1,156 research outputs found
Unified derivation of phase-field models for alloy solidification from a grand-potential functional
In the literature, two quite different phase-field formulations for the
problem of alloy solidification can be found. In the first, the material in the
diffuse interfaces is assumed to be in an intermediate state between solid and
liquid, with a unique local composition. In the second, the interface is seen
as a mixture of two phases that each retain their macroscopic properties, and a
separate concentration field for each phase is introduced. It is shown here
that both types of models can be obtained by the standard variational procedure
if a grand-potential functional is used as a starting point instead of a
free-energy functional. The dynamical variable is then the chemical potential
instead of the composition. In this framework, a complete analogy with
phase-field models for the solidification of a pure substance can be
established. This analogy is then exploited to formulate quantitative
phase-field models for alloys with arbitrary phase diagrams. The precision of
the method is illustrated by numerical simulations with varying interface
thickness.Comment: 36 pages, 1 figur
Spin Asymmetry and Gerasimov-Drell-Hearn Sum Rule for the Deuteron
An explicit evaluation of the spin asymmetry of the deuteron and the
associated GDH sum rule is presented which includes photodisintegration, single
and double pion and eta production as well. Photodisintegration is treated with
a realistic retarded potential and a corresponding meson exchange current. For
single pion and eta production the elementary operator from MAID is employed
whereas for double pion production an effective Lagrangean approach is used. A
large cancellation between the disintegration and the meson production channels
yields for the explicit GDH integral a value of 27.31 b to be compared to
the sum rule value 0.65 b.Comment: 4 pages, 5 figures, revtex
Models of OH Maser Variations in U Her
Arecibo spectra of the mainline OH maser emission from U Her over more than a
decade show variations of the OH emission over these time scales. These
observations are combined with high spatial resolution VLBA maps to investigate
the causes of the variations in the velocities of the maser components. Global
properties of the dust shell, such as accelerations, variations in the pump and
shell-wide magnetic field changes are examined as possibilities, and
eliminated. A possible solution to the problem involving plasma turbulence and
the local magnetic field is introduced, and the relevant time scales of the
turbulence are calculated. The turbulent velocity field yields time scales of
the turbulence are calculated. The turbulent velocity field yields time scales
that are too long (of order centuries), while the turbulent magnetic field
produces variations on appropriate time scales of a few years. A line-of-sight
model of the turbulence is developed and investigated. The complete exploration
of this solution requires extensive theoretical and observational work.
Possible avenues of investigation of the plasma turbulence model are presented.Comment: 23 pages, 17 figures, ApJ: accepted Sept, 199
Non-locality and Medium Effects in the Exclusive Photoproduction of Eta Mesons on Nuclei
A relativistic model for the quasifree exclusive photoproduction of
mesons on nuclei is extended to include both non-local and medium effects. The
reaction is assumed to proceed via the dominant contribution of the
S(1535) resonance. The complicated integrals resulting from the
non-locality are simplified using a modified version of a method given by
Cooper and Maxwell. The non-locality effects are found to affect the magnitude
of the cross section. Some possibilities reflecting the effects of the medium
on the propagation and properties of the intermediate S resonance are
studied. The effects of allowing the S to interact with the medium via
mean field scalar and vector potentials are considered. Both broadening of
width and reduction in mass of the resonance lead to a suppression of the
calculated cross sections.Comment: 19 pages, 7 figure
CO in OH/IR stars close to the Galactic centre
Aims: A pilot project has been carried out to measure circumstellar CO
emission from three OH/IR stars close to the Galactic centre. The intention was
to find out whether it would be possible to conduct a large-scale survey for
mass-loss rates using, for example, the Atacama Large Millimeter Array (ALMA).
Such a survey would increase our understanding of the evolution of the Galactic
bulge.
Methods: Two millimetre-wave instruments were used: the Nobeyama Millimeter
Array at 115 GHz and the Submillimeter Array at 230 GHz. An interferometer is
necessary as a `spatial filter' in this region of space because of the
confusion with interstellar CO emission.
Results: Towards two of the stars, CO emission was detected with positions
and radial velocities coinciding within the statistical errors with the
corresponding data of the associated OH sources. However, for one of the stars
the line profile is not what one expects for an unresolved expanding
circumstellar envelope. We believe that this CO envelope is partially resolved
and that this star therefore is a foreground star not belonging to the bulge.
Conclusions: The results of the observations have shown that it is possible
to detect line profiles of circumstellar CO from late-type stars both within
and in the direction of the Galactic bulge. ALMA will be able to detect CO
emission in short integrations with sensitivity sufficient to estimate
mass-loss rates from a large number of such stars.Comment: 5 pages, 3 figure
Incoherent Photoproduction of -mesons from the Deuteron near Threshold
Incoherent photoproduction of the -meson on the deuteron is studied for
photon energies from threshold to 800 MeV. The dominant contribution, the
N-N amplitude, is described within an isobar model. The final
state interaction derived from the CD-Bonn potential is included and found to
be important for the description of the production cross section close to
threshold. Possible effects from the final state interaction are
discussed.Comment: 11 pages, revtex, including 6 figure
Nuclear dependence of the coherent eta photoproduction reaction in a relativistic approach
We study the nuclear (or A) dependence of the coherent eta photoproduction
reaction in a relativistic impulse approximation approach. We use a standard
relativistic parameterization of the elementary amplitude, based on a set of
four Lorentz- and gauge-invariant amplitudes, to calculate the coherent
production cross section from He-4, C-12, and Ca-40. In contrast to
nonrelativistic treatments, our approach maintains the full relativistic
structure of the process. The nuclear structure affects the process through the
ground-state tensor density. This density is sensitive to relativistic effects
and depends on A in a different manner than the vector density used in
nonrelativistic approaches. This peculiar dependence results in He-4 having a
cross section significantly smaller than that of C-12---in contrast to existent
nonrelativistic calculations. Distortion effects are incorporated through an
eta-nucleus optical potential that is computed in a simple ``t rho''
approximation.Comment: 4 pages, 3 postscript figures. The replace is due to a misspelling in
the Authors' lis
Influence of image segmentation on one-dimensional fluid dynamics predictions in the mouse pulmonary arteries
Computational fluid dynamics (CFD) models are emerging as tools for assisting
in diagnostic assessment of cardiovascular disease. Recent advances in image
segmentation has made subject-specific modelling of the cardiovascular system a
feasible task, which is particularly important in the case of pulmonary
hypertension (PH), which requires a combination of invasive and non-invasive
procedures for diagnosis. Uncertainty in image segmentation can easily
propagate to CFD model predictions, making uncertainty quantification crucial
for subject-specific models. This study quantifies the variability of
one-dimensional (1D) CFD predictions by propagating the uncertainty of network
geometry and connectivity to blood pressure and flow predictions. We analyse
multiple segmentations of an image of an excised mouse lung using different
pre-segmentation parameters. A custom algorithm extracts vessel length, vessel
radii, and network connectivity for each segmented pulmonary network. We
quantify uncertainty in geometric features by constructing probability
densities for vessel radius and length, and then sample from these
distributions and propagate uncertainties of haemodynamic predictions using a
1D CFD model. Results show that variation in network connectivity is a larger
contributor to haemodynamic uncertainty than vessel radius and length
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