2,489 research outputs found
Hispanic Membership Survey
This paper reports the findings of the first survey designed for the Hispanic membership of the Pacific Union Conference. Due to their diversity, the Hispanic members comprise a special group, very different from the other ethnic groups of the North American Division. Its purpose was to gather information that would help leaders to make better decisions. Questionnaires were used for collecting the data.https://digitalcommons.andrews.edu/hrsa/1074/thumbnail.jp
Internal energy fluctuations of a granular gas under steady uniform shear flow
The stochastic properties of the total internal energy of a dilute granular
gas in the steady uniform shear flow state are investigated. A recent theory
formulated for fluctuations about the homogeneous cooling state is extended by
analogy with molecular systems. The theoretical predictions are compared with
molecular dynamics simulation results. Good agreement is found in the limit of
weak inelasticity, while systematic and relevant discrepancies are observed
when the inelasticity increases. The origin of this behavior is discussed
The Enskog equation for confined elastic hard spheres
A kinetic equation for a system of elastic hard spheres or disks confined by
a hard wall of arbitrary shape is derived. It is a generalization of the
modified Enskog equation in which the effects of the confinement are taken into
account and it is supposed to be valid up to moderate densities. From the
equation, balance equations for the hydrodynamic fields are derived,
identifying the collisional transfer contributions to the pressure tensor and
heat flux. A Lyapunov functional, , is identified. For any
solution of the kinetic equation, decays monotonically in time
until the system reaches the inhomogeneous equilibrium distribution, that is a
Maxwellian distribution with a the density field consistent with equilibrium
statistical mechanics
Rheological effects in the linear response and spontaneous fluctuations of a sheared granular gas
The decay of a small homogeneous perturbation of the temperature of a dilute
granular gas in the steady uniform shear flow state is investigated. Using
kinetic theory based on the inelastic Boltzmann equation, a closed equation for
the decay of the perturbation is derived. The equation involves the generalized
shear viscosity of the gas in the time-dependent shear flow state, and
therefore it predicts relevant rheological effects beyond the quasi-elastic
limit. A good agreement is found when comparing the theory with molecular
dynamics simulation results. Moreover, the Onsager postulate on the regression
of fluctuations is fulfilled
Mesoscopic Theory of Critical Fluctuations in Isolated Granular Gases
Fluctuating hydrodynamics is used to describe the total energy fluctuations
of a freely evolving gas of inelastic hard spheres near the threshold of the
clustering instability. They are shown to be governed by vorticity fluctuations
only, that also lead to a renormalization of the average total energy. The
theory predicts a power-law divergent behavior of the scaled second moment of
the fluctuations, and a scaling property of their probability distribution,
both in agreement with simulations results. A more quantitative comparison
between theory and simulation for the critical amplitudes and the form of the
scaling function is also carried out
Homogeneous hydrodynamics of a collisional model of confined granular gases
The hydrodynamic equation governing the homogeneous time evolution of the
temperature in a model of confined granular gas is studied by means of the
Enskog equation. The existence of a normal solution of the kinetic equation is
assumed as a condition for hydrodynamics. Dimensional analysis implies a
scaling of the distribution function that is used to determine it in the first
Sonine approximation, with a coefficient that evolves in time through its
dependence on the temperature. The theoretical predictions are compared with
numerical results obtained by the direct simulation Monte Carlo method, and a
good agreement is found. The relevance of the normal homogeneous distribution
function to derive inhomogeneous hydrodynamic equations, for instance using the
Champan-Enskog algorithm, is indicated.Comment: Accepted in Phys. Rev.
Hydrodynamics for a model of a confined quasi-two-dimensional granular gas
The hydrodynamic equations for a model of a confined quasi-two-dimensional
gas of smooth inelastic hard spheres are derived from the Boltzmann equation
for the model, using a generalization of the Chapman-Enskog method. The heat
and momentum fluxes are calculated to Navier-Stokes order, and the associated
transport coefficients are explicitly determined as functions of the
coefficient of normal restitution and the velocity parameter involved in the
definition of the model. Also an Euler transport term contributing to the
energy transport equation is considered. This term arises from the gradient
expansion of the rate of change of the temperature due to the inelasticity of
collisions, and vanishes for elastic systems. The hydrodynamic equations are
particularized for the relevant case of a system in the homogeneous steady
state. The relationship with previous works is analyzed
Memory effects in the relaxation of a confined granular gas
The accuracy of a model to describe the horizontal dynamics of a confined
quasi-two-dimensional system of inelastic hard spheres is discussed by
comparing its predictions for the relaxation of the temperature in an
homogenous system with molecular dynamics simulation results for the original
system. A reasonably good agreement is found. Next, the model is used to
investigate the peculiarities of the nonlinear evolution of the temperature
when the parameter controlling the energy injection is instantaneously changed
while the system was relaxing. This can be considered as a non-equilibrium
generalization of the Kovacs effect. It is shown that, in the low density
limit, the effect can be accurately described by using a simple kinetic theory
based on the first Sonine approximation for the one-particle distribution
function. Some possible experimental implications are indicated
Preliminary results on SiO v=3 J=1-0 maser emission from AGB stars
We present the results of SiO maser observations at 43GHz toward two AGB
stars using the VLBA. Our preliminary results on the relative positions of the
different J=1-0 SiO masers (v=1,2 and 3) indicate that the current ideas on SiO
maser pumping could be wrong at some fundamental level. A deep revision of the
SiO pumping models could be necessary.Comment: poster, 2 pages, 2 figures, Proc. IAU Symp. 287 "Cosmic Masers: from
OH to H0", R.S. Booth, E.M.L. Humphreys and W.H.T. Vlemmings, ed
SiO masers from AGB stars in the vibrationally excited v=1,v=2, and v=3 states
The v=1 and v=2 J=1-0 (43 GHz), and v=1 J=2-1 (86 GHz) SiO masers are intense
in AGB stars and have been mapped using VLBI showing ring-like distributions.
Those of the v=1, v=2 J=1-0 masers are similar, but the spots are rarely
coincident, while the v=1 J=2-1 maser arises from a well separated region
farther out. These relative locations can be explained by models tools that
include the overlap of two IR lines of SiO and H2O. The v=3 J=1-0 line is not
directly affected by any line overlap and its spot structure and position,
relative to the other lines, is a good test to the standard pumping models. We
present single-dish and simultaneous VLBI observations of the v=1, v=2, and v=3
J=1-0 maser transitions of 28SiO in several AGB stars. The spatial distribution
of the SiO maser emission in the v=3 J=1-0 transition from AGB stars is
systematically composed of a series of spots that occupy a ring-like structure.
The overall ring structure is extremely similar to that found in the other 43
GHz transitions and is very different from the structure of the v=1 J=2-1
maser. The positions of the individual spots of the different 43 GHz lines are,
however, very rarely coincident, which in general is separated by about 0.3 AU
(between 1 and 5 mas). These results are very difficult to reconcile with
standard pumping models, which predict that the masers of rotational
transitions within a given vibrational state require very similar excitation
conditions, while the transitions of different vibrational states should appear
in different positions. However, models including line overlap tend to predict
v=1, v=2, v=3 J=1-0 population inversion to occur under very similar
conditions, while the requirements for v=1 J=2-1 appear clearly different, and
are compatible with the observational results.Comment: 9 pages, 4 figures accepted by A&
- …