2,731 research outputs found
Inclusion of pore pressure effects in discrete element modeling of rock cutting
Many rock excavation processes occur in a marine environment, like in drilling for oil/gas, dredging, trenching and deep sea mining. The presence of a fluid in and surrounding the rock can have a significant influence on the cutting process, through differences in the ambient (confining) and pore pressure. The cutting motion deforms the rock matrix, and as a result, local fluid pressure differences will occur. The magnitude of these pressure differences, and thus its effect on the cutting process, increases with larger water depths and/or higher cutting velocities. The apparent strength of the rock matrix increases with higher confining pressures, resulting in a higher cutting force. The Discrete Element Method is used successfully to simulate the rock cutting process of dry rock for various applications. In this paper, the authors extend DEM with a fully coupled fluid pressure model to simulate the mechanics of saturated rock. This is done by solving a pore pressure diffusion equation with a Smoothed Particle (SP) method. By using the SP, it is possible to convert the discontinuum properties of the DEM to a continuum, in which the fluid pressure is modeled and applied as an additional force in the DEM. Qualitative results show that the model is able to capture the increase in cutting force with increasing confining pressure, as well as deformation rate effects applied on saturated rocks
The Influence of Large-Scale Structure on Halo Shapes and Alignments
Alignments of galaxy clusters (the Binggeli effect), as well as of galaxies
themselves have long been studied both observationally and theoretically. Here
we test the influence of large-scales structures and tidal fields on the shapes
and alignments of cluster-size and galaxy-size dark matter halos. We use a
high-resolution N-body simulation of a CDM universe, together with the
results of Colberg et al. (2005), who identified filaments connecting pairs of
clusters. We find that cluster pairs connected by a filament are strongly
aligned with the cluster-cluster axis, whereas unconnected ones are not. For
smaller, galaxy-size halos, there also is an alignment signal, but its strength
is independent of whether the halo is part of an obvious large-scale structure.
Additionally, we find no measureable dependence of galaxy halo shape on
membership of a filament. We also quantify the influence of tidal fields and
find that these do correlate strongly with alignments of halos. The alignments
of most halos are thus caused by tidal fields, with cluster-size halos being
strongly aligned through the added mechanism of infall of matter from
filaments.Comment: 8 pages, 6 figures, accepted for publication in MNRA
VLT/NACO astrometry of the HR8799 planetary system. L'-band observations of the three outer planets
HR8799 is so far the only directly imaged multiple exoplanet system. The
orbital configuration would, if better known, provide valuable insight into the
formation and dynamical evolution of wide-orbit planetary systems. We present
L'-band observations of the HR8799 system obtained with NACO at VLT, adding to
the astrometric monitoring of the planets HR8799b, c and d. We investigate how
well the two simple cases of (i) a circular orbit and (ii) a face-on orbit fit
the astrometric data for HR8799d over a total time baseline of ~2 years. The
results indicate that the orbit of HR8799d is inclined with respect to our line
of sight, and suggest that the orbit is slightly eccentric or non-coplanar with
the outer planets and debris disk.Comment: 5 pages, 4 figures, 1 table, accepted for publication in A\&A.
Updated version includes minor changes made in the proof
Finite-Size Scaling in Two-Dimensional Superfluids
Using the model and a non-local updating scheme called cluster Monte
Carlo, we calculate the superfluid density of a two dimensional superfluid on
large-size square lattices up to . This technique
allows us to approach temperatures close to the critical point, and by studying
a wide range of values and applying finite-size scaling theory we are able
to extract the critical properties of the system. We calculate the superfluid
density and from that we extract the renormalization group beta function. We
derive finite-size scaling expressions using the Kosterlitz-Thouless-Nelson
Renormalization Group equations and show that they are in very good agreement
with our numerical results. This allows us to extrapolate our results to the
infinite-size limit. We also find that the universal discontinuity of the
superfluid density at the critical temperature is in very good agreement with
the Kosterlitz-Thouless-Nelson calculation and experiments.Comment: 13 pages, postscript fil
Spatial and kinematic alignments between central and satellite halos
Based on a cosmological N-body simulation we analyze spatial and kinematic
alignments of satellite halos within six times the virial radius of group size
host halos (Rvir). We measure three different types of spatial alignment: halo
alignment between the orientation of the group central substructure (GCS) and
the distribution of its satellites, radial alignment between the orientation of
a satellite and the direction towards its GCS, and direct alignment between the
orientation of the GCS and that of its satellites. In analogy we use the
directions of satellite velocities and probe three further types of alignment:
the radial velocity alignment between the satellite velocity and connecting
line between satellite and GCS, the halo velocity alignment between the
orientation of the GCS and satellite velocities and the auto velocity alignment
between the satellites orientations and their velocities. We find that
satellites are preferentially located along the major axis of the GCS within at
least 6 Rvir (the range probed here). Furthermore, satellites preferentially
point towards the GCS. The most pronounced signal is detected on small scales
but a detectable signal extends out to 6 Rvir. The direct alignment signal is
weaker, however a systematic trend is visible at distances < 2 Rvir. All
velocity alignments are highly significant on small scales. Our results suggest
that the halo alignment reflects the filamentary large scale structure which
extends far beyond the virial radii of the groups. In contrast, the main
contribution to the radial alignment arises from the adjustment of the
satellite orientations in the group tidal field. The projected data reveal good
agreement with recent results derived from large galaxy surveys. (abridged)Comment: accepted for publication in Ap
Dissipational versus Dissipationless Galaxy Formation and the Dark Matter Content of Galaxies
We examine two extreme models for the build-up of the stellar component of
luminous elliptical galaxies. In one case, we assume the build-up of stars is
dissipational, with centrally accreted gas radiating away its orbital and
thermal energy; the dark matter halo will undergo adiabatic contraction and the
central dark matter density profile will steepen. For the second model, we
assume the central galaxy is assembled by a series of dissipationless mergers
of stellar clumps that have formed far from the nascent galaxy. In order to be
accreted, these clumps lose their orbital energy to the dark matter halo via
dynamical friction, thereby heating the central dark matter and smoothing the
dark matter density cusp. The central dark matter density profiles differ
drastically between these models. For the isolated elliptical galaxy, NGC 4494,
the central dark matter densities follow the power-laws r^(-0.2) and r^(-1.7)
for the dissipational and dissipationless models, respectively. By matching the
dissipational and dissipationless models to observations of the stellar
component of elliptical galaxies, we examine the relative contributions of
dissipational and dissipationless mergers to the formation of elliptical
galaxies and look for observational tests that will distinguish between these
models. Comparisons to strong lensing brightest cluster galaxies yield median
M*/L_B ratios of 2.1+/-0.8 and 5.2+/-1.7 at z=0.39 for the dissipational and
dissipationless models, respectively. For NGC 4494, the best-fit dissipational
and dissipationless models have M*/L_B=2.97 and 3.96. Comparisons to expected
stellar mass-to-light ratios from passive evolution and population syntheses
appear to rule out a purely dissipational formation mechanism for the central
stellar regions of giant elliptical galaxies.Comment: 15 pages, 9 figures, accepted to Ap
Novel Modifications of Parallel Jacobi Algorithms
We describe two main classes of one-sided trigonometric and hyperbolic
Jacobi-type algorithms for computing eigenvalues and eigenvectors of Hermitian
matrices. These types of algorithms exhibit significant advantages over many
other eigenvalue algorithms. If the matrices permit, both types of algorithms
compute the eigenvalues and eigenvectors with high relative accuracy.
We present novel parallelization techniques for both trigonometric and
hyperbolic classes of algorithms, as well as some new ideas on how pivoting in
each cycle of the algorithm can improve the speed of the parallel one-sided
algorithms. These parallelization approaches are applicable to both
distributed-memory and shared-memory machines.
The numerical testing performed indicates that the hyperbolic algorithms may
be superior to the trigonometric ones, although, in theory, the latter seem
more natural.Comment: Accepted for publication in Numerical Algorithm
Target selection for the SUNS and DEBRIS surveys for debris discs in the solar neighbourhood
Debris discs - analogous to the Asteroid and Kuiper-Edgeworth belts in the
Solar system - have so far mostly been identified and studied in thermal
emission shortward of 100 um. The Herschel space observatory and the SCUBA-2
camera on the James Clerk Maxwell Telescope will allow efficient photometric
surveying at 70 to 850 um, which allow for the detection of cooler discs not
yet discovered, and the measurement of disc masses and temperatures when
combined with shorter wavelength photometry. The SCUBA-2 Unbiased Nearby Stars
(SUNS) survey and the DEBRIS Herschel Open Time Key Project are complimentary
legacy surveys observing samples of ~500 nearby stellar systems. To maximise
the legacy value of these surveys, great care has gone into the target
selection process. This paper describes the target selection process and
presents the target lists of these two surveys.Comment: 67 pages with full tables, 7 figures, accepted to MNRA
Astrometry with Hubble Space Telescope: A Parallax of the Fundamental Distance Calibrator RR Lyrae
We present an absolute parallax and relative proper motion for the
fundamental distance scale calibrator, RR Lyr. We obtain these with astrometric
data from FGS 3, a white-light interferometer on HST. We find mas. Spectral classifications and VRIJHKTM and DDO51 photometry of
the astrometric reference frame surrounding RR Lyr indicate that field
extinction is low along this line of sight. We estimate =0.07\pm0.03 for
these reference stars. The extinction suffered by RR Lyr becomes one of the
dominant contributors to the uncertainty in its absolute magnitude. Adopting
the average field absorption, =0.07 \pm 0.03, we obtain M_V^{RR} = 0.61
^{-0.11}_{+0.10}. This provides a distance modulus for the LMC, m-M = 18.38 -
18.53^{-0.11}_{+0.10} with the average extinction-corrected magnitude of RR Lyr
variables in the LMC, , remaining a significant uncertainty. We compare
this result to more than 80 other determinations of the distance modulus of the
LMC.Comment: Several typos corrected. To appear in The Astronomical Journal,
January 200
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