1,194 research outputs found
Sleeper end resistance of ballasted railway tracks
This paper describes model tests used to investigate how ballast shoulder width and height contribute to a railway sleeperâs resistance to lateral movement for a range of shoulder widths and heights. Deflection and resistance were measured and photographs taken during the tests.The photographs were analyzed using a digital image correlation technique to identify the zones of ballast surface disturbance, which demonstrated that a bulbed failure volume was mobilized at the ultimate limit state. An idealized three-dimensional failure mechanism is proposed, and resistances are calculated using the limit equilibrium approach. The calculation provides a reliable estimate of the measured resistance. The work identifies the optimum shoulder width and height. The calculations are extended to demonstrate that when a number of sleepers are moved simultaneously, the sleeper end resistance may be one-third less per sleeper than that indicated in tests on an isolated sleeper. Image analysis and limit equilibrium calculations show that this is caused by overlapping of mobilized failure volumes from adjacent sleepers
Efficient approximations of neutrino physics for three-dimensional simulations of stellar core collapse
Neutrino transport in spherically symmetric models of stellar core collapse
and bounce has achieved a technically complete level, rewarded by the agreement
among independent groups that a multi-dimensional treatment of the
fluid-instabilities in the post-bounce phase is indispensable to model
supernova explosions. While much effort is required to develop a reliable
neutrino transport technique in axisymmetry, we explore neutrino physics
approximations and parameterizations for an efficient three-dimensional
simulation of the fluid-instabilities in the shock-heated matter that
accumulates between the accretion shock and the protoneutron star. We
demonstrate the reliability of a simple parameterization scheme in the collapse
phase and extend our 3D magneto-hydrodynamical collapse simulations to a
preliminary postbounce evolution. The growth of magnetic fields is
investigated.Comment: 5 pages, 4 figures, in Proceedings of "Nuclei in the Cosmos IX,
Geneva, Jun 25-30", associated movies are displayed at
http://www.physik.unibas.ch/~liebend/displa
Precision measurement of cosmic magnification from 21 cm emitting galaxies
We show how precision lensing measurements can be obtained through the
lensing magnification effect in high redshift 21cm emission from galaxies.
Normally, cosmic magnification measurements have been seriously complicated by
galaxy clustering. With precise redshifts obtained from 21cm emission line
wavelength, one can correlate galaxies at different source planes, or exclude
close pairs to eliminate such contaminations.
We provide forecasts for future surveys, specifically the SKA and CLAR. SKA
can achieve percent precision on the dark matter power spectrum and the galaxy
dark matter cross correlation power spectrum, while CLAR can measure an
accurate cross correlation power spectrum. The neutral hydrogen fraction was
most likely significantly higher at high redshifts, which improves the number
of observed galaxies significantly, such that also CLAR can measure the dark
matter lensing power spectrum. SKA can also allow precise measurement of
lensing bispectrum.Comment: 11 pages, 8 figures. Accepted to MNRAS. We deleted two figures and
shortened the paper to meet MNRAS's requirement. All main results remain
unchange
Precision era of the kinetic Sunyaev-Zeldovich effect: simulations, analytical models and observations and the power to constrain reionization
The kinetic SZ effect, which is the dominant CMB source at arc-minute scales
and Ghz, probes the ionized gas peculiar momentum up to the
epoch of reionization and is a sensitive measure of the reionization history.
We ran high resolution self-similar and CDM hydro simulations and
built an analytical model to study this effect. Our model reproduces the
CDM simulation results to several percent accuracy, passes various
tests against self-similar simulations, and shows a wider range of
applicability than previous analytical models. Our model in its continuous
version is free of simulation limitations such as finite simulation box and
finite resolution and allows an accurate prediction of the kinetic SZ power
spectrum . For the WMAP cosmology, we find for the reionization redshift and
. The corresponding temperature fluctuation is several K at
these ranges. The dependence of on the reionization history allows an
accurate measurement of the reionization epoch. For the Atacama cosmology
telescope experiment, can be measured with accuracy.
scales as . Given cosmological parameters,
ACT would be able to constrain with several percent accuracy.
Some multi-reionization scenarios degenerate in the primary CMB temperature and
TE measurement can be distinguished with confidence.Comment: 14 pages, 7 figures. Accepted by MNRAS. We corrected the primary CMB
power spectrum we used. We added discussions about the effects of lensing and
relativistic SZ correctio. We withdraw a claim about the patchy reionizatio
Gravity and Non-gravity Modes in the VIRMOS-DESCART Weak Lensing Survey
We analyze the weak lensing data of the VIRMOS imaging survey using
projections (called E and B-modes) of the two independents observed correlation
functions. The E-mode contains all the lensing signal, while noise and
systematics contribute equally to the E and B modes provided that intrinsic
alignment is negligible. The mode separation allows a measurement of the signal
with a \sqrt{2} smaller error bars, and a separate channel to test for
systematic errors. We apply various transformations, including a spherical
harmonic space power spectrum C^E_l and C^B_l, which provides a direct
measurement of the projected dark matter distribution for 500<l<10^4.Comment: accepted version, minor changes, 18 pages including 6 figure
Likelihood Analysis of Cosmic Shear on Simulated and VIRMOS-DESCART Data
We present a maximum likelihood analysis of cosmological parameters from
measurements of the aperture mass up to 35 arcmin, using simulated and real
cosmic shear data. A four-dimensional parameter space is explored which
examines the mean density \Omega_M, the mass power spectrum normalization
\sigma_8, the shape parameter \Gamma and the redshift of the sources z_s.
Constraints on \Omega_M and \sigma_8 (resp. \Gamma and z_s) are then given by
marginalizing over \Gamma and z_s (resp. \Omega_M and \sigma_8). For a flat
LCDM cosmologies, using a photometric redshift prior for the sources and \Gamma
\in [0.1,0.4], we find \sigma_8=(0.57\pm0.04) \Omega_M^{(0.24\mp 0.18)
\Omega_M-0.49} at the 68% confidence level (the error budget includes
statistical noise, full cosmic variance and residual systematic). The estimate
of \Gamma, marginalized over \Omega_M \in [0.1,0.4], \sigma_8 \in [0.7,1.3] and
z_s constrained by photometric redshifts, gives \Gamma=0.25\pm 0.13 at 68%
confidence. Adopting h=0.7, a flat universe, \Gamma=0.2 and \Omega_m=0.3 we
find \sigma_8=0.98 \pm0.06 . Combined with CMB, our results suggest a non-zero
cosmological constant and provide tight constraints on \Omega_M and \sigma_8.
We finaly compare our results to the cluster abundance ones, and discuss the
possible discrepancy with the latest determinations of the cluster method. In
particular we point out the actual limitations of the mass power spectrum
prediction in the non-linear regime, and the importance for its improvement.Comment: 11 pages, submitted to A&
FISH: A 3D parallel MHD code for astrophysical applications
FISH is a fast and simple ideal magneto-hydrodynamics code that scales to ~10
000 processes for a Cartesian computational domain of ~1000^3 cells. The
simplicity of FISH has been achieved by the rigorous application of the
operator splitting technique, while second order accuracy is maintained by the
symmetric ordering of the operators. Between directional sweeps, the
three-dimensional data is rotated in memory so that the sweep is always
performed in a cache-efficient way along the direction of contiguous memory.
Hence, the code only requires a one-dimensional description of the conservation
equations to be solved. This approach also enable an elegant novel
parallelisation of the code that is based on persistent communications with MPI
for cubic domain decomposition on machines with distributed memory. This scheme
is then combined with an additional OpenMP parallelisation of different sweeps
that can take advantage of clusters of shared memory. We document the detailed
implementation of a second order TVD advection scheme based on flux
reconstruction. The magnetic fields are evolved by a constrained transport
scheme. We show that the subtraction of a simple estimate of the hydrostatic
gradient from the total gradients can significantly reduce the dissipation of
the advection scheme in simulations of gravitationally bound hydrostatic
objects. Through its simplicity and efficiency, FISH is as well-suited for
hydrodynamics classes as for large-scale astrophysical simulations on
high-performance computer clusters. In preparation for the release of a public
version, we demonstrate the performance of FISH in a suite of astrophysically
orientated test cases.Comment: 27 pages, 11 figure
Halo stochasticity in global clustering analysis
In the present work we study the statistics of haloes, which in the halo
model determines the distribution of galaxies. Haloes are known to be biased
tracer of dark matter, and at large scales it is usually assumed there is no
intrinsic stochasticity between the two fields. Following the work of Seljak &
Warren (2004), we explore how correct this assumption is and, moving a step
further, we try to qualify the nature of stochasticity. We use Principal
Component Analysis applied to the outputs of a cosmological N-body simulation
to: (1) explore the behaviour of stochasticity in the correlation between
haloes of different masses; (2) explore the behaviour of stochasticity in the
correlation between haloes and dark matter. We show results obtained using a
catalogue with 2.1 million haloes, from a PMFAST simulation with box size of
1000h^{-1}Mpc. In the relation between different populations of haloes we find
that stochasticity is not-negligible even at large scales. In agreement with
the conclusions of Tegmark & Bromley (1999) who studied the correlations of
different galaxy populations, we found that the shot-noise subtracted
stochasticity is qualitatively different from `enhanced' shot noise and,
specifically, it is dominated by a single stochastic eigenvalue. We call this
the `minimally stochastic' scenario, as opposed to shot noise which is
`maximally stochastic'. In the correlation between haloes and dark matter, we
find that stochasticity is minimized, as expected, near the dark matter peak (k
~ 0.02 h Mpc^{-1} for a LambdaCDM cosmology) and, even at large scales, it is
of the order of 15 per cent above the shot noise. Moreover, we find that the
reconstruction of the dark matter distribution is improved when we use
eigenvectors as tracers of the bias. [Abridged]Comment: 9 pages, 12 figures. Submitted to MNRA
- âŠ