827 research outputs found
White dwarf envelopes: further results of a non-local model of convection
We present results of a fully non-local model of convection for white dwarf
envelopes. We show that this model is able to reproduce the results of
numerical simulations for convective efficiencies ranging from very inefficient
to moderately efficient; this agreement is made more impressive given that no
closure parameters have been adjusted in going from the previously reported
case of A-stars to the present case of white dwarfs; for comparison, in order
to match the peak convective flux found in numerical simulations for both the
white dwarf envelopes discussed in this paper and the A-star envelopes
discussed in our previous work requires changing the mixing length parameter of
commonly used local models by a factor of 4. We also examine in detail the
overshooting at the base of the convection zone, both in terms of the
convective flux and in terms of the velocity field: we find that the flux
overshoots by approximately 1.25 H_P and the velocity by approximately 2.5 H_P.
Due to the large amount of overshooting found at the base of the convection
zone the new model predicts the mixed region of white dwarf envelopes to
contain at least 10 times more mass than local mixing length theory (MLT)
models having similar photospheric temperature structures. This result is
consistent with the upper limit given by numerical simulations which predict an
even larger amount of mass to be mixed by convective overshooting. Finally, we
attempt to parametrise some of our results in terms of local MLT-based models,
insofar as is possible given the limitations of MLTComment: Accepted for publication in MNRAS; 11 pages, 5 figures, 3 table
A-star envelopes: a test of local and non-local models of convection
We present results of a fully non-local, compressible model of convection for
A-star envelopes. This model quite naturally reproduces a variety of results
from observations and numerical simulations which local models based on a
mixing length do not. Our principal results, which are for models with Teff
between 7200 K and 8500 K, are the following: First, the photospheric
velocities and filling factors are in qualitative agreement with those derived
from observations of line profiles of A-type stars. Second, the He II and H I
convection zones are separated in terms of convective flux and thermal
interaction, but joined in terms of the convective velocity field, in agreement
with numerical simulations. In addition, we attempt to quantify the amount of
overshooting in our models at the base of the He II convection zone.Comment: 5 pages with 4 figures (1a, 1b, 2 and 3), MNRAS (letter), in prin
Effects of resolution and helium abundance in A star surface convection simulations
We present results from 2D radiation-hydrodynamical simulations of fully
compressible convection for the surface layers of A-type stars with the ANTARES
code. Spectroscopic indicators for photospheric convective velocity fields show
a maximum of velocities near Teff ~8000 K. In that range the largest values are
measured for the subgroup of Am stars. Thus far, no prognostic model, neither
theoretical nor numerical, is able to exactly reproduce the line profiles of
sharp line A and Am stars in that temperature range. In general, the helium
abundance of A stars is not known from observations. Hence, we have considered
two extreme cases for our simulations: a solar helium abundance as an upper
limit and zero helium abundance as a lower limit. The simulation for the helium
free case is found to differ from the case with solar helium abundance by
larger velocities, larger flow structures, and by a sign reversal of the flux
of kinetic energy inside the hydrogen ionisation zone. Both simulations show
extended shock fronts emerging from the optical surface, as well as mixing far
below the region of partial ionisation of hydrogen, and vertical oscillations
emerging after initial perturbations have been damped. We discuss problems
related to the rapid radiative cooling at the surface of A-type stars such as
resolution and efficient relaxation. The present work is considered as a step
towards a systematic study of convection in A- to F-type stars, encouraged by
the new data becoming available for these objects from both asteroseismological
missions and from high resolution spectroscopy.Comment: submitted to CoAst, preprint version with 26 pages (29 pages in CoAst
layout), 8 figures, 1 tabl
Turbulent convection: comparing the moment equations to numerical simulations
The non-local hydrodynamic moment equations for compressible convection are
compared to numerical simulations. Convective and radiative flux typically
deviate less than 20% from the 3D simulations, while mean thermodynamic
quantities are accurate to at least 2% for the cases we have investigated. The
moment equations are solved in minutes rather than days on standard
workstations. We conclude that this convection model has the potential to
considerably improve the modelling of convection zones in stellar envelopes and
cores, in particular of A and F stars.Comment: 10 pages (6 pages of text including figure captions + 4 figures),
Latex 2e with AAS Latex 5.0 macros, accepted for publication in ApJ
Angle dependence of Andreev scattering at semiconductor-superconductor interfaces
We study the angle dependence of the Andreev scattering at a
semiconductor-superconductor interface, generalizing the one-dimensional theory
of Blonder, Tinkham and Klapwijk. An increase of the momentum parallel to the
interface leads to suppression of the probability of Andreev reflection and
increase of the probability of normal reflection. We show that in the presence
of a Fermi velocity mismatch between the semiconductor and the superconductor
the angles of incidence and transmission are related according to the
well-known Snell's law in optics. As a consequence there is a critical angle of
incidence above which only normal reflection exists. For two and
three-dimensional interfaces a lower excess current compared to ballistic
transport with perpendicular incidence is found. Thus, the one-dimensional BTK
model overestimates the barrier strength for two and three-dimensional
interfaces.Comment: 8 pages including 3 figures (revised, 6 references added
Ispitivanja statičke kompresije i rezonantne vibracije celularnih materijala dobivenih gravitacijskim sinterovanjem šupljih brončanih kugli
The cylindrical and rod-shaped specimens were prepared by gravity sintering from Cu-Sn hollow spheres. On these samples, both static compression tests and measurements of resonance frequencies were performed. The compressive stress-strain curves revealed the features characteristic for a closed-cell ductile cellular solid. The removal of in general open porosity among loosely packed closed metallic hollow spheres was recognized as the principal mode of plastic deformation. The approximative effective moduli of elasticity were determined for cellular materials under consideration by means of the measurements of resonance frequencies on rod-shaped specimens.Cilindrični i šipkasti uzorci su pripremljeni gravitacionim sinterovanjem šupljih Cu-Sn kugli. Na tim uzorcima su provedena ispitivanja statičke kompresije i mjerenja rezonantnih frekvencija. Krivulja naprezanje-rastezanje nam pokazuje svojstva karakteristična za plastične celularne krute materijale sa zatvorenim ćelijama. Uklanjanje uglavnom otvorene povezanosti među labavo povezanim šupljim metalnim kuglama prepoznajemo kao glavni način plastične deformacije. Za celularne materijale koji se razmatraju određeni su približno učinkoviti moduli elastičnosti mjerenjem frekvencija rezonancije na šipkasto oblikovanim uzorcima
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Cost-effectiveness of community-based screening and treatment of moderate acute malnutrition in Mali.
IntroductionModerate acute malnutrition (MAM) causes substantial child morbidity and mortality, accounting for 4.4% of deaths and 6.0% of disability-adjusted life years (DALY) lost among children under 5 each year. There is growing consensus on the need to provide appropriate treatment of MAM, both to reduce associated morbidity and mortality and to halt its progression to severe acute malnutrition. We estimated health outcomes, costs and cost-effectiveness of four dietary supplements for MAM treatment in children 6-35 months of age in Mali.MethodsWe conducted a cluster-randomised MAM treatment trial to describe nutritional outcomes of four dietary supplements for the management of MAM: ready-to-use supplementary foods (RUSF; PlumpySup); a specially formulated corn-soy blend (CSB) containing dehulled soybean flour, maize flour, dried skimmed milk, soy oil and a micronutrient pre-mix (CSB++; Super Cereal Plus); Misola, a locally produced, micronutrient-fortified, cereal-legume blend (MI); and locally milled flour (LMF), a mixture of millet, beans, oil and sugar, with a separate micronutrient powder. We used a decision tree model to estimate long-term outcomes and calculated incremental cost-effectiveness ratios (ICERs) comparing the health and economic outcomes of each strategy.ResultsCompared to no MAM treatment, MAM treatment with RUSF, CSB++, MI and LMF reduced the risk of death by 15.4%, 12.7%, 11.9% and 10.3%, respectively. The ICER was US347 per DALY averted for RUSF compared with no MAM treatment.ConclusionMAM treatment with RUSF is cost-effective across a wide range of willingness-to-pay thresholds.Trial registrationNCT01015950
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