759 research outputs found
3D simulations of shear instabilities in magnetized flows
We present results of three-dimensional (3D) simulations of the
magnetohydrodynamic Kelvin-Helmholtz instability in a stratified shear layer.
The magnetic field is taken to be uniform and parallel to the shear flow. We
describe the evolution of the fluid flow and the magnetic field for a range of
initial conditions. In particular, we investigate how the mixing rate of the
fluid depends on the Richardson number and the magnetic field strength. It was
found that the magnetic field can enhance as well as suppress mixing. Moreover,
we have performed two-dimensional (2D) simulations and discuss some interesting
differences between the 2D and 3D results.Comment: submitted to MNRAS, figures in colour and higher quality at
http://www.mpa-garching.mpg.de/~maria/greenreports/mpa00/reports_00.htm
On the Explosion Mechanism of SNe Type Ia
In this article we discuss the first simulations of two- and
three-dimensional Type Ia supernovae with an improved hydrodynamics code. After
describing the various enhancements, the obtained results are compared to those
of earlier code versions, observational data and the findings of other
researchers in this field.Comment: 7 pages, 4 figure
Numerical dissipation and the bottleneck effect in simulations of compressible isotropic turbulence
The piece-wise parabolic method (PPM) is applied to simulations of forced
isotropic turbulence with Mach numbers . The equation of state
is dominated by the Fermi pressure of an electron-degenerate fluid. The
dissipation in these simulations is of purely numerical origin. For the
dimensionless mean rate of dissipation, we find values in agreement with known
results from mostly incompressible turbulence simulations. The calculation of a
Smagorinsky length corresponding to the rate of numerical dissipation supports
the notion of the PPM supplying an implicit subgrid scale model. In the
turbulence energy spectra of various flow realisations, we find the so-called
bottleneck phenomenon, i.e., a flattening of the spectrum function near the
wavenumber of maximal dissipation. The shape of the bottleneck peak in the
compensated spectrum functions is comparable to what is found in turbulence
simulations with hyperviscosity. Although the bottleneck effect reduces the
range of nearly inertial length scales considerably, we are able to estimate
the value of the Kolmogorov constant. For steady turbulence with a balance
between energy injection and dissipation, it appears that .
However, a smaller value is found in the case of transonic turbulence with a
large fraction of compressive components in the driving force. Moreover, we
discuss length scales related to the dissipation, in particular, an effective
numerical length scale , which can be regarded as the
characteristic smoothing length of the implicit filter associated with the PPM.Comment: 23 pages, 7 figures. Revised version accepted by Comp. Fluids. Not
all figures included due to size restriction. Complete PDF available at
http://www.astro.uni-wuerzburg.de/%7Eschmidt/Paper/NumDiss_CF.pd
Type Ia Supernova Explosion Models: Homogeneity versus Diversity
Type Ia supernovae (SN Ia) are generally believed to be the result of the
thermonuclear disruption of Chandrasekhar-mass carbon-oxygen white dwarfs,
mainly because such thermonuclear explosions can account for the right amount
of Ni-56, which is needed to explain the light curves and the late-time
spectra, and the abundances of intermediate-mass nuclei which dominate the
spectra near maximum light. Because of their enormous brightness and apparent
homogeneity SN Ia have become an important tool to measure cosmological
parameters. In this article the present understanding of the physics of
thermonuclear explosions is reviewed. In particular, we focus our attention on
subsonic (``deflagration'') fronts, i.e. we investigate fronts propagating by
heat diffusion and convection rather than by compression. Models based upon
this mode of nuclear burning have been applied very successfully to the SN Ia
problem, and are able to reproduce many of their observed features remarkably
well. However, the models also indicate that SN Ia may differ considerably from
each other, which is of importance if they are to be used as standard candles.Comment: 11 pages, 4 figures. To appear in Proc. 10th Ann. Astrophys. Conf.
"Cosmic Explosions", Univ. of Maryland 1999, eds. S.S. Holt and W.W. Zhan
R-Process in Collapsing O/Ne/Mg Cores
Several circumstantial arguments point to the formation of the third
r-process peak at A about 190, near platinum, in stars of mass of about 8-10
solar masses: 1) The delayed production of europium with respect to iron
imposes a time scale that restricts the progenitor stars to less than about 10
solar masses; 2) the r-process demands a dominant robust mechanism at least for
barium and above, since the relative abundance pattern of those r-process
elements in low-metallicity stars is consistent with the solar pattern; 3)
stars of about 8-10 solar masses produce nearly identical degenerate O/Ne/Mg
cores that collapse due to electron capture; and 4) the resulting low-mass
cores may produce both an r-process in a prompt explosion and a subsequent
r-process in a neutrino driven wind. The prompt explosion of an O/Ne/Mg core
yields low entropy and low electron fraction, and hence may produce a
reasonable r-process peak at A about 190 as well as all of the r-process
elements with Z greater than 56. The possible differences in the
neutrino-driven wind and associated r-process due to the low-mass neutron stars
expected in this mass range are also discussed.Comment: 16 pages, LaTeX aasms4; to be published in ApJ Letter
A localised subgrid scale model for fluid dynamical simulations in astrophysics II: Application to type Ia supernovae
The dynamics of the explosive burning process is highly sensitive to the
flame speed model in numerical simulations of type Ia supernovae. Based upon
the hypothesis that the effective flame speed is determined by the unresolved
turbulent velocity fluctuations, we employ a new subgrid scale model which
includes a localised treatment of the energy transfer through the turbulence
cascade in combination with semi-statistical closures for the dissipation and
non-local transport of turbulence energy. In addition, subgrid scale buoyancy
effects are included. In the limit of negligible energy transfer and transport,
the dynamical model reduces to the Sharp-Wheeler relation. According to our
findings, the Sharp-Wheeler relation is insuffcient to account for the
complicated turbulent dynamics of flames in thermonuclear supernovae. The
application of a co-moving grid technique enables us to achieve very high
spatial resolution in the burning region. Turbulence is produced mostly at the
flame surface and in the interior ash regions. Consequently, there is a
pronounced anisotropy in the vicinity of the flame fronts. The localised
subgrid scale model predicts significantly enhanced energy generation and less
unburnt carbon and oxygen at low velocities compared to earlier simulations.Comment: 13 pages, 10 figures, accepted for publication in Astron. Astrophys.;
3D visualisations not included; complete PDF version can be downloaded from
http://www.astro.uni-wuerzburg.de/%7Eschmidt/Paper/SGSModel_II_AA.pd
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