440 research outputs found
Fresh concrete pumping arrest investigation for thixotropy by a CFD modelling apporach
Concrete pumping operations determine construction speed, finishing
quality, durability and even structural integrity. When pumping operations
cannot be continued, most problems occur due to complex time-dependent
transformations. This causes significant industrial costs (e.g. material and delay).
Since time-dependent aspects are currently not fully understood and cannot be
predicted, a way to quantify time-dependent aspects is needed. Therefore, we
make an attempt by numerical simulation by comparing thixotropic cases with
different pumping arresting times. After an introduction to fresh concrete rheology
and numerical modelling, ten representative thixotropy cases are analysed.
Despite some unresolved numerical instabilities, the numerical framework
allows to estimate pumping pressure peaks after resting time. The results
evaluate a thixotropy model, which is generally applicable for less thixotropic
SCC’s. It is clear that flow re-initiation after rest in concrete pumping is poorly
understood. Numerical simulation could be one approach for further analysis
and is potentially important for practice. Future work such as simulation of
concrete mixers, pressure increase after pumping arrest, formwork pressure
decay and leakage are therefore recommended
Facilitating Knowledge Integration and Flexibility: The Effects of Reflection and Exposure to Alternative Models.
Three experiments investigated the effect of various kinds of reflection (within-task and post-task reflection) on learning a process control task. Also, two ways of providing learners with alternative ideas about task behavior, exposure to other\u27s ideas and providing hints for task solution, were examined. The task involved a simulated sugar production factory in which the learners sought to control sugar production by manipulating workforce size. It was predicted that combining within-task reflection with exposure to alternative task ideas would lead to superior task performance through integration of experiential and reflective knowledge. Contrary to the prediction, within-task reflection consistently interfered with learning and knowledge integration by causing learners to acquire overly general and invalid rules (reflective knowledge) about the relations among task variables. These results were interpreted as evidence of a fundamental tendency (reflective abstraction error) of people to seek simple relations among variables in complex systems when engaged in within-task reflection. The most efficient learning occurred when reflection was discouraged during task performance, learners were given access to alternative ideas about task behavior, and they were given an opportunity to discuss their task experiences with other learners post-task
Spectral Mapping Reconstruction of Extended Sources
Three dimensional spectroscopy of extended sources is typically performed
with dedicated integral field spectrographs. We describe a method of
reconstructing full spectral cubes, with two spatial and one spectral
dimension, from rastered spectral mapping observations employing a single slit
in a traditional slit spectrograph. When the background and image
characteristics are stable, as is often achieved in space, the use of
traditional long slits for integral field spectroscopy can substantially reduce
instrument complexity over dedicated integral field designs, without loss of
mapping efficiency -- particularly compelling when a long slit mode for single
unresolved source followup is separately required. We detail a custom
flux-conserving cube reconstruction algorithm, discuss issues of extended
source flux calibration, and describe CUBISM, a tool which implements these
methods for spectral maps obtained with ther Spitzer Space Telescope's Infrared
Spectrograph.Comment: 11 pages, 8 figures, accepted by PAS
H_2 emission arises outside photodissociation regions in ultra-luminous infrared galaxies
Ultra-luminous infrared galaxies are among the most luminous objects in the
local universe and are thought to be powered by intense star formation. It has
been shown that in these objects the rotational spectral lines of molecular
hydrogen observed at mid-infrared wavelengths are not affected by dust
obscuration, leaving unresolved the source of excitation of this emission. Here
I report an analysis of archival Spitzer Space Telescope data on ultra-luminous
infrared galaxies and demonstrate that star formation regions are buried inside
optically thick clouds of gas and dust, so that dust obscuration affects
star-formation indicators but not molecular hydrogen. I thereby establish that
the emission of H_2 is not co-spatial with the buried starburst activity and
originates outside the obscured regions. This is rather surprising in light of
the standard view that H_2 emission is directly associated with star-formation
activity. Instead, I propose that H_2 emission in these objects traces shocks
in the surrounding material, which are in turn excited by interactions with
nearby galaxies, and that powerful large-scale shocks cooling by means of H_2
emission may be much more common than previously thought. In the early
universe, a boost in H_2 emission by this process may speed up the cooling of
matter as it collapsed to form the first stars and galaxies and would make
these first structures more readily observable.Comment: Main text and supplemental information, 21 pages including 6 figures,
2 table
Mid-Infrared IRS Spectroscopy of NGC 7331: A First Look at the SINGS Legacy
The nearby spiral galaxy NGC 7331 was spectrally mapped from 5-38um using all
modules of Spitzer's IRS spectrograph. A strong new dust emission feature,
presumed due to PAHs, was discovered at 17.1um. The feature's intensity is
nearly half that of the ubiquitous 11.3um band. The 7-14um spectral maps
revealed significant variation in the 7.7 and 11.3um PAH features between the
stellar ring and nucleus. Weak [OIV] 25.9um line emission was found to be
centrally concentrated in the nucleus, with an observed strength over 10% of
the combined neon line flux, indicating an AGN or unusually active massive star
photo-ionization. Two [SIII] lines fix the characteristic electron density in
the HII regions at n_e < ~200 cm^-3. Three detected H_2 rotational lines,
tracing warm molecular gas, together with the observed IR continuum, are
difficult to match with standard PDR models. Either additional PDR heating or
shocks are required to simultaneously match lines and continuum.Comment: 6 pages, 5 figures, accepted for publication in ApJS Spitzer Special
Issu
Warm Dust and Spatially Variable PAH Emission in the Dwarf Starburst Galaxy NGC 1705
We present Spitzer observations of the dwarf starburst galaxy NGC 1705
obtained as part of SINGS. The galaxy morphology is very different shortward
and longward of ~5 microns: short-wavelength imaging shows an underlying red
stellar population, with the central super star cluster (SSC) dominating the
luminosity; longer-wavelength data reveals warm dust emission arising from two
off-nuclear regions offset by ~250 pc from the SSC. These regions show little
extinction at optical wavelengths. The galaxy has a relatively low global dust
mass (~2E5 solar masses, implying a global dust-to-gas mass ratio ~2--4 times
lower than the Milky Way average). The off-nuclear dust emission appears to be
powered by photons from the same stellar population responsible for the
excitation of the observed H Alpha emission; these photons are unassociated
with the SSC (though a contribution from embedded sources to the IR luminosity
of the off-nuclear regions cannot be ruled out). Low-resolution IRS
spectroscopy shows moderate-strength PAH emission in the 11.3 micron band in
the eastern peak; no PAH emission is detected in the SSC or the western dust
emission complex. There is significant diffuse 8 micron emission after scaling
and subtracting shorter wavelength data; the spatially variable PAH emission
strengths revealed by the IRS data suggest caution in the interpretation of
diffuse 8 micron emission as arising from PAH carriers alone. The metallicity
of NGC 1705 falls at the transition level of 35% solar found by Engelbracht and
collaborators; the fact that a system at this metallicity shows spatially
variable PAH emission demonstrates the complexity of interpreting diffuse 8
micron emission. A radio continuum non-detection, NGC 1705 deviates
significantly from the canonical far-IR vs. radio correlation. (Abridged)Comment: ApJ, in press; please retrieve full-resolution version from
http://www.astro.wesleyan.edu/~cannon/pubs.htm
The Nature of Infrared Emission in the Local Group Dwarf Galaxy NGC 6822 As Revealed by Spitzer
We present Spitzer imaging of the metal-deficient (Z ~30% Z_sun) Local Group
dwarf galaxy NGC 6822. On spatial scales of ~130 pc, we study the nature of IR,
H alpha, HI, and radio continuum emission. Nebular emission strength correlates
with IR surface brightness; however, roughly half of the IR emission is
associated with diffuse regions not luminous at H alpha (as found in previous
studies). The global ratio of dust to HI gas in the ISM, while uncertain at the
factor of ~2 level, is ~25 times lower than the global values derived for
spiral galaxies using similar modeling techniques; localized ratios of dust to
HI gas are about a factor of five higher than the global value in NGC 6822.
There are strong variations (factors of ~10) in the relative ratios of H alpha
and IR flux throughout the central disk; the low dust content of NGC 6822 is
likely responsible for the different H alpha/IR ratios compared to those found
in more metal-rich environments. The H alpha and IR emission is associated with
high-column density (> ~1E21 cm^-2) neutral gas. Increases in IR surface
brightness appear to be affected by both increased radiation field strength and
increased local gas density. Individual regions and the galaxy as a whole fall
within the observed scatter of recent high-resolution studies of the radio-far
IR correlation in nearby spiral galaxies; this is likely the result of depleted
radio and far-IR emission strengths in the ISM of this dwarf galaxy.Comment: ApJ, in press; please retrieve full-resolution version from
http://www.astro.wesleyan.edu/~cannon/pubs.htm
Dust Masses, PAH Abundances, and Starlight Intensities in the SINGS Galaxy Sample
Physical dust models are presented for 65 galaxies in SINGS that are strongly detected in the four IRAC bands and three MIPS bands. For each galaxy we estimate (1) the total dust mass, (2) the fraction of the dust mass contributed by PAHs, and (3) the intensity of the starlight heating the dust grains. We find that spiral galaxies have dust properties resembling the dust in the local region of the Milky Way, with similar dust-to-gas ratio and similar PAH abundance. The observed SEDs, including galaxies with SCUBA photometry, can be reproduced by dust models that do not require "cold" (T ≾ 10 K) dust. The dust-to-gas ratio is observed to be dependent on metallicity. In the interstellar media of galaxies with A_O ≡ 12 + log_(10)(O/H) > 8.1, grains contain a substantial fraction of interstellar Mg, Si, and Fe. Galaxies with A_O 8.1 have a median q_(PAH) = 3.55%. The derived dust masses favor a value X_(CO) ≈ 4 × 10^(20) cm^(-2) (K km s^(-1))^(-1) for the CO-to-H_2 conversion factor. Except for some starbursting systems (Mrk 33, Tol 89, NGC 3049), dust in the diffuse ISM dominates the IR power
Toward a global description of the nucleus-nucleus interaction
Extensive systematization of theoretical and experimental nuclear densities
and of optical potential strengths exctracted from heavy-ion elastic scattering
data analyses at low and intermediate energies are presented.The
energy-dependence of the nuclear potential is accounted for within a model
based on the nonlocal nature of the interaction.The systematics indicate that
the heavy-ion nuclear potential can be described in a simple global way through
a double-folding shape,which basically depends only on the density of nucleons
of the partners in the collision.The poissibility of extracting information
about the nucleon-nucleon interaction from the heavy-ion potential is
investigated.Comment: 12 pages,12 figure
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