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1.
Magneto-vibratory separation of glass and bronze granular mixtures immersed in a paramagnetic liquid
López-Alcaraz P Catherall AT Hill RJ Leaper MC Swift MR King PJ 《The European physical journal. E, Soft matter》2007,24(2):145-156
A fluid-immersed granular mixture may spontaneously separate when subjected to vertical vibration, separation occurring when
the ratio of particle inertia to fluid drag is sufficiently different between the component species of the mixture. Here,
we describe how fluid-driven separation is influenced by magneto-Archimedes buoyancy, the additional buoyancy force experienced
by a body immersed in a paramagnetic fluid when a strong inhomogeneous magnetic field is applied. In our experiments glass
and bronze mixtures immersed in paramagnetic aqueous solutions of MnCl2 have been subjected to sinusoidal vertical vibration. In the absence of a magnetic field the separation is similar to that
observed when the interstitial fluid is water. However, at modest applied magnetic fields, magneto-Archimedes buoyancy may
balance the inertia/fluid-drag separation mechanism, or it may dominate the separation process. We identify the vibratory
and magnetic conditions for four granular configurations, each having distinctive granular convection. Abrupt transitions
between these states occur at well-defined values of the magnetic and vibrational parameters. In order to gain insight into
the dynamics of the separation process we use computer simulations based on solutions of the Navier-Stokes' equations. The
simulations reproduce the experimental results revealing the important role of convection and gap formation in the stability
of the different states. 相似文献
2.
Goujon C Dalloz-Dubrujeaud B Thomas N 《The European physical journal. E, Soft matter》2007,23(2):199-215
Experiments were performed to provide insight into the
flow behavior and structure of bimodal mixtures of grains in
gravity-driven, free-surface flows. Unsteady unconfined flows
were produced by releasing instantaneously a dry granular mass,
composed of two particle sizes, over a rough inclined plane. As a
result of size segregation, the small particles are found at the
bottom of the flow and final deposit, the large particles are
found at the free surface, but also on the lateral borders and at
the front of the flow. The lateral and vertical inhomogeneous
repartitions of particles lead to two main effects that are
completely absent in monodispersed flows. The outline effect
results from the accumulation of large beads on the periphery of
the flow depending on the value of the relative friction of each
particle species on the plane. This effect in turn causes a
narrowing of the flow and/or an increase of length of the final
deposit. The interface effect results of the interaction between
layers of different size particles and causes the modification of
the thickness of the deposit. These effects occur simultaneously
and their combination leads to a great variety of behaviors. In
this investigation, evidence of the diversity of behaviors is
presented as the size ratio, relative friction and concentration
of each particle species are varied. 相似文献
3.
Molecular (or granular) dynamics methods are used to study the gravity-driven flow of granular material through a horizontal
aperture in three dimensions. The grains are spherical and modeled using a short-range repulsive interaction, together with
normal and tangential frictional damping forces. The material is contained in a rough-walled cylindrical container with a
circular hole in its base, and to permit flow measurements under steady-state conditions a continuous feed approach is employed
in which exiting grains are replaced at the upper surface of the material. The dependence of flow velocity and discharge rate
on aperture diameter is found to agree with experiment; other quantities such as the kinetic energy and pressure distributions
are also examined.
Received 5 June 2000 and Received in final form 21 September 2000 相似文献
4.
The silo discharge process is studied by molecular
dynamics simulations. The development of the velocity profile and
the probability density function for the displacements in the
horizontal and vertical axis are obtained. The PDFs obtained at
the beginning of the discharge reveal non-Gaussian statistics and
superdiffusive behaviors. When the stationary flow is developed,
the PDFs at shorter temporal scales are non-Gaussian too. For big
orifices a well-defined transition between ballistic and diffusive
regime is observed. In the case of a small outlet orifice, no
well-defined transition is observed. We use a nonlinear diffusion
equation introduced in the framework of non-extensive
thermodynamics in order to describe the movements of the grains.
The solution of this equation gives a well-defined relationship
(γ= 2/(3-q)) between the anomalous diffusion exponent
γ and the entropic parameter q introduced by the
non-extensive formalism to fit the PDF of the fluctuations. 相似文献
5.
Goujon C Thomas N Dalloz-Dubrujeaud B 《The European physical journal. E, Soft matter》2003,11(2):147-157
Recent studies have pointed out the importance of the
basal friction on the dynamics of granular flows. We present
experimental results on the influence of the roughness of the
inclined plane on the dynamics of a monodisperse dry granular
flow. We found experimentally that there exists a maximum of the
friction for a given relative roughness. This maximum is shown
to be independent of the slope angle. This behavior is observed
for four planes with different bump sizes (given by the size of
the beads glued on the plane) from 200 m to 2 mm. The relative
roughness corresponding to the maximum of the friction can be
predicted with a geometrical model of stability of one single
bead on the plane. The main parameters are the size of the bumps
and the size of the flowing beads. In order to obtain a higher
precision, the model also takes into account the spacing between
the bumps of the rough plane. Experimental results and model are
in good agreement for all the planes we studied. Other
parameters, like the sphericity of the beads, or irregularities
in the thickness of the layer of glued particles, are shown to
be of influence on the friction. 相似文献
6.
Dispersive flow of disks through a
two-dimensional Galton board 总被引:1,自引:0,他引:1
We report here an experimental and numerical study of the
flow properties of disks driven by gravity through a hexagonal
lattice of obstacles, i.e. a
Galton board. During the fall, particles experience dissipative
collisions that scatter them in random directions. A
driven-diffusion regime can be achieved under certain
conditions. A characteristic length of the motion and its
dependence on geometrical parameters of the system is analyzed
in the steady regime. The influence of collective effects on the
dispersion process is investigated by comparison between single-
and many-particle flows. The characterization of the dynamics
and the diffusive properties of the flow in a system like a
Galton board can be expanded to other granular systems,
particularly static solid particle mixers and will give some
insight in understanding granular mixing. 相似文献
7.
The avalanche mixing of granular solids in a slowly rotated 2D upright drum is studied. We demonstrate that the account of
the difference δ between the angle of marginal stability and the angle of repose of the granular material leads to a restricted
value of the mixing time τ for a half filled drum. The process of mixing is described by a linear discrete difference equation.
We show that the mixing looks like linear diffusion of fractions with the diffusion coefficient vanishing when δ is an integer
part of π. Introduction of fluctuations of δ suppresses the singularities of τ(δ) and smoothes the dependence τ(δ).
Received 27 October 2000 and Received in final form 13 March 2001 相似文献
8.
Soulié F El Youssoufi MS Cherblanc F Saix C 《The European physical journal. E, Soft matter》2006,21(4):349-357
We investigate the macroscopic mechanical behaviour of wet polydisperse granular media. Capillary bonding between two grains
of unequal diameters is described by a realistic force law implemented in a molecular-dynamics algorithm together with a protocol
for the distribution of water in the bulk. Axial-compression tests are simulated for granular samples at different levels
of water content, and compared to experiments performed in similar conditions. We find good agreement between numerical and
experimental data in terms of the rupture strength as a function of water content. Our results show the importance of the
distribution of water for the mechanical behaviour. 相似文献
9.
Z. M. Jakšić S. B. Vrhovac B. M. Panić Z. Nikolić B. M. Jelenković 《The European physical journal. E, Soft matter》2008,27(4):345-356
We study experimentally the creeping penetration of guest (percolating) grains through densely packed granular media in two dimensions. The evolution of the system of the guest grains during the penetration is studied by image analysis. To quantify the changes in the internal structure of the packing, we use Voronoï tessellation and a certain shape factor which is a clear indicator of the presence of different underlying substructures (domains). We first consider the impact of the effective gravitational acceleration on upward penetration of grains. It is found that the higher effective gravity increases the resistance to upward penetration and enhances structural organization in the system of the percolating grains. We also focus our attention on the dependence of the structural rearrangements of percolating grains on some parameters like polydispersity and the initial packing fraction of the host granular system. It is found that the anisotropy of penetration is larger in the monodisperse case than in the bidisperse one, for the same value of the packing fraction of the host medium. Compaction of initial host granular packing also increases anisotropy of penetration of guest grains. When a binary mixture of large and small guest grains is penetrated into the host granular medium, we observe size segregation patterns. 相似文献
10.
P. Entel R. Meyer K. Kadau H.C. Herper E. Hoffmann 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,5(3):379-388
Results are presented of first-principles total-energy calculations and molecular-dynamics simulations of structural transformations
in magnetic transition metal alloys like Fe1-xNix. While first-principles calculations allow to identify those structures having the lower total energy, molecular-dynamics
simulations can be used to trace out the dependence of the transformation on temperature, composition, concentration of defects
etc. We have used the method of the semi-empiric embedded-atom potential in the molecular-dynamics simulations which yields remarkable
good results for the structural changes.
Received: 3 February 1998 / Revised: 4 May 1998 / Accepted: 24 June 1998 相似文献
11.
The airborne transport of particles on a granular surface by the
saltation mechanism is studied through numerical simulation of
particles dragged by turbulent air flow. We calculate the
saturated flux qs and show that its dependence on the wind
strength u* is consistent with several empirical relations
obtained from experimental measurements. We propose and explain
a new relation for fluxes close to the threshold velocity ut,
namely, qs=a(u*-ut)α with α≈2. We
also obtain the distortion of the velocity profile of the wind due to the
drag of the particles and find a novel dynamical
scaling relation. We also obtain a new expression for the dependence of
the height of the saltation layer as function of the strength of the wind. 相似文献
12.
Fragmentation of grains in a two-dimensional packing 总被引:1,自引:0,他引:1
J.A. Åström H.J. Herrmann 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,5(3):551-554
A numerical model of fragmentation of a two-dimensional granular medium under pressure is investigated. The fragmentation
process is found to be strongly dependent on the type of force used as the criterion for breaking the grains. The fragmentation
mode affects the process less dramatically. There is a power-law divergence in the pressure when the medium approaches the
full packing limit, . Both log-normal and power-law fragment-size distributions are found. Gravity is demonstrated to be an important factor.
Received: 14 December 1997 / Accepted: 17 March 1998 相似文献
13.
The collision of frictional granular particles may be described by an interaction force whose normal component is that of viscoelastic spheres while the tangential part is described by the model by Cundall and Strack (Géotechnique 29, 47 (1979)) being the most popular tangential collision model in Molecular Dynamics simulations. Albeit being a rather complicated model, governed by 5 phenomenological parameters and 2 independent initial conditions, we find that it is described by 3 independent parameters only. Surprisingly, in a wide range of parameters the corresponding coefficient of tangential restitution, epsilont, is well described by the simple Coulomb law with a cut-off at epsilont = 0. A more complex behavior of the coefficient of restitution as a function on the normal and tangential components of the impact velocity, gn and gt, including negative values of epsilont, is found only for very small ratio gt/gn. For the analysis presented here we neglect dissipation of the interaction in normal direction. 相似文献
14.
Núñez A Darias R Pinto R Paredes V R Medina E 《The European physical journal. E, Soft matter》2002,9(4):327-334
We study a model of concentrated suspensions under shear in two dimensions. Interactions between suspended particles are dominated
by direct-contact viscoelastic forces and the particles are neutrally bouyant. The bimodal suspensions consist of a variable
proportion between large and small droplets, with a fixed global suspended fraction. Going beyond the assumptions of the classical
theory of Farris (R.J. Farris, Trans. Soc. Rheol. 12, 281 (1968)), we discuss a shear viscosity minimum, as a function of the small-to-large-particle ratio, in shear geometries
imposed by external body forces and boundaries. Within a linear-response scheme, we find the dependence of the viscosity minimum
on the imposed shear and the microscopic drop friction parameters. We also discuss the viscosity minimum under dynamically
imposed shear applied by boundaries. We find a reduction of macroscopic viscosity with the increase of the microscopic friction
parameters that is understood using a simple two-drop model. Our simulation results are qualitatively consistent with recent
experiments in concentrated bimodal emulsions with a highly viscous or rigid suspended component.
Received 28 June 2002
RID="a"
ID="a"e-mail: ernesto@pion.ivic.ve 相似文献
15.
We present a mesoscopic model for simulating the dynamics of a non-volatile liquid on a solid substrate. The wetting properties
of the solid can be tuned from complete wetting to total non-wetting. This model opens the way to study the dynamics of drops
and liquid thin films at mesoscopic length scales of the order of the nanometer. As particular applications, we analyze the
kinetics of spreading of a liquid drop wetting a solid substrate and the dewetting of a liquid film on a hydrophobic substrate.
In all these cases, very good agreement is found between simulations and theoretical predictions. 相似文献
16.
Granular flows in a rotating drum: the scaling law
between velocity and thickness of the flow 总被引:1,自引:0,他引:1
The flow of dry granular material in a half-filled rotating drum is studied. The thickness of the flowing zone is measured
for several rotation speeds, drum
sizes and beads sizes (size ratio between drum and beads ranging from 47
to 7400). Varying the rotation speed, a scaling law linking
mean velocity vs. thickness of the flow, v∼hm, is deduced for each couple
(beads, drum). The
obtained exponent m is not always equal to 1, the value previously reported for a drum in litterature, but varies with the
geometry of the system.
For small size ratios, exponents higher
than 1 are obtained due to a saturation of the flowing zone thickness.
The exponent of the power law decreases with the size ratio, leading
to exponents lower than 1 for high size ratios. These exponents imply that
the velocity gradient of a dry granular flow in a rotating drum
is not constant. More fundamentally, these results show that the flow of a
granular material in a rotating drum is very sensible to the geometry, and that
the deduction of the “rheology” of a granular medium flowing in such a
geometry is not obvious. 相似文献
17.
Pica Ciamarra M Coniglio A Nicodemi M 《The European physical journal. E, Soft matter》2007,22(3):227-234
We overview the physics of a granular mixture subject to horizontal oscillations,
recently investigated via experiments and molecular dynamics simulations.
First we discuss the rich phenomenology exhibited by this system, which encompasses
both segregation and dynamical instabilities.
Then we show that the phenomenology can be explained
via an effective interaction approach, by which the driven,
non-thermal, granular mixture in mapped into a monodispersed thermal
system of particles interacting via an effective potential.
After determining the effective interaction we discuss its
microscopic origin and investigate how it induces the observed
phenomenology. Finally, as much as in thermal fluids, from the
effective interaction we derive a Cahn-Hilliard dynamics equation,
which appears to capture the essential characteristics of the dynamics
of the granular mixture. 相似文献
18.
V. Garzó 《The European physical journal. E, Soft matter》2009,29(3):261-274
A theory based on a solution of the inelastic Enskog equation that goes beyond the weak dissipation limit is used to determine
the thermal diffusion factor of a binary granular mixture under gravity. The Enskog equation that aims to describe moderate
densities neglects velocity correlations but retains spatial correlations arising from volume exclusion effects. As expected,
the thermal diffusion factor provides a segregation criterion that shows the transition between the Brazil-nut effect (BNE)
and the reverse Brazil-nut effect (RBNE) by varying the parameters of the system (masses, sizes, composition, density and
coefficients of restitution). The form of the phase diagrams for the BNE/RBNE transition is illustrated in detail in the tracer
limit case, showing that the phase diagrams depend sensitively on the value of gravity relative to the thermal gradient. Two
specific situations are considered: i) absence of gravity, and ii) homogeneous temperature. In the latter case, after some
approximations, our results are consistent with previous theoretical results derived from the Enskog equation. Our results
also indicate that the influence of dissipation on thermal diffusion is more important in the absence of gravity than in the
opposite limit. The present analysis, which is based on a preliminary short report of the author (Phys. Rev. E 78, 020301(R) (2008)), extends previous theoretical results derived in the dilute limit case. 相似文献
19.
W. Kob J.-L. Barrat 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,13(2):319-333
By means of molecular dynamics computer simulations we investigate the out of equilibrium relaxation dynamics of a simple
glass former, a binary Lennard-Jones system, after a quench to low temperatures. We find that one-time quantities, such as
the energy or the structure factor, show only a weak time dependence. By comparing the out of equilibrium structure factor
with equilibrium data we find evidence that during the aging process the system remains in that part of phase space that mode-coupling
theory classifies as liquid like. Two-times correlation functions show a strong time and waiting time dependence. For large and times corresponding to the early -relaxation regime the correlators approach the Edwards-Anderson value by means of a power-law in time. For large but fixed
values of the relaxation dynamics in the -relaxation regime seems to be independent of the observable and temperature. The -relaxation shows a power-law dependence on time with an exponent which is independent of but depends on the observable. We find that at long times the correlation functions can be expressed as and compute the function h(t). This function is found to show a t-dependence which is a bit stronger than a logarithm and to depend on the observable considered. If the system is quenched
to very low temperatures the relaxation dynamics at long times shows fast drops as a function of time. We relate these drops
to relatively local rearrangements in which part of the sample relaxes its stress by a collective motion of 50-100 particles.
Finally we discuss our measurements of the time dependent response function. We find that at long times the correlation functions
and the response are not related by the usual fluctuation dissipation theorem but that this relation is similar to the one
found for spin glasses with one step replica symmetry breaking.
Received 17 May 1999 相似文献
20.
T. Gleim W. Kob 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,13(1):83-86
We present a detailed analysis of the -relaxation dynamics of a simple glass former, a binary Lennard-Jones system with a stochastic dynamics. By testing the various
predictions of mode-coupling theory, including the recently proposed corrections to the asymptotic scaling laws, we come to
the conclusion that in this time regime the dynamics is described very well by this theory.
Received 5 February 1999 and Received in final form 7 June 1999 相似文献