199 research outputs found
Compaction dynamics of a granular media under vertical tapping
We report new experimental results on granular compaction under consecutive
vertical taps. The evolution of the mean volume fraction and of the mean
potential energy of a granular packing presents a slow densification until a
final steady-state, and is reminiscent to usual relaxation in glasses via a
stretched exponential law. The intensity of the taps seems to rule the
characteristic time of the relaxation according to an Arrhenius's type relation
>. Finally, the analysis of the vertical volume fraction profile reveals an
almost homogeneous densification in the packing.Comment: 7 pages, 4 figures, to appear in Europhysics Letter
Granular circulation in a cylindrical pan: simulations of reversing radial and tangential flows
Granular flows due to simultaneous vertical and horizontal excitations of a
flat-bottomed cylindrical pan are investigated using event-driven molecular
dynamics simulations. In agreement with recent experimental results, we observe
a transition from a solid-like state, to a fluidized state in which circulatory
flow occurs simultaneously in the radial and tangential directions. By going
beyond the range of conditions explored experimentally, we find that each of
these circulations reverse their direction as a function of the control
parameters of the motion. We numerically evaluate the dynamical phase diagram
for this system and show, using a simple model, that the solid-fluid transition
can be understood in terms of a critical value of the radial acceleration of
the pan bottom; and that the circulation reversals are controlled by the phase
shift relating the horizontal and vertical components of the vibrations. We
also discuss the crucial role played by the geometry of the boundary
conditions, and point out a relationship of the circulation observed here and
the flows generated in vibratory conveyors.Comment: 10 pages, 8 figure
Packing Fractions and Maximum Angles of Stability of Granular Materials
In two-dimensional rotating drum experiments, we find two separate influences
of the packing fraction of a granular heap on its stability. For a fixed grain
shape, the stability increases with packing fraction. However, in determining
the relative stability of different grain shapes, those with the lowest average
packing fractions tend to form the most stable heaps. We also show that only
the configuration close to the surface of the pile figures prominently.Comment: 4 pages, 4 figure
On the existence of stationary states during granular compaction
When submitted to gentle mechanical taps a granular packing slowly compacts
until it reaches a stationary state that depends on the tap characteristics.
The properties of such stationary states are experimentally investigated. The
influence of the initial state, taps properties and tapping protocol are
studied. The compactivity of the packings is determinated. Our results strongly
support the idea that the stationary states are genuine thermodynamic states.Comment: to be published in EPJE. The original publication will be available
at www.europhysj.or
Effect of boundary conditions on diffusion in two-dimensional granular gases
We analyze the influence of boundary conditions on numerical simulations of
the diffusive properties of a two dimensional granular gas. We show in
particular that periodic boundary conditions introduce unphysical correlations
in time which cause the coefficient of diffusion to be strongly dependent on
the system size. On the other hand, in large enough systems with hard walls at
the boundaries, diffusion is found to be independent of the system size. We
compare the results obtained in this case with Langevin theory for an elastic
gas. Good agreement is found. We then calculate the relaxation time and the
influence of the mass for a particle of radius in a sea of particles of
radius . As granular gases are dissipative, we also study the influence of
an external random force on the diffusion process in a forced dissipative
system. In particular, we analyze differences in the mean square velocity and
displacement between the elastic and inelastic cases.Comment: 15 figures eps figures, include
Power law velocity fluctuations due to inelastic collisions in numerically simulated vibrated bed of powder}
Distribution functions of relative velocities among particles in a vibrated
bed of powder are studied both numerically and theoretically. In the solid
phase where granular particles remain near their local stable states, the
probability distribution is Gaussian. On the other hand, in the fluidized
phase, where the particles can exchange their positions, the distribution
clearly deviates from Gaussian. This is interpreted with two analogies;
aggregation processes and soft-to-hard turbulence transition in thermal
convection. The non-Gaussian distribution is well-approximated by the
t-distribution which is derived theoretically by considering the effect of
clustering by inelastic collisions in the former analogy.Comment: 7 pages, using REVTEX (Figures are inculded in text body)
%%%Replacement due to rivision (Europhys. Lett., in press)%%
Observations of present-day activity at super-fast spreading : volcanic, hydrothermal and tectonic studies of the EPR 17-19°S
Experimental and computational studies of jamming
Jamming is a common feature of out of equilibrium systems showing slow
relaxation dynamics. Here we review our efforts in understanding jamming in
granular materials using experiments and computer simulations. We first obtain
an estimation of an effective temperature for a slowly sheared granular
material very close to jamming. The measurement of the effective temperature is
realized in the laboratory by slowly shearing a closely-packed ensemble of
spherical beads confined by an external pressure in a Couette geometry. All the
probe particles, independent of their characteristic features, equilibrate at
the same temperature, given by the packing density of the system. This suggests
that the effective temperature is a state variable for the nearly jammed
system. Then we investigate numerically whether the effective temperature can
be obtained from a flat average over the jammed configuration at a given energy
in the granular packing, as postulated by the thermodynamic approach to grains.Comment: 20 pages, 9 figure
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