dust (passing sieve ASTM N°30) and to study its influence on the prediction of classification parameters in an elutriator device. Three types of restitution coefficients were evaluated in the computational fluid dynamic model: experimental, elastic, and half the experimental value. The influence of process variables, such as the inclination angle of the equipment and the air inlet velocity, on the efficiency of the obtained fine and coarse fractions was also studied. The results indicate that the coefficient of restitution significantly influences the efficiency parameters calculated for the inclined tests, given the greater contact that exists between the particles and the equipment wall. The experimental value of the coefficient achieved almost total congruence with the coarse fraction collected for the tested velocities. For the vertical tests, the variation of the coefficient did not show significant differences for both collected fractions. The results obtained in this work allowed to gain a deeper insight into the studied gas-solid systems, and to improve the numerical predictions on fine particles classification processes widely used in the ceramic, mining and metallurgical industries.

Analytical and DEM studies of thermal stress in granular materials

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Analytical and DEM studies of thermal stress in granular materials

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Mobilisation of friction in unstationary flows down a model topography

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