共查询到20条相似文献,搜索用时 17 毫秒
1.
J.E.S. Socolar D.G. Schaeffer P. Claudin 《The European physical journal. E, Soft matter》2002,7(4):353-370
A theory of stress fields in two-dimensional granular materials based on directed force chain networks is presented. A general
Boltzmann equation for the densities of force chains in different directions is proposed and a complete solution is obtained
for a special case in which chains lie along a discrete set of directions. The analysis and results demonstrate the necessity
of including nonlinear terms in the Boltzmann equation. A line of nontrivial fixed-point solutions is shown to govern the
properties of large systems. In the vicinity of a generic fixed point, the response to a localized load shows a crossover
from a single, centered peak at intermediate depths to two propagating peaks at large depths that broaden diffusively.
Received 16 January 2002 相似文献
2.
This paper deals with the quasi-static regime of deformation of granular matter. It investigates the size of the Representative
Elementary Volume (REV), which is the minimum packing size above which the macroscopic mechanical behaviour of granular materials
can be defined from averaging. The first part uses typical results from recent literature and finds that the minimum REV contains
in general 10 grains; this result holds true either for most experiments or for Discrete Element Method (DEM) simulation.
This appears to be quite small. However, the second part gives a counterexample, which has been found when investigating uniaxial
compression of glass spheres which exhibit stick-slip; we show in this case that the minimum REV becomes 107 grains. This makes the system not computable by DEM. Moreover, similarity between the Richter law of seism and the exponential
statistics of stick-slip is stressed.
Received 8 March 2002 and Received in final form 12 July 2002 相似文献
3.
D.A. Head A.V. Tkachenko T.A. Witten 《The European physical journal. E, Soft matter》2001,6(1):99-105
By employing the adaptive network simulation method, we demonstrate that the ensemble-averaged stress caused by a local force
for packings of frictionless rigid beads is concentrated along rays whose slope is consistent with unity: forces propagate
along lines at 45 degrees to the horizontal or vertical. This slope is shown to be independent of polydispersity or the degree
to which the system is sheared. Further confirmation of this result comes from fitting the components of the stress tensor
to the null stress “constitutive equation”. The magnitude of the response is also shown to fall off with the -1/2 power of
distance. We argue that our findings are a natural consequence of a system that preserves its volume under small perturbations.
Received 30 April 2001 and Received in final form 16 August 2001 相似文献
4.
S. Aumaître A. Alastuey S. Fauve 《The European Physical Journal B - Condensed Matter and Complex Systems》2006,54(2):263-266
Using simple scaling arguments and two-dimensional numerical simulations of a granular gas excited by vibrating one of the
container boundaries, we study a double limit of small 1-r and large L, where r is the restitution coefficient and L the size
of the container. We show that if the particle density n0 and (1-r2)(n0 Ld) where d is the particle diameter, are kept constant and small enough, the granular temperature, i.e. the mean value of
the kinetic energy per particle, 〈E 〉/N, tends to a constant whereas the mean dissipated power per particle, 〈D 〉/N, decreases
like
when N increases, provided that (1-r2)(n0 Ld)2 < 1. The relative fluctuations of E, D and the power injected by the moving boundary, I, have simple properties in that regime.
In addition, the granular temperature can be determined from the fluctuations of the power I(t) injected by the moving boundary. 相似文献
5.
E. Kolb T. Mazozi E. Clément J. Duran 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,8(3):483-491
We present series of experiments on the resistance force encountered by a bottom piston pushing a vertical granular column
confined in a two-dimensional cell. We show that, due to the presence of friction at the boundaries and between the grains,
the signal shows many complex features. At slow driving velocities, we observe a transition to a stick-slip dynamic instability.
Depending on the granular material used, the elementary stick-slip events may either be well characterized or largely distributed.
We present a statistical study on the waiting time between events and the distribution of energy release as a function of
the spring stiffness and the driving velocity.
Received 5 August 1998 and Received in final form 22 October 1998 相似文献
6.
D. Serero G. Reydellet P. Claudin É. Clément D. Levine 《The European physical journal. E, Soft matter》2001,6(2):169-179
We measured the vertical pressure response function of a layer of sand submitted to a localized normal force at its surface.
We found that this response profile depends on the way the layer has been prepared: all profiles show a single centered peak
whose width scales with the thickness of the layer, but a dense packing gives a wider peak than a loose one. We calculate
the prediction of isotropic elastic theory in the presence of a bottom boundary and compare it to the data. We found that
the theory gives the right scaling and the correct qualitative shape, but fails to really fit the data.
Received 3 August 2001 相似文献
7.
P. Visco A. Puglisi A. Barrat F. van Wijland E. Trizac 《The European Physical Journal B - Condensed Matter and Complex Systems》2006,51(3):377-387
We investigate the behavior of energy fluctuations in several
models of granular gases maintained in a non-equilibrium steady state. In
the case of a gas heated from a boundary, the inhomogeneities of the system
play a predominant role. Interpreting the total kinetic energy as a sum of
independent but not identically distributed random variables, it is possible
to compute the probability density function (pdf) of the total energy.
Neglecting correlations and using the analytical expression for the
inhomogeneous temperature profile obtained from the granular hydrodynamic
equations, we recover results that have previously been observed
numerically and that had been attributed to the presence of correlations.
In order to separate the effects of spatial inhomogeneities from those
ascribable to velocity correlations, we have also considered two models of
homogeneously thermostated gases: in this framework it is possible to reveal
the presence of non-trivial effects due to velocity correlations between
particles. Such correlations stem from the inelasticity of collisions.
Moreover, the observation that the pdf of the total energy tends to a
Gaussian in the large system limit suggests that they are also due to the
finite size of the system. 相似文献
8.
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10.
We have experimentally studied granular arches through electrical measurements. The packing is composed of 2d metallic pentagons and is submitted to small taps. Large electrical fluctuations are observed and they are distributed along
power laws. This indicates the presence of long-time memory effects even the packing density remains constant around a value
ρ = 0.72±0.02. Large electrical fluctuations should be associated with the breaking/creation of granular arches.
Received 3 October 2000 相似文献
11.
Exactly solvable models through the generalized empty interval method,for multi-species interactions
A. Aghamohammadi M. Alimohammadi M. Khorrami 《The European Physical Journal B - Condensed Matter and Complex Systems》2003,31(3):371-378
Multi-species reaction-diffusion systems, with nearest-neighbor interaction on a one-dimensional lattice are considered. Necessary
and sufficient constraints on the interaction rates are obtained, that guarantee the closedness of the time evolution equation
for E
n(t)'s, the expectation value of the product of certain linear combination of the number operators on n consecutive sites at time t. The constraints are solved for the single-species left-right-symmetric systems. Also, examples of multi-species system for
which the evolution equations of E
n(t)'s are closed, are given.
Received 25 September 2002 / Received in final form 3 December 2002 Published online 14 February 2003
RID="a"
ID="a"e-mail: mamwad@iasbs.ac.ir 相似文献
12.
A new and general approach is proposed to analyze the dynamics of a colloidal particle interacting with a nearby wall. This
analysis can be used to determine the acting forces even when the system is non-stationary. As an illustration, we use total
internal reflection microscopy to investigate the forces acting on a polystyrene sulfate latex particle as it is receding
from a charged glass surface.
Received 10 October 2002 Published online: 16 April 2003
RID="a"
ID="a"Present address: Department of Polymer Physics, BASF Aktiengesellschaft, 67056 Ludwigshafen, Germany
RID="b"
ID="b"Present address: Arryx. Inc., Chicago, IL 60601, USA 相似文献
13.
A. Namazi N. Eissfeldt P. Wagner A. Schadschneider 《The European Physical Journal B - Condensed Matter and Complex Systems》2002,30(4):559-570
The Krauss-model is a stochastic model for traffic flow which is continuous in space. For periodic boundary conditions it
is well understood and known to display a non-unique flow-density relation (fundamental diagram) for certain densities. In
many applications, however, the behaviour under open boundary conditions plays a crucial role. In contrast to all models investigated
so far, the high flow states of the Krauss-model are not metastable, but also stable. Nevertheless we find that the current
in open systems obeys an extremal principle introduced for the case of simpler discrete models. The phase diagram of the open
system will be completely determined by the fundamental diagram of the periodic system through this principle. In order to
allow the investigation of the whole state space of the Krauss-model, appropriate strategies for the injection of cars into
the system are needed. Two methods solving this problem are discussed and the boundary-induced phase transitions for both
methods are studied. We also suggest a supplementary rule for the extremal principle to account for cases where not all the
possible bulk states are generated by the chosen boundary conditions.
Received 16 September 2002 / Received in final form 4 November 2002 Published online 31 December 2002 相似文献
14.
S. Dubois S. Harel M. Grosbras 《The European Physical Journal B - Condensed Matter and Complex Systems》2001,21(4):507-510
Macroscopic resistivity measurements have been performed on TiC/Al2O3 random mixtures submitted to uniaxial compression (0-95 kN). Such a random mixture exhibits an insulator-conductor transition
which appears at increasing force while decreasing the conductive composition of the TiC/Al2O3 mixture. It is demonstrated that the conductivity behavior may be understood in the framework of a site percolation model.
Finally, the rate of creation of the contacts between conductive grains is extracted from the macroscopic resistivity measurements.
Received 6 December 2000 and Received in final form 30 March 2001 相似文献
15.
I.M. Sokolov 《The European Physical Journal B - Condensed Matter and Complex Systems》2001,22(3):369-373
We consider two-particle dispersion in a velocity field, where the relative two-point velocity scales according to v
2(r) ∝r
α and the corresponding correlation time scales as τ(r) ∝r
β, and fix α = 2/3, as typical for turbulent flows. We show that two generic types of dispersion behavior arize: For α/2 +
β < 1 the correlations in relative velocities decouple and the diffusion approximation holds. In the opposite case, α/2 +
β > 1, the relative motion is strongly correlated. The case of Kolmogorov flows corresponds to a marginal, nongeneric situation.
In this case, depending on the particular parameters of the flow, the dispersion behavior can be rather diffusive or rather
ballistic.
Received 13 March 2001 相似文献
16.
S. Dubois E. Béré P. Grosbras A. Straboni 《The European Physical Journal B - Condensed Matter and Complex Systems》2002,26(1):35-39
Resistivity measurements have been performed on silicon powder beds with different thicknesses which were submitted to uniaxial
pressure (0-640 MPa). For the smallest thicknesses of the Si pellets, the pressure dependence of conductivity may be described
using models based on effective medium theory or strongest stresses network. For largest Si thicknesses, it exists an inhomogeneous
distribution of pressure in the granular medium as a consequence of arching effects. It is shown that the conductivity variation
with Si powder thickness may be understood using a distribution of conductivity induced by the distribution of pressure. Finally,
it is shown that the conductivity of the sintered Si pellets is correlated with the uniaxial compression step.
Received 14 September 2001 and Received in final form 18 December 2001 相似文献
17.
J. Dunkel W. Ebeling U. Erdmann 《The European Physical Journal B - Condensed Matter and Complex Systems》2001,24(4):511-524
We investigate the stochastic dynamics of an one-dimensional ring with N self-driven Brownian particles. In this model neighboring particles interact via conservative Morse potentials. The influence of the surrounding heat bath is modeled by Langevin-forces (white noise) and
a constant viscous friction coefficient γ. The Brownian particles are provided with internal energy depots which may lead to active motions of the particles. The depots
are realized by an additional nonlinearly velocity-dependent friction coefficient γ
1(v) in the equations of motions. In the first part of the paper we study the partition functions of time averages and thermodynamical
quantities (e.g. pressure) characterizing the stationary physical system. Numerically calculated non-equilibrium phase diagrams are represented.
The last part is dedicated to transport phenomena by including a homogeneous external force field that breaks the symmetry
of the model. Here we find enhanced mobility of the particles at low temperatures.
Received 21 July 2001 相似文献
18.
Solomon Fekade Mulugeta Bekele 《The European Physical Journal B - Condensed Matter and Complex Systems》2002,26(3):369-374
We analyze the relaxation behavior of a bistable system when the background temperature profile is inhomogeneous due to the
presence of a localized hot region (blowtorch) on one side of the potential barrier. Since the diffusion equation for inhomogeneous
medium is model-dependent, we consider two physical models to study the kinetics of such system. Using a conventional stochastic
method, we obtain the escape and equilibration rates of the system for the two physical models. For both models, we find that
the hot region enhances the escape rate from the well where it is placed while it retards the escape rate from the other well. However, the value of the escape rate from the well where the hot region is placed differs
for the two models while that of the escape rate from the other well is identical for both. This work, for the first time,
gives a detailed report of the similarities and differences of the escape rates and, hence, exposes the common and distinct
features of the two known physical models in determining the way the bistable system relaxes.
Received 25 September 2001 相似文献
19.
T. Aspelmeier J. Magnin W. Graupner U.C. Täuber 《The European Physical Journal B - Condensed Matter and Complex Systems》2002,28(4):441-450
We investigate random walks on a lattice with imperfect traps. In one dimension, we perturbatively compute the survival probability
by reducing the problem to a particle diffusing on a closed ring containing just one single trap. Numerical simulations reveal
this solution, which is exact in the limit of perfect traps, to be remarkably robust with respect to a significant lowering
of the trapping probability. We demonstrate that for randomly distributed traps, the long-time asymptotics of our result recovers
the known stretched exponential decay. We also study an anisotropic three-dimensional version of our model. We discuss possible
applications of some of our findings to the decay of excitons in semiconducting organic polymer materials, and emphasize the
crucial influence of the spatial trap distribution on the kinetics.
Received 23 July 2001 / Received in final form 14 May 2002 Published online 13 August 2002 相似文献
20.
M.E. Fouladvand M. Nematollahi 《The European Physical Journal B - Condensed Matter and Complex Systems》2001,22(3):395-401
We propose a model for the intersection of two urban streets. The traffic status of the crossroads is controlled by a set
of traffic lights which periodically switch to red and green with a total period of T. Two different types of crossroads are discussed. The first one describes the intersection of two one-way streets, while
the second type models the intersection of a two-way street with an one-way street. We assume that the vehicles approach the
crossroads with constant rates in time which are taken as the model parameters. We optimize the traffic flow at the crossroads
by minimizing the total waiting time of the vehicles per cycle of the traffic light. This leads to the determination of the
optimum green-time allocated to each phase.
Received 19 October 2000 and Received in final form 25 May 2001 相似文献