14,321 research outputs found
Impurity in a granular gas under nonlinear Couette flow
We study in this work the transport properties of an impurity immersed in a
granular gas under stationary nonlinear Couette flow. The starting point is a
kinetic model for low-density granular mixtures recently proposed by the
authors [Vega Reyes F et al. 2007 Phys. Rev. E 75 061306]. Two routes have been
considered. First, a hydrodynamic or normal solution is found by exploiting a
formal mapping between the kinetic equations for the gas particles and for the
impurity. We show that the transport properties of the impurity are
characterized by the ratio between the temperatures of the impurity and gas
particles and by five generalized transport coefficients: three related to the
momentum flux (a nonlinear shear viscosity and two normal stress differences)
and two related to the heat flux (a nonlinear thermal conductivity and a cross
coefficient measuring a component of the heat flux orthogonal to the thermal
gradient). Second, by means of a Monte Carlo simulation method we numerically
solve the kinetic equations and show that our hydrodynamic solution is valid in
the bulk of the fluid when realistic boundary conditions are used. Furthermore,
the hydrodynamic solution applies to arbitrarily (inside the continuum regime)
large values of the shear rate, of the inelasticity, and of the rest of
parameters of the system. Preliminary simulation results of the true Boltzmann
description show the reliability of the nonlinear hydrodynamic solution of the
kinetic model. This shows again the validity of a hydrodynamic description for
granular flows, even under extreme conditions, beyond the Navier-Stokes domain.Comment: 23 pages, 11 figures; v2: Preliminary DSMC results from the Boltzmann
equation included, Fig. 11 is ne
Did DR-CAFTA Affect the Exports of the Dominican Republic to the United States?
This article evaluates the impact the Dominican Republic and Central America Free Trade Agreement (DR-CAFTA) had on the exports of the Dominican Republic (DR) to the United States. We estimate a gravity model for the DR exports to the 109 trade partners of the country from 1990 to 2014. This model quantifies the effect of the DR-CAFTA since 2007, when the agreement was ratified, and finds that the DR-CAFTA negatively affected DR exports to the US. We conduct further analysis of factors that could explain the decline in exports, in spite of the ratification of the agreement. We find that the 2008–09 crisis was not the driver of the slowdown of DR exports to the US. The increasing competition with Central American countries and other export oriented economies, like China, in the US market seem to have the leading role in the export contraction that we find in our study
Testing symmetries in effective models of higher derivative field theories
Higher derivative field theories with interactions raise serious doubts about
their validity due to severe energy instabilities. In many cases the
implementation of a direct perturbation treatment to excise the dangerous
negative-energies from a higher derivative field theory may lead to violations
of Lorentz and other symmetries. In this work we study a perturbative
formulation for higher derivative field theories that allows the construction
of a low-energy effective field theory being a genuine perturbations over the
ordinary-derivative theory and having a positive-defined Hamiltonian. We show
that some discrete symmetries are recovered in the low-energy effective theory
when the perturbative method to reduce the negative-energy degrees of freedom
from the higher derivative theory is applied. In particular, we focus on the
higher derivative Maxwell-Chern-Simons model which is a Lorentz invariant and
parity-odd theory in 2+1 dimensions. The parity violation arises in the
effective action of QED as a quantum correction from the massive fermionic
sector. We obtain the effective field theory which remains Lorentz invariant,
but parity invariant to the order considered in the perturbative expansion.Comment: 13 pages, Sec. III, additional references added, P symmetry revised,
accepted for publication in PR
Computer simulations of an impurity in a granular gas under planar Couette flow
We present in this work results from numerical solutions, obtained by means
of the direct simulation Monte Carlo (DSMC) method, of the Boltzmann and
Boltzmann--Lorentz equations for an impurity immersed in a granular gas under
planar Couette flow. The DSMC results are compared with the exact solution of a
recent kinetic model for the same problem. The results confirm that, in steady
states and over a wide range of parameter values, the state of the impurity is
enslaved to that of the host gas: it follows the same flow velocity profile,
its concentration (relative to that of the granular gas) is constant in the
bulk region, and the impurity/gas temperature ratio is also constant. We
determine also the rheological properties and nonlinear hydrodynamic transport
coefficients for the impurity, finding a good semi-quantitative agreement
between the DSMC results and the theoretical predictions.Comment: 23 pages, 11 figures; v2: minor change
Combined Relativistic and static analysis for all Delta B=2 operators
We analyse matrix elements of Delta B=2 operators by combining QCD results
with the ones obtained in the static limit of HQET. The matching of all the QCD
operators to HQET is made at NLO order. To do that we have to include the
anomalous dimension matrix up to two loops, both in QCD and HQET, and the one
loop matching for all the Delta B=2 operators. The matrix elements of these
operators are relevant for the prediction of the B-\bar B mixing, B_s meson
width difference and supersymmetric effects in Delta B=2 transitions.Comment: 3 pages, 1 figure. Lattice2001(heavyquark
Enhanced moments of Eu in single crystals of the metallic helical antiferromagnet EuCo2 yAs2
The compound EuCo{2-y}As2 with the tetragonal ThCr2Si2 structure is known to
contain Eu{+2} ions with spin S = 7/2 that order below a temperature TN = 47 K
into an antiferromagnetic (AFM) proper helical structure with the ordered
moments aligned in the tetragonal ab plane, perpendicular to the helix axis
along the c axis, with no contribution from the Co atoms. Here we carry out a
detailed investigation of the properties of single crystals. Enhanced ordered
and effective moments of the Eu spins are found in most of our crystals.
Electronic structure calculations indicate that the enhanced moments arise from
polarization of the d bands, as occurs in ferromagnetic Gd metal. Electrical
resistivity measurements indicate metallic behavior. The low-field in-plane
magnetic susceptibilities chi{ab}(T < TN) for several crystals are reported
that are fitted well by unified molecular field theory (MFT), and the Eu-Eu
exchange interactions Jij are extracted from the fits. High-field magnetization
M data for magnetic fields H||ab reveal what appears to be a first-order
spin-flop transition followed at higher field by a second-order metamagnetic
transition of unknown origin, and then by another second-order transition to
the paramagnetic (PM) state. For H||c, the magnetization shows only a
second-order transition from the canted AFM to the PM state, as expected. The
critical fields for the AFM to PM transition are in approximate agreement with
the predictions of MFT. Heat capacity Cp measurements in zero and high H are
reported. Phase diagrams for H||c and H||ab versus T are constructed from the
high-field M(H,T) and Cp(H,T) measurements. The magnetic part Cmag(T, H = 0) of
Cp(T, H = 0) is extracted and is fitted rather well below TN by MFT, although
dynamic short-range AFM order is apparent in Cmag(T) up to about 70 K, where
the molar entropy attains its high-T limit of R ln8.Comment: 29 pages, 30 figures including 62 subfigures, 8 tables, 84 reference
Granular mixtures modeled as elastic hard spheres subject to a drag force
Granular gaseous mixtures under rapid flow conditions are usually modeled by
a multicomponent system of smooth inelastic hard spheres with constant
coefficients of normal restitution. In the low density regime an adequate
framework is provided by the set of coupled inelastic Boltzmann equations. Due
to the intricacy of the inelastic Boltzmann collision operator, in this paper
we propose a simpler model of elastic hard spheres subject to the action of an
effective drag force, which mimics the effect of dissipation present in the
original granular gas. The Navier--Stokes transport coefficients for a binary
mixture are obtained from the model by application of the Chapman--Enskog
method. The three coefficients associated with the mass flux are the same as
those obtained from the inelastic Boltzmann equation, while the remaining four
transport coefficients show a general good agreement, especially in the case of
the thermal conductivity. Finally, the approximate decomposition of the
inelastic Boltzmann collision operator is exploited to construct a model
kinetic equation for granular mixtures as a direct extension of a known kinetic
model for elastic collisions.Comment: The title has been changed, 4 figures, and to be published in Phys.
Rev.
An exact solution of the inelastic Boltzmann equation for the Couette flow with uniform heat flux
In the steady Couette flow of a granular gas the sign of the heat flux
gradient is governed by the competition between viscous heating and inelastic
cooling. We show from the Boltzmann equation for inelastic Maxwell particles
that a special class of states exists where the viscous heating and the
inelastic cooling exactly compensate each other at every point, resulting in a
uniform heat flux. In this state the (reduced) shear rate is enslaved to the
coefficient of restitution , so that the only free parameter is the
(reduced) thermal gradient . It turns out that the reduced moments of
order are polynomials of degree in , with coefficients that
are nonlinear functions of . In particular, the rheological properties
() are independent of and coincide exactly with those of the
simple shear flow. The heat flux () is linear in the thermal gradient
(generalized Fourier's law), but with an effective thermal conductivity
differing from the Navier--Stokes one. In addition, a heat flux component
parallel to the flow velocity and normal to the thermal gradient exists. The
theoretical predictions are validated by comparison with direct Monte Carlo
simulations for the same model.Comment: 16 pages, 4 figures,1 table; v2: minor change
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