255 research outputs found
Towards scaling laws for DDT in obstructed channels
In a coal mine, natural gas can leak through walls and accumulate in enclosed regions that are no longer being mined or ventilated. If there is an accidental spark in the region containing this gas, it can ignite a flame that may transition to detonation (DDT). An important problem is to assess if, when, and where DDT can occur, and thus provide information needed to design strong enough barriers to protect active mining areas. We describe results of numerical simulations of flame acceleration and DDT in obstacle-laden channels to find a scaling law for DDT, the distance the deflagration travels before a detonation forms as a function of channel size d. The scaling law is derived for a stoichiometric natural-gas air mixture in a channel with blockage ratio 0.3 and channel sizes ranging from 0.17 to 3.0 m
Synthetic Spectra for Type Ia Supernovae at Early Epochs
We present the current status of our construction of synthetic spectra for
type Ia supernovae. These properly take into account the effects of NLTE and an
adequate representation of line blocking and blanketing. The models are based
on a sophisticated atomic database. We show that the synthetic spectrum
reproduces the observed spectrum of 'normal' SN-Ia near maximum light from the
UV to the near-IR. However, further improvements are necessary before truly
quantitative analyses of observed SN-Ia spectra can be performed. In
particular, the inner boundary condition has to be fundamentally modified. This
is due to the dominance of electron scattering over true absorption processes
coupled with the flat density structure in these objectsComment: To appear in "Proceedings of the IAU Colloquium 192 - Supernovae (10
Years of SN1993J)", eds. J.M. Marcaide and K.W. Weile
The Interaction of High-Speed Turbulence with Flames: Global Properties and Internal Flame Structure
We study the dynamics and properties of a turbulent flame, formed in the
presence of subsonic, high-speed, homogeneous, isotropic Kolmogorov-type
turbulence in an unconfined system. Direct numerical simulations are performed
with Athena-RFX, a massively parallel, fully compressible, high-order,
dimensionally unsplit, reactive-flow code. A simplified reaction-diffusion
model represents a stoichiometric H2-air mixture. The system being modeled
represents turbulent combustion with the Damkohler number Da = 0.05 and with
the turbulent velocity at the energy injection scale 30 times larger than the
laminar flame speed. The simulations show that flame interaction with
high-speed turbulence forms a steadily propagating turbulent flame with a flame
brush width approximately twice the energy injection scale and a speed four
times the laminar flame speed. A method for reconstructing the internal flame
structure is described and used to show that the turbulent flame consists of
tightly folded flamelets. The reaction zone structure of these is virtually
identical to that of the planar laminar flame, while the preheat zone is
broadened by approximately a factor of two. Consequently, the system evolution
represents turbulent combustion in the thin-reaction zone regime. The turbulent
cascade fails to penetrate the internal flame structure, and thus the action of
small-scale turbulence is suppressed throughout most of the flame. Finally, our
results suggest that for stoichiometric H2-air mixtures, any substantial flame
broadening by the action of turbulence cannot be expected in all subsonic
regimes.Comment: 30 pages, 9 figures; published in Combustion and Flam
Depiction of Characters’ Social Background through their Sociolect in Charles Dickens’ Novel Hard Times and its Russian Translation: Politeness Formulae
The present research is devoted to the study of politeness formulae conveying the characters’ social background and origin in Charles Dickens’ novel “Hard Times” and in its Russian translation. The goal of the research is to determine the features that characterise the upper-class, middle-class, and lower-class sociolects in the original novel and to compare them to the features found in the translation
Explosion models for thermonuclear supernovae resulting from different ignition conditions
We have explored in three dimensions the fate of a massive white dwarf as a
function of different initial locations of carbon ignition, with the aid of a
SPH code. The calculated models cover a variety of possibilities ranging from
the simultaneous ignition of the central volume of the star to the off-center
ignition in multiple scattered spots. In the former case, there are discussed
the possibility of a transition to a detonation when the mean density of the
nuclear flame decreases below 2x10**7 g cm**-3, and its consequences. In the
last case, the dependence of the results on the number of initial igniting
spots and the chance of some of these models to evolve to the pulsating delayed
detonation scenario are also outlined.Comment: 5 pages, 1 figure, proceedings of IAU Colloquium 192, 'Supernovae (10
years of SN1993J)', 22-26 April 2003, Valencia, Spai
C+O detonations in thermonuclear supernovae: Interaction with previously burned material
In the context of explosion models for Type Ia Supernovae, we present one-
and two-dimensional simulations of fully resolved detonation fronts in
degenerate C+O White Dwarf matter including clumps of previously burned
material. The ability of detonations to survive the passage through sheets of
nuclear ashes is tested as a function of the width and composition of the ash
region. We show that detonation fronts are quenched by microscopically thin
obstacles with little sensitivity to the exact ash composition. Front-tracking
models for detonations in macroscopic explosion simulations need to include
this effect in order to predict the amount of unburned material in delayed
detonation scenarios.Comment: 6 pages, 9 figures, uses isotope.sty, accepted for publication in A&
Three-dimensional modeling of Type Ia supernovae - The power of late time spectra
Late time synthetic spectra of Type Ia supernovae, based on three-dimensional
deflagration models, are presented. We mainly focus on one
model,"c3_3d_256_10s", for which the hydrodynamics (Roepke 2005) and
nucleosynthesis (Travaglio et al. 2004) was calculated up to the homologous
phase of the explosion. Other models with different ignition conditions and
different resolution are also briefly discussed. The synthetic spectra are
compared to observed late time spectra. We find that while the model spectra
after 300 to 500 days show a good agreement with the observed Fe II-III
features, they also show too strong O I and C I lines compared to the observed
late time spectra. The oxygen and carbon emission originates from the
low-velocity unburned material in the central regions of these models. To get
agreement between the models and observations we find that only a small mass of
unburned material may be left in the center after the explosion. This may be a
problem for pure deflagration models, although improved initial conditions, as
well as higher resolution decrease the discrepancy. The relative intensity from
the different ionization stages of iron is sensitive to the density of the
emitting iron-rich material. We find that clumping, with the presence of low
density regions, is needed to reproduce the observed iron emission, especially
in the range between 4000 and 6000 AA. Both temperature and ionization depend
sensitively on density, abundances and radioactive content. This work therefore
illustrates the importance of including the inhomogeneous nature of realistic
three-dimensional explosion models. We briefly discuss the implications of the
spectral modeling for the nature of the explosion.Comment: 20 pages, 9 figures, resolution of Fig 1 is reduced to meet astro-ph
file size restriction, submitted to A&
Double-detonation supernovae of sub-Chandrasekhar mass white dwarfs
In the "double-detonation sub-Chandrasekhar" model for type Ia supernovae, a
carbon-oxygen (C + O) white dwarf accumulates sufficient amounts of helium such
that a detonation ignites in that layer before the Chandrasekhar mass is
reached. This detonation is thought to trigger a secondary detonation in the C
+ O core. By means of one- and two-dimensional hydrodynamic simulations, we
investigate the robustness of this explosion mechanism for generic 1-M_sun
models and analyze its observable predictions. Also a resolution dependence in
numerical simulations is analyzed. The propagation of thermonuclear detonation
fronts, both in helium and in the carbon-oxygen mixture, is computed by means
of both a level-set function and a simplified description for nuclear
reactions. The decision whether a secondary detonation is triggered in the
white dwarf's core or not is made based on criteria given in the literature. In
a parameter study involving different initial flame geometries for He-shell
masses of 0.2 and 0.1 M_sun, we find that a secondary detonation ignition is a
very robust process. Converging shock waves originating from the detonation in
the He shell generate the conditions for a detonation near the center of the
white dwarf in most of the cases considered. Finally, we follow the complete
evolution of three selected models with 0.2 M_sun of He through the
C/O-detonation phase and obtain nickel-masses of about 0.40 to 0.45 M_sun.
Although we have not done a complete scan of the possible parameter space, our
results show that sub-Chandrasekhar models are not good candidates for normal
or sub-luminous type Ia supernovae. The chemical composition of the ejecta
features significant amounts of nickel in the outer layers at high expansion
velocities, which is inconsistent with near-maximum spectra. (abbreviated)Comment: 11 pages, 10 figures, PDFLaTeX, accepted for publication in A&
Stylistics of Scientific Discourse in Charles Darwin’s “The Origin of Species” and Its Russian Translations
The present research is devoted to the study of diachronic development of the Russian academic style based on three translations of Charles Darwin’s “The Origin of the Species” into Russian conducted in different time periods. The original text and the translations are analysed with the goal to identify the characteristic features of the style at different points in time and to determine its developmental tendencies
On the Stability of Thermonuclear Burning Fronts in Type Ia Supernovae
The propagation of cellularly stabilized thermonuclear flames is investigated
by means of numerical simulations. In Type Ia supernova explosions the
corresponding burning regime establishes at scales below the Gibson length. The
cellular flame stabilization - which is a result of an interplay between the
Landau-Darrieus instability and a nonlinear stabilization mechanism - is
studied for the case of propagation into quiescent fuel as well as interaction
with vortical fuel flows. Our simulations indicate that in thermonuclear
supernova explosions stable cellular flames develop around the Gibson scale and
that deflagration-to-detonation transition is unlikely to be triggered from
flame evolution effects here.Comment: 6 pages, 2 figures, to appear in the proceedings of the IAU
Colloquium 192, "Supernovae (10 years of SN1993J)", 22-26 April 2003,
Valencia, Spain, Eds. J.M. Marcaide and K.W. Weiler, Springer Verla
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