498 research outputs found
Determination of Corresponding Temperature Distribution within CFRP during Laser Cutting
AbstractLaser cutting of carbon fiber reinforced plastics as a thermal process results in a thermal load on the material. Due to the high thermal conductivity of carbon fibers, residual heat is conducted along the fibers, away from the laser interaction zone. Common temperature measurement techniques, such as pyrometry and infrared thermography only allow for observation of the temperature development on the surface of the material. In order to achieve information about the temperature distribution within the material during the cutting process, thermochromes and thermocouples were implemented during the laminating process of CFRP. The cutting tests were performed with a single mode fiber laser emitting a continuous wave and at a wavelength of λ = 1080 nm
X-ray emission from Planetary Nebulae. I. Spherically symmetric numerical simulations
(abridged) The interaction of a fast wind with a spherical Asymptotic Giant
Branch (AGB) wind is thought to be the basic mechanism for shaping
Pre-Planetary Nebulae (PPN) and later Planetary Nebulae (PN). Due to the large
speed of the fast wind, one expects extended X-ray emission from these objects,
but X-ray emission has only been detected in a small fraction of PNs and only
in one PPN. Using numerical simulations we investigate the constraints that can
be set on the physical properties of the fast wind (speed, mass-flux, opening
angle) in order to produce the observed X-ray emission properties of PPNs and
PNs. We combine numerical hydrodynamical simulations including radiative
cooling using the code FLASH with calculations of the X-ray properties of the
resulting expanding hot bubble using the atomic database ATOMDB. In this first
study, we compute X-ray fluxes and spectra using one-dimensional models.
Comparing our results with analytical solutions, we find some agreements and
many disagreements. In particular, we test the effect of different time
histories of the fast wind on the X-ray emission and find that it is determined
by the final stage of the time history during which the fast wind velocity has
its largest value. The disagreements which are both qualitative and
quantitative in nature argue for the necessity of using numerical simulations
for understanding the X-ray properties of PNs.Comment: 17 pages, accepted for publication in ApJ (July 27, 2006), uses
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A conservative orbital advection scheme for simulations of magnetized shear flows with the PLUTO code
Explicit numerical computations of super-fast differentially rotating disks
are subject to the time-step constraint imposed by the Courant condition. When
the bulk orbital velocity largely exceeds any other wave speed the time step is
considerably reduced and a large number of steps may be necessary to complete
the computation.
We present a robust numerical scheme to overcome the Courant limitation by
extending the algorithm previously known as FARGO (Fast Advection in Rotating
Gaseous Objects) to the equations of magnetohydrodynamics (MHD). The proposed
scheme conserves total angular momentum and energy to machine precision and
works in Cartesian, cylindrical, or spherical coordinates. The algorithm is
implemented in the PLUTO code for astrophysical gasdynamics and is suitable for
local or global simulations of accretion or proto-planetary disk models.
By decomposing the total velocity into an average azimuthal contribution and
a residual term, the algorithm solves the MHD equations through a linear
transport step in the orbital direction and a standard nonlinear solver applied
to the MHD equations written in terms of the residual velocity. Since the
former step is not subject to any stability restriction, the Courant condition
is computed only in terms of the residual velocity, leading to substantially
larger time steps. The magnetic field is advanced in time using the constrained
transport method in order to preserve the divergence-free condition.
Conservation of total energy and angular momentum is enforced at the discrete
level by properly expressing the source terms in terms of upwind fluxes
available during the standard solver.
Our results show that applications of the proposed orbital-advection scheme
to problems of astrophysical relevance provides, at reduced numerical cost,
equally accurate and less dissipative results than standard time-marching
schemes.Comment: 16 pages, 13 figures. Accepted for publication in A&
Young stellar object jet models: From theory to synthetic observations
Astronomical observations, analytical solutions and numerical simulations
have provided the building blocks to formulate the current theory of young
stellar object jets. Although each approach has made great progress
independently, it is only during the last decade that significant efforts are
being made to bring the separate pieces together. Building on previous work
that combined analytical solutions and numerical simulations, we apply a
sophisticated cooling function to incorporate optically thin energy losses in
the dynamics. On the one hand, this allows a self-consistent treatment of the
jet evolution and on the other, it provides the necessary data to generate
synthetic emission maps. Firstly, analytical disk and stellar outflow solutions
are properly combined to initialize numerical two-component jet models inside
the computational box. Secondly, magneto-hydrodynamical simulations are
performed in 2.5D, following properly the ionization and recombination of a
maximum of ions. Finally, the outputs are post-processed to produce
artificial observational data. The first two-component jet simulations, based
on analytical models, that include ionization and optically thin radiation
losses demonstrate promising results for modeling specific young stellar object
outflows. The generation of synthetic emission maps provides the link to
observations, as well as the necessary feedback for the further improvement of
the available models.Comment: accepted for publication A&A, 20 pages, 11 figure
Mid-IR period-magnitude relations for AGB stars
Asymptotic Giant Branch variables are found to obey period-luminosity
relations in the mid-IR similar to those seen at K_S (2.14 microns), even at 24
microns where emission from circumstellar dust is expected to be dominant.
Their loci in the M, logP diagrams are essentially the same for the LMC and for
NGC6522 in spite of different ages and metallicities. There is no systematic
trend of slope with wavelength. The offsets of the apparent magnitude vs. logP
relations imply a difference between the two fields of 3.8 in distance modulus.
The colours of the variables confirm that a principal period with log P > 1.75
is a necessary condition for detectable mass-loss. At the longest observed
wavelength, 24 microns, many semi-regular variables have dust shells comparable
in luminosity to those around Miras. There is a clear bifurcation in LMC
colour-magnitude diagrams involving 24 micron magnitudes.Comment: 5 pages, 4 figure
Galactic bulge giants: probing stellar and galactic evolution I. Catalogue of Spitzer IRAC and MIPS sources
Aims: We aim at measuring mass-loss rates and the luminosities of a
statistically large sample of Galactic bulge stars at several galactocentric
radii. The sensitivity of previous infrared surveys of the bulge has been
rather limited, thus fundamental questions for late stellar evolution, such as
the stage at which substantial mass-loss begins on the red giant branch and its
dependence on fundamental stellar properties, remain unanswered. We aim at
providing evidence and answers to these questions. Methods: To this end, we
observed seven 15 times 15 arcmin^2 fields in the nuclear bulge and its
vicinity with unprecedented sensitivity using the IRAC and MIPS imaging
instruments on-board the Spitzer Space Telescope. In each of the fields, tens
of thousands of point sources were detected. Results: In the first paper based
on this data set, we present the observations, data reduction, the final
catalogue of sources, and a detailed comparison to previous mid-IR surveys of
the Galactic bulge, as well as to theoretical isochrones. We find in general
good agreement with other surveys and the isochrones, supporting the high
quality of our catalogue.Comment: 21 pages, accepted for publication in A&A. A version with
high-resolution figures, as well as the data catalogues (including cross-id
with GLIMPSE and GALCEN) and image mosaics are available at the anonymous
ftp://ftp.ster.kuleuven.be/dist/stefan/Spitzer
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New insights into lithology and hydrogeology of the northern Newark Rift Basin
The marginal facies of the Triassic rift basins in the eastern United States are poorly documented, particularly on the hinge or hanging wall margins. This study presents a lithological description and multiscale petrophysical analysis of basement rocks, overlying marginal facies of the early synrift strata, and the basal contact of the Palisade Sill that were drilled and cored in the northeastern part of the Newark Basin, near its terminus. The expression of the Stockton Formation differs from that in the central basin in having thinner layers, with uncertain temporal relationship to the type area. The bottom 50 m is lithologically distinct with brick-red to dark-purple mudstones and sandstones, abundant gypsum-filled fractures, and a thin zone with anomalously high uranium concentration, not associated with organic-rich mudstones as other occurrences in the basin. The crystalline basement is apparently Fordham gneiss, overlain by a thin sandstone layer and a dark-purple hydrophilic mudstone. Despite the abundance of coarse-grained strata and multiple sets of tectonic fractures, hydraulically transmissive zones are sparse, and do not uniquely correlate to fracture and/or matrix characteristics. Enhanced transmissivity may exist along intrusion boundaries due to enhanced thermal fracturing, but more hydraulic data are needed to verify it. Comparison of petrophysical data in two boreholes ∼210 m apart shows no direct correlation of individual lithological units and their hydraulic properties, although the overall formation characteristics are similar. The results highlight challenges for outcrop correlation at the marginal edges of the rift basins and estimating reservoir properties of these heterogeneous formations
The universal Glivenko-Cantelli property
Let F be a separable uniformly bounded family of measurable functions on a
standard measurable space, and let N_{[]}(F,\epsilon,\mu) be the smallest
number of \epsilon-brackets in L^1(\mu) needed to cover F. The following are
equivalent:
1. F is a universal Glivenko-Cantelli class.
2. N_{[]}(F,\epsilon,\mu)0 and every probability
measure \mu.
3. F is totally bounded in L^1(\mu) for every probability measure \mu.
4. F does not contain a Boolean \sigma-independent sequence.
It follows that universal Glivenko-Cantelli classes are uniformity classes
for general sequences of almost surely convergent random measures.Comment: 26 page
The Atlantic Ocean at the last glacial maximum: 1. Objective mapping of the GLAMAP sea-surface conditions
Recent efforts of the German paleoceanographic community have resulted in a unique data set of reconstructed sea-surface temperature for the Atlantic Ocean during the Last Glacial Maximum, plus estimates for the extents of glacial sea ice. Unlike prior attempts, the contributing research groups based their data on a common definition of the Last Glacial Maximum chronozone and used the same modern reference data for calibrating the different transfer techniques. Furthermore, the number of processed sediment cores was vastly increased. Thus the new data is a significant advance not only with respect to quality, but also to quantity. We integrate these new data and provide monthly data sets of global sea-surface temperature and ice cover, objectively interpolated onto a regular 1°x1° grid, suitable for forcing or validating numerical ocean and atmosphere models. This set is compared to an existing subjective interpolation of the same base data, in part by employing an ocean circulation model. For the latter purpose, we reconstruct sea surface salinity from the new temperature data and the available oxygen isotope measurements
Osteoporosis drug treatment: duration and management after discontinuation. A position statement from the SVGO/ASCO.
Antiosteoporotic drugs are recommended in patients with fragility fractures and in patients considered to be at high fracture risk on the basis of clinical risk factors and/or low bone mineral density. As first-line treatment most patients are started with an antiresorptive treatment, i.e. drugs that inhibit osteoclast development and/or function (bisphosphonates, denosumab, oestrogens or selective oestrogen receptor modulators). In the balance between benefits and risks of antiresorptive treatment, uncertainties remain regarding the optimal treatment duration and the management of patients after drug discontinuation. Based on the available evidence, this position statement will focus on the long-term management of osteoporosis therapy, formulating decision criteria for clinical practice
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