63,732 research outputs found
Star/horizon simulator used to test space guidance system
Star/horizon simulator is used for alignment and optical plus photoelectric tests of the sextant for the Apollo guidance and navigation system optical unit assembly. The unit is basically a refractive collimator with a two inch objective lens system and a twenty-four inch focal length
Thermonuclear supernova simulations with stochastic ignition
We apply an ad hoc model for dynamical ignition in three-dimensional
numerical simulations of thermonuclear supernovae assuming pure deflagrations.
The model makes use of the statistical description of temperature fluctuations
in the pre-supernova core proposed by Wunsch & Woosley (2004). Randomness in
time is implemented by means of a Poisson process. We are able to vary the
explosion energy and nucleosynthesis depending on the free parameter of the
model which controls the rapidity of the ignition process. However, beyond a
certain threshold, the strength of the explosion saturates and the outcome
appears to be robust with respect to number of ignitions. In the most energetic
explosions, we find about 0.75 solar masses of iron group elements. Other than
in simulations with simultaneous multi-spot ignition, the amount of unburned
carbon and oxygen at radial velocities of a few 1000 km/s tends to be reduced
for an ever increasing number of ignition events and, accordingly, more
pronounced layering results.Comment: 7 pages, 6 figures, accepted for publication in Astron. Astrophys.;
PDF version with full resolution figures available from
http://www.astro.uni-wuerzburg.de/~schmidt/Paper/StochIgnt_AA.pd
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
Chandra observations of the galaxy cluster Abell 1835
We present the analysis of 30 ksec of Chandra observations of the galaxy
cluster Abell 1835. Overall, the X-ray image shows a relaxed morphology,
although we detect substructure in in the inner 30 kpc radius. Spectral
analysis shows a steep drop in the X-ray gas temperature from ~12 keV in the
outer regions of the cluster to ~4 keV in the core. The Chandra data provide
tight constraints on the gravitational potential of the cluster which can be
parameterized by a Navarro, Frenk & White (1997) model. The X-ray data allow us
to measure the X-ray gas mass fraction as a function of radius, leading to a
determination of the cosmic matter density of \Omega_m=0.40+-0.09 h_50^-0.5.
The projected mass within a radius of ~150 kpc implied by the presence of
gravitationally lensed arcs in the cluster is in good agreement with the mass
models preferred by the Chandra data. We find a radiative cooling time of the
X-ray gas in the centre of Abell 1835 of about 3x10^8 yr. Cooling flow model
fits to the Chandra spectrum and a deprojection analysis of the Chandra image
both indicate the presence of a young cooling flow (~6x10^8 yr) with an
integrated mass deposition rate of 230^+80_-50 M_o yr^-1 within a radius of 30
kpc. We discuss the implications of our results in the light of recent RGS
observations of Abell 1835 with XMM-Newton.Comment: 15 pages, 15 figures, accepted by MNRA
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
Cosmological constraints from the X-ray gas mass fraction in relaxed lensing clusters observed with Chandra
We present precise measurements of the X-ray gas mass fraction for a sample
of luminous, relatively relaxed clusters of galaxies observed with the Chandra
Observatory, for which independent confirmation of the mass results is
available from gravitational lensing studies. Parameterizing the total
(luminous plus dark matter) mass profiles using the model of Navarro, Frenk &
White (1997), we show that the X-ray gas mass fractions in the clusters
asymptote towards an approximately constant value at a radius r_2500, where the
mean interior density is 2500 times the critical density of the Universe at the
redshifts of the clusters. Combining the Chandra results on the X-ray gas mass
fraction and its apparent redshift dependence with recent measurements of the
mean baryonic matter density in the Universe and the Hubble Constant determined
from the Hubble Key Project, we obtain a tight constraint on the mean total
matter density of the Universe, Omega_m = 0.30^{+0.04}_{-0.03}, and measure a
positive cosmological constant, Omega_Lambda = 0.95^{+0.48}_{-0.72}. Our
results are in good agreement with recent, independent findings based on
analyses of anisotropies in the cosmic microwave background radiation, the
properties of distant supernovae, and the large-scale distribution of galaxies.Comment: Accepted for publication in MNRAS Letters (6 pages, 3 figures
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