193 research outputs found
Exact solution of a one-dimensional Boltzmann equation for a granular tracer particle
We consider a one-dimensional system consisting of a granular tracer particle
of mass in a bath of thermalized particles each of mass . When the mass
ratio, , is equal to the coefficient of restitution, , the system
maps to a a one-dimensional elastic gas. In this case, Boltzmann equation can
be solved exactly. We also obtain expressions for the velocity autocorrelation
function and the diffusion coefficient. Numerical simulations of the Boltzmann
equation are performed for where no analytical solution is
available. It appears that the dynamical features remain qualitatively similar
to those found in the exactly solvable case.Comment: 17 pages, 3 figures, Accepted in Physica
Shear and Bulk Viscosities of a Gluon Plasma in Perturbative QCD: Comparison of Different Treatments for the gg<->ggg Process
The leading order contribution to the shear and bulk viscosities, \eta and
\zeta, of a gluon plasma in perturbative QCD includes the gg -> gg (22)
process, gg ggg (23) process and multiple scattering processes known as the
Landau-Pomeranchuk-Migdal (LPM) effect. Complete leading order computations for
\eta and \zeta were obtained by Arnold, Moore and Yaffe (AMY) and Arnold, Dogan
and Moore (ADM), respectively, with the inelastic processes computed by an
effective g gg gluon splitting. We study how complementary calculations
with 22 and 23 processes and a simple treatment to model the LPM effect compare
with the results of AMY and ADM. We find that our results agree with theirs
within errors. By studying the contribution of the 23 process to \eta, we find
that the minimum angle \theta among the final state gluons in the fluid local
rest frame has a distribution that is peaked at \theta \sim \sqrt{\alpha_{s}},
analogous to the near collinear splitting asserted by AMY and ADM. However, the
average of \theta is much bigger than its peak value, as its distribution is
skewed with a long tail. The same \theta behavior is also seen if the 23 matrix
element is taken to the soft gluon bremsstrahlung limit in the center-of-mass
(CM) frame. This suggests that the soft gluon bremsstrahlung in the CM frame
still has some near collinear behavior in the fluid local rest frame. We also
generalize our result to a general SU(N_c) pure gauge theory and summarize the
current viscosity computations in QCD.Comment: ReVTex 4, 18 pages, 7 figures, accepted version in Phys. Rev.
Tagged particle in a sheared suspension: effective temperature determines density distribution in a slowly varying external potential beyond linear response
We consider a sheared colloidal suspension under the influence of an external
potential that varies slowly in space in the plane perpendicular to the flow
and acts on one selected (tagged) particle of the suspension. Using a
Chapman-Enskog type expansion we derive a steady state equation for the tagged
particle density distribution. We show that for potentials varying along one
direction only, the tagged particle distribution is the same as the equilibrium
distribution with the temperature equal to the effective temperature obtained
from the violation of the Einstein relation between the self-diffusion and
tagged particle mobility coefficients. We thus prove the usefulness of this
effective temperature for the description of the tagged particle behavior
beyond the realm of linear response. We illustrate our theoretical predictions
with Brownian dynamics computer simulations.Comment: Accepted for publication in Europhys. Let
Shear viscosity of a superfluid Fermi gas in the unitarity limit
We compute the shear viscosity of a superfluid atomic Fermi gas in the
unitarity limit. The unitarity limit is characterized by a divergent scattering
length between the atoms, and it has been argued that this will result in a
very small viscosity. We show that in the low temperature T limit the shear
viscosity scales as xi^5/T^5, where the universal parameter 'xi' relates the
chemical potential and the Fermi energy, mu=xi E_F. Combined with the high
temperature expansions of the viscosity our results suggest that the viscosity
has a minimum near the critical temperature T_c. A naive extrapolation
indicates that the minimum value of the ratio of viscosity over entropy density
is within a factor of ~ 5 of the proposed lower bound hbar/(4\pi k_B).Comment: 9 pages, 7 figures, LaTeX2
N-body decomposition of bipartite networks
In this paper, we present a method to project co-authorship networks, that
accounts in detail for the geometrical structure of scientists collaborations.
By restricting the scope to 3-body interactions, we focus on the number of
triangles in the system, and show the importance of multi-scientists (more than
2) collaborations in the social network. This motivates the introduction of
generalized networks, where basic connections are not binary, but involve
arbitrary number of components. We focus on the 3-body case, and study
numerically the percolation transition.Comment: 5 pages, submitted to PR
Generalized Fokker-Planck equation, Brownian motion, and ergodicity
Microscopic theory of Brownian motion of a particle of mass in a bath of
molecules of mass is considered beyond lowest order in the mass ratio
. The corresponding Langevin equation contains nonlinear corrections to
the dissipative force, and the generalized Fokker-Planck equation involves
derivatives of order higher than two. These equations are derived from first
principles with coefficients expressed in terms of correlation functions of
microscopic force on the particle. The coefficients are evaluated explicitly
for a generalized Rayleigh model with a finite time of molecule-particle
collisions. In the limit of a low-density bath, we recover the results obtained
previously for a model with instantaneous binary collisions. In general case,
the equations contain additional corrections, quadratic in bath density,
originating from a finite collision time. These corrections survive to order
and are found to make the stationary distribution non-Maxwellian.
Some relevant numerical simulations are also presented
Strong Shock Waves and Nonequilibrium Response in a One-dimensional Gas: a Boltzmann Equation Approach
We investigate the nonequilibrium behavior of a one-dimensional binary fluid
on the basis of Boltzmann equation, using an infinitely strong shock wave as
probe. Density, velocity and temperature profiles are obtained as a function of
the mixture mass ratio \mu. We show that temperature overshoots near the shock
layer, and that heavy particles are denser, slower and cooler than light
particles in the strong nonequilibrium region around the shock. The shock width
w(\mu), which characterizes the size of this region, decreases as w(\mu) ~
\mu^{1/3} for \mu-->0. In this limit, two very different length scales control
the fluid structure, with heavy particles equilibrating much faster than light
ones. Hydrodynamic fields relax exponentially toward equilibrium, \phi(x) ~
exp[-x/\lambda]. The scale separation is also apparent here, with two typical
scales, \lambda_1 and \lambda_2, such that \lambda_1 ~ \mu^{1/2} as \mu-->0$,
while \lambda_2, which is the slow scale controlling the fluid's asymptotic
relaxation, increases to a constant value in this limit. These results are
discussed at the light of recent numerical studies on the nonequilibrium
behavior of similar 1d binary fluids.Comment: 9 pages, 8 figs, published versio
Alignment of Rods and Partition of Integers
We study dynamical ordering of rods. In this process, rod alignment via
pairwise interactions competes with diffusive wiggling. Under strong diffusion,
the system is disordered, but at weak diffusion, the system is ordered. We
present an exact steady-state solution for the nonlinear and nonlocal kinetic
theory of this process. We find the Fourier transform as a function of the
order parameter, and show that Fourier modes decay exponentially with the wave
number. We also obtain the order parameter in terms of the diffusion constant.
This solution is obtained using iterated partitions of the integer numbers.Comment: 6 pages, 4 figure
One-particle and few-particle billiards
We study the dynamics of one-particle and few-particle billiard systems in containers of various shapes. In few-particle systems, the particles collide elastically both against the boundary and against each other. In the one-particle case, we investigate the formation and destruction of resonance islands in (generalized) mushroom billiards, which are a recently discovered class of Hamiltonian systems with mixed regular-chaotic dynamics. In the few-particle case, we compare the dynamics in container geometries whose counterpart one-particle billiards are integrable, chaotic, and mixed. One of our findings is that two-, three-, and four-particle billiards confined to containers with integrable one-particle counterparts inherit some integrals of motion and exhibit a regular partition of phase space into ergodic components of positive measure. Therefore, the shape of a container matters not only for noninteracting particles but also for interacting particles
Singular forces and point-like colloids in lattice Boltzmann hydrodynamics
We present a second-order accurate method to include arbitrary distributions
of force densities in the lattice Boltzmann formulation of hydrodynamics. Our
method may be used to represent singular force densities arising either from
momentum-conserving internal forces or from external forces which do not
conserve momentum. We validate our method with several examples involving point
forces and find excellent agreement with analytical results. A minimal model
for dilute sedimenting particles is presented using the method which promises a
substantial gain in computational efficiency.Comment: 22 pages, 9 figures. Submitted to Phys. Rev.
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