598 research outputs found
PDF model based on Langevin equation for polydispersed two-phase flows applied to a bluff-body gas-solid flow,
The aim of the paper is to discuss the main characteristics of a complete
theoretical and numerical model for turbulent polydispersed two-phase flows,
pointing out some specific issues. The theoretical details of the model have
already been presented [Minier and Peirano, Physics Reports, Vol. 352/1-3, 2001
]. Consequently, the present work is mainly focused on complementary aspects,
that are often overlooked and that require particular attention. In particular,
the following points are analysed : the necessity to add an extra term in the
equation for the velocity of the fluid seen in the case of twoway coupling, the
theoretical and numerical evaluations of particle averages and the fulfilment
of the particle mass-continuity constraint. The theoretical model is developed
within the PDF formalism. The important-physical choice of the state vector
variables is first discussed and the model is then expressed as a stochastic
differential equation (SDE) written in continuous time (Langevin equations) for
the velocity of the fluid seen. The interests and limitations of Langevin
equations, compared to the single-phase case, are reviewed. From the numerical
point of view, the model corresponds to an hybrid Eulerian/Lagrangian approach
where the fluid and particle phases are simulated by different methods.
Important aspects of the Monte Carlo particle/mesh numerical method are
emphasised. Finally, the complete model is validated and its performance is
assessed by simulating a bluff-body case with an important recirculation zone
and in which two-way coupling is noticeable.Comment: 23 pages, 10 figure
NG7538 IRS1 N: modeling a circumstellar maser disk
We present an edge-on Keplerian disk model to explain the main component of
the 12.2 and 6.7 GHz methanol maser emission detected toward NGC7538-IRS1 N.
The brightness distribution and spectrum of the line of bright masers are
successfully modeled with high amplification of background radio continuum
emission along velocity coherent paths through a maser disk. The bend seen in
the position-velocity diagram is a characteristic signature of differentially
rotating disks. For a central mass of 30 solar masses, suggested by other
observations, our model fixes the masing disk to have inner and outer radii of
about 270 AU and 750 AU.Comment: To appear in The Proceedings of the 2004 European Workshop: "Dense
Molecular Gas around Protostars and in Galatic Nuclei", Eds. Y. Hagiwara,
W.A. Baan, H.J. van Langevelde, 2004, a special issue of ApSS, Kluwe
VLBA imaging of a periodic 12.2 GHz methanol maser flare in G9.62+0.20E
The class II methanol maser source G9.62+0.20E undergoes periodic flares at
both 6.7 and 12.2 GHz. The flare starting in 2001 October was observed at seven
epochs over three months using the VLBA at 12.2 GHz. High angular resolution
images (beam size 1.7 x 0.6 mas) were obtained, enabling us to observe
changes in 16 individual maser components. It was found that while existing
maser spots increased in flux density, no new spots developed and no changes in
morphology were observed. This rules out any mechanism which disturbs the
masing region itself, implying that the flares are caused by a change in either
the seed or pump photon levels. A time delay of 1--2 weeks was observed between
groups of maser features. These delays can be explained by light travel time
between maser groups. The regularity of the flares can possibly be explained by
a binary system.Comment: 11 pages, accepted for publication in MNRA
Opportunities for maser studies with the Square Kilometre Array
The Square Kilometre Array (SKA) is the radio telescope of the next
generation, providing an increase in sensitivity and angular resolution of two
orders of magnitude over existing telescopes. Currently, the SKA is expected to
span the frequency range 0.1-25 GHz with capabilities including a wide
field-of-view and measurement of polarised emission. Such a telescope has
enormous potential for testing fundamental physical laws and producing
transformational discoveries. Important science goals include using H2O
megamasers to make precise estimates of H0, which will anchor the extragalactic
distance scale, and to probe the central structures of accretion disks around
supermassive black holes in AGNs, to study OH megamasers associated with
extreme starburst activity in distant galaxies and to study with unprecedented
precision molecular gas and star formation in our Galaxy.Comment: 5 pages, to appear in: IAU Symposium 242 Astrophysical Masers and
their Environment
19F NMR study of ligand dynamics in carboxylate-bridged diiron(II) complexes supported by a macrocyclic ligand
A series of asymmetrically carboxylate-bridged diiron(II) complexes featuring fluorine atoms as NMR spectroscopic probes, [Fe[subscript 2](PIM)(Ar[superscript 4F-Ph]CO[subscript 2])[subscript 2]] (10), [Fe[subscript 2](F[subscript 2]PIM)(Ar[superscript Tol]CO[subscript 2])[subscript 2]] (11), and [Fe[subscript 2](F[subscript 2]PIM)(Ar[superscript 4F-Ph]CO[subscript 2])[superscript 2]] (12), were prepared and characterized by X-ray crystallography, Mössbauer spectroscopy, and VT [superscript19]F NMR spectroscopy. These complexes are part of a rare family of syn-N diiron(II) complexes, [Fe[subscript 2](X[subscript 2]PIM)(RCO[subscript 2])[superscript 2]], that are structurally very similar to the active site of the hydroxylase enzyme component of reduced methane monooxygenase (MMOH[subscript red]). Solution characterization of these complexes demonstrates that they undergo intramolecular carboxylate rearrangements, or carboxylate shifts, a dynamic feature relevant to the reactivity of the diiron centers in bacterial multicomponent monooxygenasesNational Institute of General Medical Sciences (U.S.) (Grant GM 32114)National Science Foundation (U.S.). Graduate Research Fellowship Program (Grant 1122374
The Extended Methanol Maser Emission in W51
The European VLBI Network (EVN) has been used to make phase referenced,
wide-field (several arcminute) spectral line observations of the 6.7-GHz
methanol maser emission towards W51. In the W51main region, the bulk of the
methanol is offset from an UCHII region. This probably indicates the methanol
emission arises at the interface of the expanding UCHII region and not from an
edge-on circumstellar disc, as previously suggested. Near the W51 IRS2 region,
the methanol emission is associated with a very compact, extremely embedded
source supporting the hypothesis that methanol masers trace the earliest stages
of massive star formation. As well as these two previously well studied sites
of star formation, many previously unknown regions star formation are detected,
demonstrating that methanol masers are powerful means of detection young
massive stars.Comment: 5 pages, 3 figure
Dynamics of the 6.7 and 12.2 GHz methanol masers around Cepheus A HW2
The 6.7 GHz methanol maser is exclusively associated with high-mass star
formation. However, it remains unclear what structures harbour the methanol
masers. Cepheus A is one of the closest regions of massive star formation,
making it an excellent candidate for detailed studies. We determine the
dynamics of maser spots in the high-mass star-forming region Cepheus A in order
to infer where and when the maser emission occurs. Very long baseline
interferometry (VLBI) observations of the 6.7 and 12.2 GHz methanol masers
allows for mapping their spatial and velocity distribution. Phase-referencing
is used to determine the astrometric positions of the maser emission, and
multi-epoch observations can reveal 3D motions. The 6.7 GHz methanol masers are
found in a filamentary structure over ~1350 AU, straddling the waist of the
radio jet HW2. The positions agree well with previous observations of both the
6.7 and 12.2 GHz methanol masers. The velocity field of the maser spots does
not show any sign of rotation, but is instead consistent with an infall
signature. The 12.2 GHz methanol masers are closely associated with the 6.7 GHz
methanol masers, and the parallax that we derive confirms previous
measurements. We show that the methanol maser emission very likely arises in a
shock interface in the equatorial region of Cepheus A HW2 and presents a model
in which the maser emission occurs between the infalling gas and the accretion
disk/process.Comment: 9 pages, 5 figures; accepted for publication in Astronomy and
Astrophysic
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