21,453 research outputs found
Investigation of the fiber reinforcement of a cobalt base alloy for application at elevated temperature
Technique developed for incorporating alumina and silicon carbide fibers in cobalt base alloy for application at high temperature
Adaptive mesh refinement with spectral accuracy for magnetohydrodynamics in two space dimensions
We examine the effect of accuracy of high-order spectral element methods,
with or without adaptive mesh refinement (AMR), in the context of a classical
configuration of magnetic reconnection in two space dimensions, the so-called
Orszag-Tang vortex made up of a magnetic X-point centered on a stagnation point
of the velocity. A recently developed spectral-element adaptive refinement
incompressible magnetohydrodynamic (MHD) code is applied to simulate this
problem. The MHD solver is explicit, and uses the Elsasser formulation on
high-order elements. It automatically takes advantage of the adaptive grid
mechanics that have been described elsewhere in the fluid context [Rosenberg,
Fournier, Fischer, Pouquet, J. Comp. Phys. 215, 59-80 (2006)]; the code allows
both statically refined and dynamically refined grids. Tests of the algorithm
using analytic solutions are described, and comparisons of the Orszag-Tang
solutions with pseudo-spectral computations are performed. We demonstrate for
moderate Reynolds numbers that the algorithms using both static and refined
grids reproduce the pseudo--spectral solutions quite well. We show that
low-order truncation--even with a comparable number of global degrees of
freedom--fails to correctly model some strong (sup--norm) quantities in this
problem, even though it satisfies adequately the weak (integrated) balance
diagnostics.Comment: 19 pages, 10 figures, 1 table. Submitted to New Journal of Physic
Characterization of soft-bottom benthic habitats of the Aland islands, northern Baltic sea
Sediment surface and profile imaging (SPI) was used in combination with grab sampling of sediment (sediment type, organic content, benthic infauna) and hydrography (temperature, oxygen saturation of bottom water) to analyze and describe the soft-bottom benthic habitats of the Aland archipelago (60 degrees 00\u27 to 60 degrees 30\u27 N, 19 degrees 30\u27 to 20 degrees 30\u27 E) in the northern Baltic Sea. The SPI analysis covered 42 stations (5 to 263 m depth), from inner sheltered bays to open coastal waters, with varying sediment types (soft mud with high organic content to sandy substrates with low organic content; loss on ignition: 0.5 to 12.4%). Clustering of the sampled stations (sediment properties) yielded 3 distinct categories of sedimentary habitats: (1) inner archipelago areas and bays with high organic content of the sediment and reduced oxygen saturation in the bottom water, (2) archipelago waters with intermediate values of all analyzed parameters, and (3) open coastal sediments with low organic content and high oxygen saturation (2 deep offshore stations formed an additional group based primarily on depth). Visual analysis of the images provided information on several additional abiotic and biotic characteristics of the sediment, and significant correlations were found mainly between oxygen saturation, organic content, sediment type, shear strength (penetration of gear), surface relief and the depth of the redox potential discontinuity layer in the sediment. The sediment properties were also reflected in the zoobenthos. The correlations between parameters measured are discussed in relation to applicability of the SPI method, monitoring demands, and basic understanding of the sediment-animal relationships
Ruthenium and osmium carbonyl clusters incorporating stannylene and stannyl ligands
The reaction of [Ruâ (CO)ââ] with PhâSnSPh in refluxing benzene furnished the bimetallic Ru-Sn compound [Ruâ(CO)â(Îź-SPh)â(Îź3-SnPhâ)(SnPhâ)â] 1 which consists of a SnPhâ stannylene bonded to three Ru atoms to give a planar tetra-metal core, with two peripheral SnPhâ ligands. The stannylene ligand forms a very short bond to one Ru atom [Sn-Ru 2.538(1) Ă
] and very long bonds to the other two [Sn-Ru 3.074(1) Ă
]. The germanium compound [Ruâ(CO)â(Îź-SPh)â(Îźâ-GePhâ)(GePhâ)â] 2 was obtained from the reaction of [Ruâ (CO)ââ] with PhâGeSPh and has a similar structure to that of 1 as evidenced by spectroscopic data. Treatment of [Osâ(CO)ââ(MeCN)â] with PhâSnSPh in refluxing benzene yielded the bimetallic Os-Sn compound [Osâ(CO)â(Îź-SPh)(Îźâ-SnPhâ)(MeCN)(Ćš-CâHâ
)] 3. Cluster 3 has a superficially similar planar metal core, but with a different bonding mode with respect to that of 1. The PhâSn group is bonded most closely to Os(2) and Os(3) [2.7862(3) and 2.7476(3) Ă
respectively] with a significantly longer bond to Os(1), 2.9981(3) Ă
indicating a weak back-donation to the Sn. The reaction of the bridging dppm compound [Ruâ(CO)ââ(Îź-dppm)] with PhâSnSPh afforded [Ruâ(CO)â(Îź-dppm)(Îźâ-S)(Îźâ-SPh)(SnPhâ)] 5. Compound 5 contains an open triangle of Ru atoms simultaneously capped by a sulfido and a PhS ligand on opposite sides of the cluster with a dppm ligand bridging one of the Ru-Ru edges and a PhâSn group occupying an axial position on the Ru atom not bridged by the dppm ligand
Molecular gas heating in Arp 299
Understanding the heating and cooling mechanisms in nearby (Ultra) luminous
infrared galaxies can give us insight into the driving mechanisms in their more
distant counterparts. Molecular emission lines play a crucial role in cooling
excited gas, and recently, with Herschel Space Observatory we have been able to
observe the rich molecular spectrum. CO is the most abundant and one of the
brightest molecules in the Herschel wavelength range. CO transitions are
observed with Herschel, and together, these lines trace the excitation of CO.
We study Arp 299, a colliding galaxy group, with one component harboring an AGN
and two more undergoing intense star formation. For Arp 299 A, we present PACS
spectrometer observations of high-J CO lines up to J=20-19 and JCMT
observations of CO and HCN to discern between UV heating and alternative
heating mechanisms. There is an immediately noticeable difference in the
spectra of Arp 299 A and Arp 299 B+C, with source A having brighter high-J CO
transitions. This is reflected in their respective spectral energy line
distributions. We find that photon-dominated regions (PDRs) are unlikely to
heat all the gas since a very extreme PDR is necessary to fit the high-J CO
lines. In addition, this extreme PDR does not fit the HCN observations, and the
dust spectral energy distribution shows that there is not enough hot dust to
match the amount expected from such an extreme PDR. Therefore, we determine
that the high-J CO and HCN transitions are heated by an additional mechanism,
namely cosmic ray heating, mechanical heating, or X-ray heating. We find that
mechanical heating, in combination with UV heating, is the only mechanism that
fits all molecular transitions. We also constrain the molecular gas mass of Arp
299 A to 3e9 Msun and find that we need 4% of the total heating to be
mechanical heating, with the rest UV heating
Evidence for Proportionate Partition Between the Magnetic Field and Hot Gas in Turbulent Cassiopeia A
We present a deep X-ray observation of the young Galactic supernova remnant
Cas A, acquired with the ROSAT High Resolution Imager. This high dynamic range
(232 ks) image reveals low-surface-brightness X-ray structure, which appears
qualitatively similar to corresponding radio features. We consider the
correlation between the X-ray and radio morphologies and its physical
implications. After correcting for the inhomogeneous absorption across the
remnant, we performed a point by point (4" resolution) surface brightness
comparison between the X-ray and radio images. We find a strong (r = 0.75)
log-log correlation, implying an overall relationship of . This is
consistent with proportionate partition (and possibly equipartition) between
the local magnetic field and the hot gas --- implying that Cas A's plasma is
fully turbulent and continuously amplifying the magnetic field.Comment: 8 pages with embedded bitmapped figures, Accepted by ApJ Letters
5/1/9
The association of attention deficit hyperactivity disorder with socioeconomic disadvantage: alternative explanations and evidence
addresses: ESRC Centre for Genomics in Society (Egenis) & Institute of Health Research, University of Exeter Medical School, Exeter, UK.OnlineOpen Article. This is a copy of an article published in the Journal of Child Psychology and Psychiatry. This journal is available online at: http://onlinelibrary.wiley.com/journal/10.1111/(ISSN)1469-7610Studies throughout Northern Europe, the United States and Australia have found an association between childhood attention deficit hyperactivity disorder (ADHD) and family socioeconomic disadvantage. We report further evidence for the association and review potential causal pathways that might explain the link.ESRCâs Secondary Data
Analysis InitiativeNational Institute for Health Research (NIHR) Collaboration
for Leadership in Applied Health Research
and Care (CLAHRC) for the South West Peninsul
A 10 GHz Quasi-Optical Grid Amplifier Using Integrated HBT Differential Pairs
We report the fabrication and testing of a 10 GHz grid amplifier utilizing sixteen GaAs chips each
containing an HBT differential pair plus integral bias/feedback resistors. The overall amplifier consists of
a 4x4 array of unit cells on an RT Duroid⢠board having a relative permittivity of 2.2. Each unit cell
consists of an emitter-coupled differential pair at the center, an input antenna which extends horizontally
in both directions from the two base leads, an output antenna which extends vertically in both directions
from the two collector leads, and high inductance bias lines. In operation, the active grid array is placed
between a pair of crossed polarizers. The horizontally polarized input wave passes through the input
polarizer and couples to the input leads. An amplified current then flows on the vertical leads, which
radiate a vertically polarized amplified signal through the output polarizer. The polarizers serve dual
functions, providing both input-output isolation as well as independent impedance matching for the input
and output ports. The grid thus functions essentially as a free-space beam amplifier. Calculations indicate
that output powers of several watts per square centimeter of grid area should be attainable with optimized
structures
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