388 research outputs found
Turbulent Mixing in Stars: Theoretical Hurdles
A program is outlined, and first results described, in which fully
three-dimensional, time dependent simulations of hydrodynamic turbulence are
used as a basis for theoretical investigation of the physics of turbulence in
stars. The inadequacy of the treatment of turbulent convection as a diffusive
process is discussed. A generalization to rotation and magnetohydrodynamics is
indicated, as are connection to simulations of 3D stellar atmospheres.Comment: 5 pages, 1 figure, IAU Symposium 265, 200
Local helioseismology and correlation tracking analysis of surface structures in realistic simulations of solar convection
We apply time-distance helioseismology, local correlation tracking and
Fourier spatial-temporal filtering methods to realistic supergranule scale
simulations of solar convection and compare the results with high-resolution
observations from the SOHO Michelson Doppler Imager (MDI). Our objective is to
investigate the surface and sub-surface convective structures and test
helioseismic measurements. The size and grid of the computational domain are
sufficient to resolve various convective scales from granulation to
supergranulation. The spatial velocity spectrum is approximately a power law
for scales larger than granules, with a continuous decrease in velocity
amplitude with increasing size. Aside from granulation no special scales exist,
although a small enhancement in power at supergranulation scales can be seen.
We calculate the time-distance diagram for f- and p-modes and show that it is
consistent with the SOHO/MDI observations. From the simulation data we
calculate travel time maps for surface gravity waves (f-mode). We also apply
correlation tracking to the simulated vertical velocity in the photosphere to
calculate the corresponding horizontal flows. We compare both of these to the
actual large-scale (filtered) simulation velocities. All three methods reveal
similar large scale convective patterns and provide an initial test of
time-distance methods.Comment: 15 pages, 9 figures (.ps format); accepted to ApJ (tentatively
scheduled to appear in March 10, 2007 n2 issue); included files ms.bbl,
aabib.bst, aabib.sty, aastex.cl
Proyecto técnico de nueva Instalación de una fábrica de yogur en el TM de Requena
[ES] Este Trabajo Final de Grado tiene como objetivo la inscripción en el Registro de
Establecimientos Agroalimentarios (en adelante R.E.A.) de una nueva instalación de una
fábrica de yogur. Se trata de un proyecto técnico en el que se detalla el objeto del proyecto, la
legislación aplicable, el titular de la industria, la distribución de las superficies del terreno en el
que se localiza la instalación, un programa productivo de materias primas, productos
obtenidos, y un cuadro de capacidades anuales, entre otros. Además, se refleja en dicho
proyecto la maquinaria que tiene la fábrica, el proceso industrial del producto (con el
consiguiente diagrama), y un estudio económico-financiero de la actividad y los ingresos y
gastos. El producto del proyecto es yogur, tanto líquido como firme. De la misma forma, se
desarrollan dos instalaciones: la eléctrica y la de refrigeración.
La inscripción de una instalación o industria en el R.E.A. es obligatoria para los titulares de
empresas con actividades y establecimientos incluidos en el ámbito de aplicación del
Reglamento del R.E.A. Dicha inscripción es importante al tratarse de un requisito indispensable
para que las industrias agroalimentarias puedan acogerse a los auxilios económicos de todo
tipo que se arbitren mediante programas de ayudas regulados por convocatorias de la
Generalitat, y financiados por fondos comunitarios, nacionales o de ámbito autonómico
(Decreto 97/2005, de 20 mayo).[EN] The aim of this project is the registration in the Registro de Establecimientos Agroalimentarios
(hereinafter R.E.A.) of a new facility of a yogurt factory. This is a technical project in which the
aim of the project is detailed, the applicable legislation, the owner of the industry, the
distribution of the areas of land on which the facility is located, a production program of raw
materials, obtained products, and a table of annual capacity, among others. In addition, the
machinery that the factory has is reflected, the manufacturing process of the product (with the
resulting diagram), and an economic and financial study of the activity and income and
expenses. The product of the project is yogurt, firm and drinkable. Also, two installations are
developed: electrical and cold installations.
The registration of a facility or industry in the R.E.A. is mandatory for owners of companies with
activities and facilities included in the scope of the R.E.A. Regulation. This registration is
important as this is a prerequisite for food processing industries to be eligible for economic aids
of all kinds that are arbitrated by aid programs regulated by official announcements of the
Generalitat, and financed by EU, national or regional funds (Decree 97/2005 May 20th).Nordlund Sierra, DP. (2016). Proyecto técnico de nueva instalación de una fábrica de yogur en el TM de Requena. http://hdl.handle.net/10251/76539.TFG
Infarct quantification using 3D inversion recovery and 2D phase sensitive inversion recovery, validation in patients and ex vivo
Turbulent Cells in Stars: I. Fluctuations in Kinetic Energy and Luminosity
Three-dimensional (3D) hydrodynamic simulations of shell oxygen burning
(Meakin and Arnett, 2007b) exhibit bursty, recurrent fluctuations in turbulent
kinetic energy. These are shown to be due to a general instability of the
convective cell, requiring only a localized source of heating or cooling. Such
fluctuations are shown to be suppressed in simulations of stellar evolution
which use mixing-length theory (MLT).
Quantitatively similar behavior occurs in the model of a convective roll
(cell) of Lorenz (1963), which is known to have a strange attractor that gives
rise to chaotic fluctuations in time of velocity and, as we show, luminosity.
Study of simulations suggests that the behavior of a Lorenz convective roll may
resemble that of a cell in convective flow. We examine some implications of
this simplest approximation, and suggest paths for improvement.
Using the Lorenz model as representative of a convective cell, a
multiple-cell model of a convective layer gives total luminosity fluctuations
which are suggestive of irregular variables (red giants and supergiants
(Schwarzschild 1975)), and of the long secondary period feature in semi-regular
AGB variables (Stothers 2010, Wood, Olivier and Kawaler 2004). This
"tau-mechanism" is a new source for stellar variability, which is inherently
non-linear (unseen in linear stability analysis), and one closely related to
intermittency in turbulence. It was already implicit in the 3D global
simulations of Woodward, Porter and Jacobs (2003). This fluctuating behavior is
seen in extended 2D simulations of CNeOSi burning shells (Arnett and Meakin
2011b), and may cause instability which leads to eruptions in progenitors of
core collapse supernovae PRIOR to collapse.Comment: 30 pages, 13 figure
Towards Realistic Progenitors of Core-Collapse Supernovae
Two-dimensional (2D) hydrodynamical simulations of progenitor evolution of a
23 solar mass star, close to core collapse (about 1 hour, in 1D), with
simultaneously active C, Ne, O, and Si burning shells, are presented and
contrasted to existing 1D models (which are forced to be quasi-static).
Pronounced asymmetries, and strong dynamical interactions between shells are
seen in 2D. Although instigated by turbulence, the dynamic behavior proceeds to
sufficiently large amplitudes that it couples to the nuclear burning. Dramatic
growth of low order modes is seen, as well as large deviations from spherical
symmetry in the burning shells. The vigorous dynamics is more violent than that
seen in earlier burning stages in the 3D simulations of a single cell in the
oxygen burning shell, or in 2D simulations not including an active Si shell.
Linear perturbative analysis does not capture the chaotic behavior of
turbulence (e.g., strange attractors such as that discovered by Lorenz), and
therefore badly underestimates the vigor of the instability. The limitations of
1D and 2D models are discussed in detail. The 2D models, although flawed
geometrically, represent a more realistic treatment of the relevant dynamics
than existing 1D models, and present a dramatically different view of the
stages of evolution prior to collapse. Implications for interpretation of
SN1987A, abundances in young supernova remnants, pre-collapse outbursts,
progenitor structure, neutron star kicks, and fallback are outlined. While 2D
simulations provide new qualitative insight, fully 3D simulations are needed
for a quantitative understanding of this stage of stellar evolution. The
necessary properties of such simulations are delineated.Comment: 26 pages, 1 table, 4 figure
Turbulent Convection in Stellar Interiors. II. The Velocity Field
We analyze stellar convection with the aid of 3D hydrodynamic simulations,
introducing the turbulent cascade into our theoretical analysis. We devise
closures of the Reynolds-decomposed mean field equations by simple physical
modeling of the simulations (we relate temperature and density fluctuations via
coefficients); the procedure (CABS, Convection Algorithms Based on Simulations)
is terrestrially testable and is amenable to systematic improvement. We develop
a turbulent kinetic energy equation which contains both nonlocal and time
dependent terms, and is appropriate if the convective transit time is shorter
than the evolutionary time scale. The interpretation of mixing-length theory
(MLT) as generally used in astrophysics is incorrect; MLT forces the mixing
length to be an imposed constant. Direct tests show that the damping associated
with the flow is that suggested by Kolmogorov. The eddy size is approximately
the depth of the convection zone, and this dissipation length corresponds to
the "mixing length". New terms involving local heating by turbulent dissipation
should appear in the stellar evolution equations. The enthalpy flux
("convective luminosity") is directly connected to the buoyant acceleration,
and hence the velocity scale. MLT tends to systematically underestimate this
velocity scale. Quantitative comparison with a variety of 3D simulations
reveals a previously recognized consistency. Examples of application to stellar
evolution will be presented in subsequent papers in this series.Comment: 47 pages, 7 figures, accepted by Ap
Pattern formation on ion-irradiated Si surface at energies where sputtering is negligible
The effect of low energy irradiation, where the sputtering is imperceptible, has not been deeply studied in the pattern formation. In this work, we want to address this question by analyzing the nanoscale topography formation on a Si surface, which is irradiated at room temperature by Arthorn ions near the displacement threshold energy, for incidence angles ranging from 0 degrees to 85 degrees. The transition from the smooth to ripple patterned surface, i.e., the stability/instability bifurcation angle is observed at 55 degrees, whereas the ripples with their wave-vector is parallel to the ion beam projection in the angular window of 60 degrees-70 degrees, and with 90 degrees rotation with respect to the ion beam projection at the grazing angles of incidence. A similar irradiation setup has been simulated by means of molecular dynamics, which made it possible, first, to quantify the effect of the irradiation in terms of erosion and redistribution using sequential irradiation and, second, to evaluate the ripple wavelength using the crater function formalism. The ripple formation results can be solely attributed to the mass redistribution based mechanism, as erosion due to ion sputtering near or above the threshold energy is practically negligible. Published by AIP Publishing.Peer reviewe
Improving atomic displacement and replacement calculations with physically realistic damage models
Atomic collision processes are fundamental to numerous advanced materials technologies such as electron microscopy, semiconductor processing and nuclear power generation. Extensive experimental and computer simulation studies over the past several decades provide the physical basis for understanding the atomic-scale processes occurring during primary displacement events. The current international standard for quantifying this energetic particle damage, the Norgett-Robinson-Torrens displacements per atom (NRT-dpa) model, has nowadays several well-known limitations. In particular, the number of radiation defects produced in energetic cascades in metals is only similar to 1/3 the NRT-dpa prediction, while the number of atoms involved in atomic mixing is about a factor of 30 larger than the dpa value. Here we propose two new complementary displacement production estimators (athermal recombination corrected dpa, arc-dpa) and atomic mixing (replacements per atom, rpa) functions that extend the NRT-dpa by providing more physically realistic descriptions of primary defect creation in materials and may become additional standard measures for radiation damage quantification.Peer reviewe
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