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Closure Relation (closure + relation)

Distribution by Scientific Domains

Physics and Astronomy	50%
Chemistry	50%

Selected Abstracts

Variational treatment of the vibrational Hamiltonian for NH3 and H2NO

JOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 5 2002
Philippe Marsal
Abstract The full vibrational Hamiltonian for the inversion of NH3 and H2NO has been diagonalized in a basis set that is the direct product of functions of the inversion coordinate and of harmonic vibrational functions independent of this inversion coordinate. The kinetic part of the Hamiltonian matrix is constructed with the use of the closure relation for these vibrational functions. The method is tested with the potential function which is supposed to be harmonic for the vibrations orthogonal to the inversion coordinate: the first computed levels are in good agreement with experimental levels for NH3. For higher levels, anharmonic terms should be included. © 2002 Wiley Periodicals, Inc. J Comput Chem 23: 541,547, 2002; DOI 10.1002/jcc.10033 [source]

An implementation of radiative transfer in the cosmological simulation code gadget

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 3 2009
Margarita Petkova
ABSTRACT We present a novel numerical implementation of radiative transfer in the cosmological smoothed particle hydrodynamics (SPH) simulation code gadget. It is based on a fast, robust and photon-conserving integration scheme where the radiation transport problem is approximated in terms of moments of the transfer equation and by using a variable Eddington tensor as a closure relation, following the Optically Thin Variable Eddington Tensor suggestion of Gnedin & Abel. We derive a suitable anisotropic diffusion operator for use in the SPH discretization of the local photon transport, and we combine this with an implicit solver that guarantees robustness and photon conservation. This entails a matrix inversion problem of a huge, sparsely populated matrix that is distributed in memory in our parallel code. We solve this task iteratively with a conjugate gradient scheme. Finally, to model photon sink processes we consider ionization and recombination processes of hydrogen, which is represented with a chemical network that is evolved with an implicit time integration scheme. We present several tests of our implementation, including single and multiple sources in static uniform density fields with and without temperature evolution, shadowing by a dense clump and multiple sources in a static cosmological density field. All tests agree quite well with analytical computations or with predictions from other radiative transfer codes, except for shadowing. However, unlike most other radiative transfer codes presently in use for studying re-ionization, our new method can be used on-the-fly during dynamical cosmological simulation, allowing simultaneous treatments of galaxy formation and the re-ionization process of the Universe. [source]

Searching for Structural Variability in Sgr A*

ASTRONOMISCHE NACHRICHTEN, Issue S1 2003
Zhi-Qiang Shen
Abstract A model fitting procedure for estimating the parameters of an elliptical Gaussian model that describes the radio emission of Sgr A* observed by the VLBA is presented. By (implicitly) using the amplitude closure relation while fitting the amplitude, the procedure can minimize the calibration errors in millimeter wavelength VLBI measurements, which are crucial to our ongoing effort to search for the structural variability in Sgr A*. The preliminary results from the application to seven-epoch ,7mm VLBA observations seem to show the sign of change to the source apparent structure in at least one epoch over the period of 7 years (1994,2001). Because of the large uncertainties in the determination of minor axis caused mainly by the poor spatial resolution along the north-south direction with the VLBA, these results are suggestive but not conclusive. This demonstrates the necessity of adding the NRAO GBT antenna to the future ,7mm VLBA observations, which can greatly improve the resolution in the north-south by a factor of 3. [source]

Comparative analysis of CFD models of dense gas,solid systems

AICHE JOURNAL, Issue 5 2001
B. G. M. van Wachem
Many gas,solid CFD models have been put forth by academic researchers, government laboratories, and commercial vendors. These models often differ in terms of both the form of the governing equations and the closure relations, resulting in much confusion in the literature. These various forms in the literature and in commercial codes are reviewed and the resulting hydrodynamics through CFD simulations of fluidized beds compared. Experimental data on fluidized beds of Hilligardt and Werther (1986), Kehoe and Davidson (1971), Darton et al.(1977), and Kuipers (1990) are used to quantitatively assess the various treatments. Predictions based on the commonly used governing equations of Ishii (1975) do not differ from those of Anderson and Jackson (1967) in terms of macroscopic flow behavior, but differ on a local scale. Flow predictions are not sensitive to the use of different solid stress models or radial distribution functions, as different approaches are very similar in dense flow regimes. The application of a different drag model, however, significantly impacts the flow of the solids phase. A simplified algebraic granular energy-balance equation is proposed for determining the granular temperature, instead of solving the full granular energy balance. This simplification does not lead to significantly different results, but it does reduce the computational effort of the simulations by about 20%. [source]