397 research outputs found
Cluster-Growth in Freely Cooling Granular Media
When dissipative particles are left alone, their fluctuation energy decays
due to collisional interactions, clusters build up and grow with time until the
system size is reached. When the effective dissipation is strong enough, this
may lead to the `inelastic collapse', i.e. the divergence of the collision
frequency of some particles. The cluster growth is an interesting physical
phenomenon, whereas the inelastic collapse is an intrinsic effect of the
inelastic hard sphere (IHS) model used to study the cluster growth - involving
only a negligible number of particles in the system. Here, we extend the IHS
model by introducing an elastic contact energy and the related contact duration
t_c. This avoids the inelastic collapse and allows to examine the long-time
behavior of the system. For a quantitative description of the cluster growth,
we propose a burning - like algorithm in continuous space, that readily
identifies all particles that belong to the same cluster. The criterion for
this is here chosen to be only the particle distance.
With this method we identify three regimes of behavior. First, for short
times a homogeneous cooling state (HCS) exists, where a mean-field theory works
nicely, and the clusters are tiny and grow very slowly. Second, at a certain
time which depends on the system's properties, cluster growth starts and the
clusters increase in size and mass until, in the third regime, the system size
is reached and most of the particles are collected in one huge cluster.Comment: 16 pages, 21 figures. Chaos 9(3) (in press, 1999
The energy flux into a fluidized granular medium at a vibrating wall
We study the power input of a vibrating wall into a fluidized granular
medium, using event driven simulations of a model granular system. The system
consists of inelastic hard disks contained between a stationary and a vibrating
elastic wall, in the absence of gravity. Two scaling relations for the power
input are found, both involving the pressure. The transition between the two
occurs when waves generated at the moving wall can propagate across the system.
Choosing an appropriate waveform for the vibrating wall removes one of these
scalings and renders the second very simple.Comment: 5 pages, revtex, 7 postscript figure
Phase transition in inelastic disks
This letter investigates the molecular dynamics of inelastic disks without
external forcing. By introducing a new observation frame with a rescaled time,
we observe the virtual steady states converted from asymptotic energy
dissipation processes. System behavior in the thermodynamic limit is carefully
investigated. It is found that a phase transition with symmetry breaking occurs
when the magnitude of dissipation is greater than a critical value.Comment: 9 pages, 6 figure
Towards a continuum theory of clustering in a freely cooling inelastic gas
We performed molecular dynamics simulations to investigate the clustering
instability of a freely cooling dilute gas of inelastically colliding disks in
a quasi-one-dimensional setting. We observe that, as the gas cools, the shear
stress becomes negligibly small, and the gas flows by inertia only. Finite-time
singularities, intrinsic in such a flow, are arrested only when close-packed
clusters are formed. We observe that the late-time dynamics of this system are
describable by the Burgers equation with vanishing viscosity, and predict the
long-time coarsening behavior.Comment: 7 pages, 5 eps figures, to appear in Europhys. Let
Dynamics of Freely Cooling Granular Gases
We study dynamics of freely cooling granular gases in two-dimensions using
large-scale molecular dynamics simulations. We find that for dilute systems the
typical kinetic energy decays algebraically with time, E(t) ~ t^{-1}, in the
long time limit. Asymptotically, velocity statistics are characterized by a
universal Gaussian distribution, in contrast with the exponential high-energy
tails characterizing the early homogeneous regime. We show that in the late
clustering regime particles move coherently as typical local velocity
fluctuations, Delta v, are small compared with the typical velocity, Delta v/v
~ t^{-1/4}. Furthermore, locally averaged shear modes dominate over acoustic
modes. The small thermal velocity fluctuations suggest that the system can be
heuristically described by Burgers-like equations.Comment: 4 pages, 5 figure
Deviations from plastic barriers in BiSrCaCuO thin films
Resistive transitions of an epitaxial BiSrCaCuO thin
film were measured in various magnetic fields (), ranging from 0
to 22.0 T. Rounded curvatures of low resistivity tails are observed in
Arrhenius plot and considered to relate to deviations from plastic barriers. In
order to characterize these deviations, an empirical barrier form is developed,
which is found to be in good agreement with experimental data and coincide with
the plastic barrier form in a limited magnetic field range. Using the plastic
barrier predictions and the empirical barrier form, we successfully explain the
observed deviations.Comment: 5 pages, 6 figures; PRB 71, 052502 (2005
Spatial Correlations in Compressible Granular Flows
For a freely evolving granular fluid, the buildup of spatial correlations in
density and flow field is described using fluctuating hydrodynamics. The theory
for incompressible flows is extended to the general, compressible case,
including longitudinal velocity and density fluctuations, and yields
qualitatively different results for long range correlations. The structure
factor of density fluctuations shows a maximum at finite wavenumber, shifting
in time to smaller wavenumbers and corresponding to a growing correlation
length. It agrees well with two-dimensional molecular dynamics simulations.Comment: 12 pages, Latex, 3 figure
Influence of Canal Geometry and Dynamics on Controllability
This paper presents the results of the Task Committee on Canal Automation Algorithms with regard to the influence of canal properties on the controllability of irrigation canals. While the control provided by individual algorithms was not evaluated, studies were performed to illustrate inherent hydraulic limitations—the inability of canal pools to recover rapidly from disturbances or flow perturbations. Studies were performed in nondimensional form to develop a better understanding of how pool properties influence pool response. Three such studies were performed. First, nondimensional backwater curves were developed for a range of canal conditions. The second study involved the propagation of waves initiated at the upstream end of a canal pool, as this is influenced by downstream boundary conditions. Finally, the response of pools to downstream withdrawals was examined in terms of their sluggish recovery even when the correct flow change is applied upstream. These results will help in understanding how canal properties influence the ability of operators to effectively control a canal either manually or automatically, and should influence future design practices
Fast diffusion of a Lennard-Jones cluster on a crystalline surface
We present a Molecular Dynamics study of large Lennard-Jones clusters
evolving on a crystalline surface. The static and the dynamic properties of the
cluster are described. We find that large clusters can diffuse rapidly, as
experimentally observed. The role of the mismatch between the lattice
parameters of the cluster and the substrate is emphasized to explain the
diffusion of the cluster. This diffusion can be described as a Brownian motion
induced by the vibrationnal coupling to the substrate, a mechanism that has not
been previously considered for cluster diffusion.Comment: latex, 5 pages with figure
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