11,672 research outputs found
Simulations of dense granular flow: Dynamic Arches and Spin Organization
We present a numerical model for a two dimensional (2D) granular assembly,
falling in a rectangular container when the bottom is removed. We observe the
occurrence of cracks splitting the initial pile into pieces, like in
experiments. We study in detail various mechanisms connected to the
`discontinuous decompaction' of this granular material. In particular, we focus
on the history of one single long range crack, from its origin at one side
wall, until it breaks the assembly into two pieces. This event is correlated to
an increase in the number of collisions, i.e. strong pressure, and to a
momentum wave originated by one particle. Eventually, strong friction reduces
the falling velocity such that the crack may open below the slow, high pressure
`dynamic arch'. Furthermore, we report the presence of large, organized
structures of the particles' angular velocities in the dense parts of the
granulate when the number of collisions is large.Comment: Submitted to J. Phys.
Simulations of Pattern Formation in Vibrated Granular Media
We present simulations of peak pattern formation in vibrated two-dimensional
(2D) granulates and measure the dispersion relation of the pattern for various
frequencies, accelerations, cell sizes, and layer heights. We report the first
quantitative data from numerical simulations showing an interesting dependence
of the pattern wavelength on the acceleration and the system size. Our results
are related to recent experimental findings and theoretical predictions for
gravity waves.Comment: 6 pages PS-file including figures, (version accepted at Europhys.
Lett. 26.10.96
Properties of holographic dark energy at the Hubble length
We consider holographic cosmological models of dark energy in which the
infrared cutoff is set by the Hubble's radius. We show that any interacting
dark energy model, regardless of its detailed form, can be recast as a non
interacting model in which the holographic parameter evolves slowly
with time. Two specific cases are analyzed. We constrain the parameters of both
models with observational data, and show that they can be told apart at the
perturbative level.Comment: 4 pages, 6 figures. Contribution to the Proceedings ERE201
Quiescent and coherent cores from gravoturbulent fragmentation
[abridged] We investigate the velocity structure of protostellar cores that
result from non-magnetic numerical models of the gravoturbulent fragmentation
of molecular cloud material. A large fraction of the cores analyzed are
``quiescent'', and more than half are identified as ``coherent''. The fact that
dynamically evolving cores in highly supersonic turbulent flows can be
quiescent may be understood because cores lie at the stagnation points of
convergent turbulent flows, where compression is at a maximum, and relative
velocity differences are at a minimum. The coherence may be due to an
observational effect related to the length and concentration of the material
contributing to the line. The velocity dispersion of the our cores often has
its local maximum at small offsets from the column density maximum, suggesting
that the core is the dense region behind a shock. Such a configuration is often
found in observations of molecular cloud cores, and argues in favor of the
gravoturbulent scenario of stellar birth as it is not expected in
star-formation models based on magnetic mediation. Cores with collapsed objects
tend to be near equipartition between their gravitational and kinetic energies,
while cores without collapsed objects tend to be gravitationally unbound,
suggesting that gravitational collapse occurs immediately after gravity becomes
dominant. Finally, cores in simulations driven at large scales are more
frequently quiescent and coherent, and have more realistic ratios of , supporting the notion that molecular cloud turbulence is driven at
large scales.Comment: ApJ, in pres
Effect of friction in a toy model of granular compaction
We proposed a toy model of granular compaction which includes some resistance
due to granular arches. In this model, the solid/solid friction of contacting
grains is a key parameter and a slipping threshold Wc is defined. Realistic
compaction behaviors have been obtained. Two regimes separated by a critical
point Wc* of the slipping threshold have been emphasized : (i) a slow
compaction with lots of paralyzed regions, and (ii) an inverse logarithmic
dynamics with a power law scaling of grain mobility. Below the critical point
Wc*, the physical properties of this frozen system become independent of Wc.
Above the critical point Wc*, i.e. for low friction values, the packing
properties behave as described by the classical Janssen theory for silos
Ripples in Tapped or Blown Powder
We observe ripples forming on the surface of a granular powder in a container
submitted from below to a series of brief and distinct shocks. After a few
taps, the pattern turns out to be stable against any further shock of the same
amplitude. We find experimentally that the characteristic wavelength of the
pattern is proportional to the amplitude of the shocks. Starting from
consideration involving Darcy's law for air flow through the porous granulate
and avalanche properties, we build up a semi-quantitative model which fits
satisfactorily the set of experimental observations as well as a couple of
additional experiments.Comment: 7 pages, four postscript figures, submitted PRL 11/19/9
Block to granular-like transition in dense bubble flows
We have experimentally investigated 2-dimensional dense bubble flows
underneath inclined planes. Velocity profiles and velocity fluctuations have
been measured. A broad second-order phase transition between two dynamical
regimes is observed as a function of the tilt angle . For low
values, a block motion is observed. For high values, the velocity
profile becomes curved and a shear velocity gradient appears in the flow.Comment: Europhys. Lett. (2003) in pres
Solution of the quasi-one-dimensional linearized Euler equations using flow invariants and the Magnus expansion
Les funcions de base flotants en cà lculs ab initio: Prppietats estructurals, grau de flotació i el teorema del virial
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