4,292 research outputs found
The effects of packing structure on the effective thermal conductivity of granular media: A grain scale investigation
Structural characteristics are considered to be the dominant factors in
determining the effective properties of granular media, particularly in the
scope of transport phenomena. Towards improved heat management, thermal
transport in granular media requires an improved fundamental understanding. In
this study, the effects of packing structure on heat transfer in granular media
are evaluated at macro- and grain-scales. At the grain-scale, a gas-solid
coupling heat transfer model is adapted into a discrete-element-method to
simulate this transport phenomenon. The numerical framework is validated by
experimental data obtained using a plane source technique, and the
Smoluschowski effect of the gas phase is found to be captured by this
extension. By considering packings of spherical SiO2 grains with an
interstitial helium phase, vibration induced ordering in granular media is
studied, using the simulation methods developed here, to investigate how
disorder-to-order transitions of packing structure enhance effective thermal
conductivity. Grain-scale thermal transport is shown to be influenced by the
local neighbourhood configuration of individual grains. The formation of an
ordered packing structure enhances both global and local thermal transport.
This study provides a structure approach to explain transport phenomena, which
can be applied in properties modification for granular media.Comment: 11 figures, 29 page
Analysis of Granular Packing Structure by Scattering of THz Radiation
Scattering methods are widespread used to characterize the structure and
constituents of matter on small length scales. This motivates this introductory
text on identifying prospective approaches to scattering-based methods for
granular media. A survey to light scattering by particles and particle
ensembles is given. It is elaborated why the established scattering methods
using X-rays and visible light cannot in general be transferred to granular
media. Spectroscopic measurements using Terahertz radiation are highlighted as
they to probe the scattering properties of granular media, which are sensitive
to the packing structure. Experimental details to optimize spectrometer for
measurements on granular media are discussed. We perform transmission
measurements on static and agitated granular media using Fourier-transform
spectroscopy at the THz beamline of the BessyII storage ring. The measurements
demonstrate the potential to evaluate degrees of order in the media and to
track transient structural states in agitated bulk granular media.Comment: 12 Pages, 9 Figures, 56 Reference
Numerical Investigation of Evaporation Induced Self-Assembly of Sub-Micron Particles Suspended in Water
Self-assembly of sub-micron particles suspended in a water film is
investigated numerically. The liquid medium is allowed to evaporate leaving
only the sub-micron particles. A coupled CFD-DEM approach is used for the
simulation of fluid-particle interaction. Momentum exchange and heat transfer
between particles and fluid and among particles are considered. A history
dependent contact model is used to compute the contact force among sub-micron
particles. Simulation is done using the open source software package CFDEM
which basically comprises of two other open source packages OpenFOAM and
LIGGGHTS. OpenFOAM is a widely used solver for CFD related problems. LIGGGHTS,
a modification of LAMMPS, is used for DEM simulation of granular materials. The
final packing structure of the sub-micron particles is discussed in terms of
distribution of coordination number and radial distribution function (RDF). The
final packing structure shows that particles form clusters and exhibit a
definite pattern as water evaporates away
Packing structure of a two-dimensional granular system through the jamming transition
We have performed a novel experiment on granular packs composed of
automatically swelling particles. By analyzing the Voronoi structure of packs
going through the jamming transition, we show that the local configuration of a
jamming pack is strikingly similar to that of a glass-forming liquid, both in
terms of their universal area distribution and the process of defect annealing.
Furthermore, we demonstrate that an unambiguous structural signature of the
jamming transition can be obtained from the pair correlation functions of a
pack. Our study provides insights into the structural properties of general
jamming systems.Comment: 5 pages, 3 figure
Packing structure of semiflexible rings
Unraveling the packing structure of dense assemblies of semiflexible rings is not only fundamental for the dynamical description of polymer rings, but also key to understand biopackaging, such as observed in circular DNA of viruses or genome folding. Here we use X-ray tomography to study the geometrical and topological features of disordered packings of rubber bands in a cylindrical container. Assemblies of short bands assume a liquidlike disordered structure, with short-range orientational order, and reveal only minor influence of the container. In the case of longer bands, the confinement causes folded configurations and the bands interpenetrate and entangle. Most of the systems are found to display a threading network which percolates the system. Surprisingly, for long bands whose diameter is more than twice the diameter of the container, we found that all bands interpenetrate each other, in a complex fully entangled structure.Fil: Gomez, Leopoldo Raimundo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Instituto de Física del Sur. Universidad Nacional del Sur. Departamento de Física. Instituto de Física del Sur; ArgentinaFil: García, Nicolás. Institut Laue Langevin; Francia. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Instituto de Física del Sur. Universidad Nacional del Sur. Departamento de Física. Instituto de Física del Sur; ArgentinaFil: Poschel, Thorsten. Universitat Erlangen-nurnberg. Institute Of Multiscale Simulation.; Alemani
Relaxing in foam
We investigate the mechanical response of an aqueous foam, and its relation
to the microscopic rearrangement dynamics of the bubble-packing structure. At
rest, even though the foam is coarsening, the rheology is demonstrated to be
linear. Under flow, shear-induced rearrangements compete with
coarsening-induced rearrangements. The macroscopic consequences are captured by
a novel rheological method in which a step-strain is superposed on an otherwise
steady flow
Structural fluctuations in thin cohesive particle layers in powder-based additive manufacturing
Producing dense and homogeneous powder layers with smooth free surface is
challenging in additive manufacturing, as interparticle cohesion can strongly
affect the powder packing structure and therefore influence the quality of the
end product. We use the Discrete Element Method to simulate the spreading
process of spherical powders and examine how cohesion influences the
characteristics of the packing structure with a focus on the fluctuation of the
local morphology. As cohesion increases, the overall packing density decreases,
and the free surface roughness increases, which is calculated from digitized
surface height distributions. Local structural fluctuations for both quantities
are examined through the local packing anisotropy on the particle scale,
obtained from Vorono\"{\i} tessellation. The distributions of these
particle-level metrics quantify the increasingly heterogeneous packing
structure with clustering and changing surface morphology.Comment: 17 pages, 8 figure
Domains growth and packing properties in driven granular media subject to gravity
We study the dynamical properties of recently introduced frustrated lattice
gas models (IFLG and Tetris) for granular media under gentle shaking. We
consider both the case where grains have inter-grain surface interactions and
the case where they have not, corresponding, for instance, to the presence or
absence of moisture in the packs. To characterise the grains packing structure,
we discuss the properties of density distribution. In particular, we consider
the phenomenon of grains domains formation under compaction. New results
amenable of experimental check are discussed along with some important
differences between the dynamics of the present models.Comment: 7 pages, 6 postscript files for figure
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