160 research outputs found
Anisotropic Energy Distribution in Three-Dimensional Vibrofluidized Granular Systems
We examine the energy distribution in a three-dimensional model granular
system contained in an open cylinder under the influence of gravity. Energy is
supplied to the system by a vibrating base. We introduce spatially resolved,
partial particle-particle ``dissipations'' for directions parallel and
perpendicular to the energy input, respectively. Energy balances show that the
total (integrated) ``dissipation'' is less than zero in the parallel direction
while greater than zero in the perpendicular directions. The energy supplied to
the perpendicular directions is dissipated by particle-wall collisions. We
further define a fractional energy transfer, which in the steady state
represents the fraction of the power supplied by the vibrating base that is
dissipated at the wall. We examine the dependence of the fractional energy
transfer on the number of particles, the velocity of the vibrating base, the
particle-particle restitution coefficient, and the particle-wall restitution
coefficient. We also explore the influence of the system parameters on the
spatially dependent partial dissipations.Comment: 10 pages, 10 figures, RevTeX forma
Transient response of photoexcited electrons: negative and oscillating current
Time-dependent current of the electrons excited in the conduction band after
ultrafast interband photogeneration is studied theoretically. The transient
photocurrent is calculated for the nonlinear regime of response to a stationary
electric field. The response demonstrates transient absolute negative
conductivity when the electrons are excited slightly below the optical phonon
energy, while the periodic oscillations of the electric current appear after
formation of the streaming distribution. The quenching of these peculiarities
by the elastic scattering of electrons is also considered.Comment: 5 pages, 3 figure
Strength distribution of repeatedly broken chains
We determine the probability distribution of the breaking strength for chains
of N links, which have been produced by repeatedly breaking a very long chain.Comment: 4 pages, 1 figur
Velocity fluctuations in forced Burgers turbulence
We propose a simple method to compute the velocity difference statistics in
forced Burgers turbulence in any dimension. Within a reasonnable assumption
concerning the nucleation and coalescence of shocks, we find in particular that
the `left' tail of the distribution decays as an inverse square power, which is
compatible with numerical data. Our results are compared to those of various
recent approaches: instantons, operator product expansion, replicas.Comment: 10 pages latex, one postcript figur
Close-packed floating clusters: granular hydrodynamics beyond the freezing point?
Monodisperse granular flows often develop regions with hexagonal close
packing of particles. We investigate this effect in a system of inelastic hard
spheres driven from below by a "thermal" plate. Molecular dynamics simulations
show, in a wide range of parameters, a close-packed cluster supported by a
low-density region. Surprisingly, the steady-state density profile, including
the close-packed cluster part, is well described by a variant of Navier-Stokes
granular hydrodynamics (NSGH). We suggest a simple explanation for the success
of NSGH beyond the freezing point.Comment: 4 pages, 5 figures. To appear in Phys. Rev. Let
The secondary minimum in YY Her: Evidence for a tidally distorted giant
We present and analyze quiescent UBVRI light curves of the classical
symbiotic binary YY Her. We show that the secondary minimum, which is clearly
visible only in the quiescent VRI light curves, is due to ellipsoidal
variability of the red giant component. Our simple light curve analysis, by
fitting of the Fourier cosine series, resulted in a self-consistent
phenomenological model of YY Her, in which the periodic changes can be
described by a combination of the ellipsoidal changes and a sinusoidal changes
of the nebular continuum and line emission.Comment: 5 pages, 2 figures, to appear in Astronomy & Astrophysic
Nontrivial Velocity Distributions in Inelastic Gases
We study freely evolving and forced inelastic gases using the Boltzmann
equation. We consider uniform collision rates and obtain analytical results
valid for arbitrary spatial dimension d and arbitrary dissipation coefficient
epsilon. In the freely evolving case, we find that the velocity distribution
decays algebraically, P(v,t) ~ v^{-sigma} for sufficiently large velocities. We
derive the exponent sigma(d,epsilon), which exhibits nontrivial dependence on
both d and epsilon, exactly. In the forced case, the velocity distribution
approaches a steady-state with a Gaussian large velocity tail.Comment: 4 pages, 1 figur
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
Dynamics of inelastically colliding rough spheres: Relaxation of translational and rotational energy
We study the exchange of kinetic energy between translational and rotational
degrees of freedom for inelastic collisions of rough spheres. Even if
equipartition holds in the initial state it is immediately destroyed by
collisions. The simplest generalisation of the homogeneous cooling state allows
for two temperatures, characterizing translational and rotational degrees of
freedom separately. For times larger than a crossover frequency, which is
determined by the Enskog frequency and the initial temperature, both energies
decay algebraically like with a fixed ratio of amplitudes, different
from one.Comment: 5 pages, RevTeX, 2 eps figures, slightly expanded discussion, new
figures with dimensionless units, added references, accepted for publication
in PRE as a Rapid Com
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