221 research outputs found
Hydrodynamics of thermal granular convection
A hydrodynamic theory is formulated for buoyancy-driven ("thermal") granular
convection, recently predicted in molecular dynamic simulations and observed in
experiment. The limit of a dilute flow is considered. The problem is fully
described by three scaled parameters. The convection occurs via a supercritical
bifurcation, the inelasticity of the collisions being the control parameter.
The theory is expected to be valid for small Knudsen numbers and nearly elastic
grain collisions.Comment: 4 pages, 4 EPS figures, some details adde
Symmetry-breaking instability in a prototypical driven granular gas
Symmetry-breaking instability of a laterally uniform granular cluster (strip
state) in a prototypical driven granular gas is investigated. The system
consists of smooth hard disks in a two-dimensional box, colliding inelastically
with each other and driven, at zero gravity, by a "thermal" wall. The limit of
nearly elastic particle collisions is considered, and granular hydrodynamics
with the Jenkins-Richman constitutive relations is employed. The hydrodynamic
problem is completely described by two scaled parameters and the aspect ratio
of the box. Marginal stability analysis predicts a spontaneous symmetry
breaking instability of the strip state, similar to that predicted recently for
a different set of constitutive relations. If the system is big enough, the
marginal stability curve becomes independent of the details of the boundary
condition at the driving wall. In this regime, the density perturbation is
exponentially localized at the elastic wall opposite to the thermal wall. The
short- and long-wavelength asymptotics of the marginal stability curves are
obtained analytically in the dilute limit. The physics of the symmetry-breaking
instability is discussed.Comment: 11 pages, 14 figure
Onset of thermal convection in a horizontal layer of granular gas
The Navier-Stokes granular hydrodynamics is employed for determining the
threshold of thermal convection in an infinite horizontal layer of granular
gas. The dependence of the convection threshold, in terms of the inelasticity
of particle collisions, on the Froude and Knudsen numbers is found. A simple
necessary condition for convection is formulated in terms of the
Schwarzschild's criterion, well-known in thermal convection of (compressible)
classical fluids. The morphology of convection cells at the onset is
determined. At large Froude numbers, the Froude number drops out of the
problem. As the Froude number goes to zero, the convection instability turns
into a recently discovered phase separation instability.Comment: 6 pages, 6 figures. An extended version. A simple and universal
necessary criterion for convection presente
Nonlinear Modulation of Multi-Dimensional Lattice Waves
The equations governing weakly nonlinear modulations of -dimensional
lattices are considered using a quasi-discrete multiple-scale approach. It is
found that the evolution of a short wave packet for a lattice system with cubic
and quartic interatomic potentials is governed by generalized Davey-Stewartson
(GDS) equations, which include mean motion induced by the oscillatory wave
packet through cubic interatomic interaction. The GDS equations derived here
are more general than those known in the theory of water waves because of the
anisotropy inherent in lattices. Generalized Kadomtsev-Petviashvili equations
describing the evolution of long wavelength acoustic modes in two and three
dimensional lattices are also presented. Then the modulational instability of a
-dimensional Stokes lattice wave is discussed based on the -dimensional
GDS equations obtained. Finally, the one- and two-soliton solutions of
two-dimensional GDS equations are provided by means of Hirota's bilinear
transformation method.Comment: Submitted to PR
Disorder Induced Phases in Higher Spin Antiferromagnetic Heisenberg Chains
Extensive DMRG calculations for spin S=1/2 and S=3/2 disordered
antiferromagnetic Heisenberg chains show a rather distinct behavior in the two
cases. While at sufficiently strong disorder both systems are in a random
singlet phase, we show that weak disorder is an irrelevant perturbation for the
S=3/2 chain, contrary to what expected from a naive application of the Harris
criterion. The observed irrelevance is attributed to the presence of a new
correlation length due to enhanced end-to-end correlations. This phenomenon is
expected to occur for all half-integer S > 1/2 chains. A possible phase diagram
of the chain for generic S is also discussed.Comment: 6 Pages and 6 figures. Final version as publishe
Observation of Scaling Violations in Scaled Momentum Distributions at HERA
Charged particle production has been measured in deep inelastic scattering
(DIS) events over a large range of and using the ZEUS detector. The
evolution of the scaled momentum, , with in the range 10 to 1280
, has been investigated in the current fragmentation region of the Breit
frame. The results show clear evidence, in a single experiment, for scaling
violations in scaled momenta as a function of .Comment: 21 pages including 4 figures, to be published in Physics Letters B.
Two references adde
D* Production in Deep Inelastic Scattering at HERA
This paper presents measurements of D^{*\pm} production in deep inelastic
scattering from collisions between 27.5 GeV positrons and 820 GeV protons. The
data have been taken with the ZEUS detector at HERA. The decay channel
(+ c.c.) has been used in the study. The
cross section for inclusive D^{*\pm} production with
and is 5.3 \pms 1.0 \pms 0.8 nb in the kinematic region
{ GeV and }. Differential cross
sections as functions of p_T(D^{*\pm}), and are
compared with next-to-leading order QCD calculations based on the photon-gluon
fusion production mechanism. After an extrapolation of the cross section to the
full kinematic region in p_T(D^{*\pm}) and (D^{*\pm}), the charm
contribution to the proton structure function is
determined for Bjorken between 2 10 and 5 10.Comment: 17 pages including 4 figure
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