536 research outputs found
Higher-order Continuum Approximation for Rarefied Gases
The Hilbert-Chapman-Enskog expansion of the kinetic equations in mean flight
times is believed to be asymptotic rather than convergent. It is therefore
inadvisable to use lower order results to simplify the current approximation as
is done in the traditional Chapman-Enskog procedure, since that is an iterative
method. By avoiding such recycling of lower order results, one obtains
macroscopic equations that are asymptotically equivalent to the ones found in
the Chapman-Enskog approach. The new equations contain higher order terms that
are discarded in the Chapman-Enskog method. These make a significant impact on
the results for such problems as ultrasound propagation. In this paper, it is
shown that these results turn out well with relatively little complication when
the expansions are carried to second order in the mean free time, for the
example of the relaxation or BGK model of kinetic theory.Comment: 20 pages, 2 figures, RevTeX 4 macro
Uniform shear flow in dissipative gases. Computer simulations of inelastic hard spheres and (frictional) elastic hard spheres
In the preceding paper (cond-mat/0405252), we have conjectured that the main
transport properties of a dilute gas of inelastic hard spheres (IHS) can be
satisfactorily captured by an equivalent gas of elastic hard spheres (EHS),
provided that the latter are under the action of an effective drag force and
their collision rate is reduced by a factor (where is
the constant coefficient of normal restitution). In this paper we test the
above expectation in a paradigmatic nonequilibrium state, namely the simple or
uniform shear flow, by performing Monte Carlo computer simulations of the
Boltzmann equation for both classes of dissipative gases with a dissipation
range and two values of the imposed shear rate .
The distortion of the steady-state velocity distribution from the local
equilibrium state is measured by the shear stress, the normal stress
differences, the cooling rate, the fourth and sixth cumulants, and the shape of
the distribution itself. In particular, the simulation results seem to be
consistent with an exponential overpopulation of the high-velocity tail. The
EHS results are in general hardly distinguishable from the IHS ones if
, so that the distinct signature of the IHS gas (higher
anisotropy and overpopulation) only manifests itself at relatively high
dissipationsComment: 23 pages; 18 figures; Figs. 2 and 9 include new simulations; two new
figures added; few minor changes; accepted for publication in PR
System of elastic hard spheres which mimics the transport properties of a granular gas
The prototype model of a fluidized granular system is a gas of inelastic hard
spheres (IHS) with a constant coefficient of normal restitution . Using
a kinetic theory description we investigate the two basic ingredients that a
model of elastic hard spheres (EHS) must have in order to mimic the most
relevant transport properties of the underlying IHS gas. First, the EHS gas is
assumed to be subject to the action of an effective drag force with a friction
constant equal to half the cooling rate of the IHS gas, the latter being
evaluated in the local equilibrium approximation for simplicity. Second, the
collision rate of the EHS gas is reduced by a factor , relative
to that of the IHS gas. Comparison between the respective Navier-Stokes
transport coefficients shows that the EHS model reproduces almost perfectly the
self-diffusion coefficient and reasonably well the two transport coefficients
defining the heat flux, the shear viscosity being reproduced within a deviation
less than 14% (for ). Moreover, the EHS model is seen to agree
with the fundamental collision integrals of inelastic mixtures and dense gases.
The approximate equivalence between IHS and EHS is used to propose kinetic
models for inelastic collisions as simple extensions of known kinetic models
for elastic collisionsComment: 20 pages; 6 figures; change of title; few minor changes; accepted for
publication in PR
T-cell intracellular antigens in health and disease
T-cell intracellular antigen 1 (TIA1) and TIA1-related/like protein (TIAR/TIAL1) are 2 proteins discovered in 1991 as components of cytotoxic T lymphocyte granules. They act in the nucleus as regulators of transcription and pre-mRNA splicing. In the cytoplasm, TIA1 and TIAR regulate and/or modulate the location, stability and/or translation of mRNAs. As knowledge of the different genes regulated by these proteins and the cellular/biological programs in which they are involved increases, it is evident that these antigens are key players in human physiology and pathology. This review will discuss the latest developments in the field, with physiopathological relevance, that point to novel roles for these regulators in the molecular and cell biology of higher eukaryotes.Ministry Economic Affairs and Competitiveness through FEDER funds (BFU2008–00354, BFU2011–29653 and BFU2014–57735-R). The CBMSO receives an institutional grant from Fundación Ramón Areces.Peer Reviewe
Photoelectron angular distributions in photodetachment from P-
The angular distributions of electrons ejected in laser photodetachment of the P- ion have been studied in the photon energy range of 0.95-3.28 eV using a photoelectron spectrometer designed to accommodate a source consisting of collinearly overlapping photon and negative ion beams. We observe the value of the asymmetry parameter β starting at zero near the threshold, falling to almost -1 about 0.5 eV above the threshold and eventually rising to a positive value. The experimental data has been fitted to a simplified model of the Cooper-Zare formula which yields a qualitative understanding of the quantum interference between the outgoing s and d waves representing the free electron. The present results are also compared with previous results for other elements involving p-electron photodetachment
Causal Relativistic Fluid Dynamics
We derive causal relativistic fluid dynamical equations from the relaxation
model of kinetic theory as in a procedure previously applied in the case of
non-relativistic rarefied gases. By treating space and time on an equal footing
and avoiding the iterative steps of the conventional Chapman-Enskog ---
CE---method, we are able to derive causal equations in the first order of the
expansion in terms of the mean flight time of the particles. This is in
contrast to what is found using the CE approach. We illustrate the general
results with the example of a gas of identical ultrarelativistic particles such
as photons under the assumptions of homogeneity and isotropy. When we couple
the fluid dynamical equations to Einstein's equation we find, in addition to
the geometry-driven expanding solution of the FRW model, a second,
matter-driven nonequilibrium solution to the equations. In only the second
solution, entropy is produced at a significant rate.Comment: 23 pages (CQG, in press
A Continuum Description of Rarefied Gas Dynamics (I)--- Derivation From Kinetic Theory
We describe an asymptotic procedure for deriving continuum equations from the
kinetic theory of a simple gas. As in the works of Hilbert, of Chapman and of
Enskog, we expand in the mean flight time of the constituent particles of the
gas, but we do not adopt the Chapman-Enskog device of simplifying the formulae
at each order by using results from previous orders. In this way, we are able
to derive a new set of fluid dynamical equations from kinetic theory, as we
illustrate here for the relaxation model for monatomic gases. We obtain a
stress tensor that contains a dynamical pressure term (or bulk viscosity) that
is process-dependent and our heat current depends on the gradients of both
temperature and density. On account of these features, the equations apply to a
greater range of Knudsen number (the ratio of mean free path to macroscopic
scale) than do the Navier-Stokes equations, as we see in the accompanying
paper. In the limit of vanishing Knudsen number, our equations reduce to the
usual Navier-Stokes equations with no bulk viscosity.Comment: 16 page
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