1,140 research outputs found
Multi-dimensional radiative transfer to analyze Hanle effect in Ca {\sc ii} K line at 3933 \AA\,
Radiative transfer (RT) studies of the linearly polarized spectrum of the Sun
(the second solar spectrum) have generally focused on the line formation, with
an aim to understand the vertical structure of the solar atmosphere using
one-dimensional (1D) model atmospheres. Modeling spatial structuring in the
observations of the linearly polarized line profiles requires the solution of
multi-dimensional (multi-D) polarized RT equation and a model solar atmosphere
obtained by magneto-hydrodynamical (MHD) simulations of the solar atmosphere.
Our aim in this paper is to analyze the chromospheric resonance line Ca {\sc
ii} K at 3933 \AA\ using multi-D polarized RT with Hanle effect and partial
frequency redistribution in line scattering. We use an atmosphere which is
constructed by a two-dimensional snapshot of the three-dimensional MHD
simulations of the solar photosphere, combined with columns of an 1D atmosphere
in the chromosphere. This paper represents the first application of polarized
multi-D RT to explore the chromospheric lines using multi-D MHD atmospheres,
with PRD as the line scattering mechanism. We find that the horizontal
inhomogeneities caused by MHD in the lower layers of the atmosphere are
responsible for strong spatial inhomogeneities in the wings of the linear
polarization profiles, while the use of horizontally homogeneous chromosphere
(FALC) produces spatially homogeneous linear polarization in the line core
Pressure and current balance conditions during electron beam injections from spacecraft
Electrostatic charging level of a conducting surface in response to injections of electron beams into space plasma is investigated by means of one-dimensional Vlasov code. Injections of Maxwellian beams into a vacuum shows that the surface can charge up to an electric potential phi sub s greater than W sub b, where W sub b is the average electron beam energy. Since Maxwellian beams have extended trails with electrons having energies greater than W sub b, it is difficult to quantify the charging level in terms of the energies of the injected electrons. In order to quantitatively understand the charging in excess of W sub b, simulations were carried out for water-bag types of beam with velocity distribution functions described by f(V) = A for V sub min approx. less than V approx. less than V sub max and f(V) = O otherwise, where A is a constant making the normalized beam density unity. It is found that V sub max does not directly determine the charging level. The pressure distribution in the electron sheath determines the electric field distribution near the surface. The electric field in turn determines the electrostatic potential of the vehicle. The pressure distribution is determined by the beam parameters such as the average beam velocity and the velocity spread of the beam
Sensitivity and optimization of composite structures using MSC/NASTRAN
Design sensitivity analysis for composites will soon be available in MSC/NASTRAN. The design variables for composites can be lamina thicknesses, orientation angles, material properties or a combination of all three. With the increasing use of composites in aerospace and automotive industries, this general capability can be used in its own right for carrying out sensitivity analysis of complicated real-life structures. As part of a research effort, the sensitivity analysis was coupled with a general purpose optimizer. This preliminary version of the optimizer is capable of dealing with minimum weight structural design with a rather general design variable linking capability at the element level or system level. Only sizing type of design variables (i.e., lamina thicknesses) can be handled by the optimizer. Test cases were run and validated by comparison with independent finite element packages. The linking of design sensitivity capability for composites in MSC/NASTRAN with an optimizer would give designers a powerful automated tool to carry out practical opitmization design of real-life complicated composite structures
Polarized Line Formation in Multi-Dimensional Media.III. Hanle Effect with Partial Frequency Redistribution
In the previous two papers, namely, \citet{anuknn11} and \citet{anuetal11} we
solved the polarized radiative transfer (RT) equation in multi-dimensional
(multi-D) geometries, with partial frequency redistribution (PRD) as the
scattering mechanism. We assumed Rayleigh scattering as the only source of
linear polarization () in both these papers. In this paper we extend
these previous works to include the effect of weak oriented magnetic fields
(Hanle effect) on line scattering. We generalize the technique of Stokes vector
decomposition in terms of the irreducible spherical tensors ,
developed in \citet{anuknn11}, to the case of RT with Hanle effect. A fast
iterative method of solution (based on the Stabilized Preconditioned
Bi-Conjugate-Gradient technique), developed in \citet{anuetal11}, is now
generalized to the case of RT in magnetized three-dimensional media. We use the
efficient short-characteristics formal solution method for multi-D media,
generalized appropriately to the present context. The main results of this
paper are the following: (1) A comparison of emergent profiles
formed in one-dimensional (1D) media, with the corresponding emergent,
spatially averaged profiles formed in multi-D media, shows that in the
spatially resolved structures, the assumption of 1D may lead to large errors in
linear polarization, especially in the line wings. (2) The multi-D RT in
semi-infinite non-magnetic media causes a strong spatial variation of the
emergent profiles, which is more pronounced in the line wings. (3)
The presence of a weak magnetic field modifies the spatial variation of the
emergent profiles in the line core, by producing significant
changes in their magnitudes.Comment: 31 pages, 14 figures, Submitted to ApJ, Under revie
Grid sensitivity capability for large scale structures
The considerations and the resultant approach used to implement design sensitivity capability for grids into a large scale, general purpose finite element system (MSC/NASTRAN) are presented. The design variables are grid perturbations with a rather general linking capability. Moreover, shape and sizing variables may be linked together. The design is general enough to facilitate geometric modeling techniques for generating design variable linking schemes in an easy and straightforward manner. Test cases have been run and validated by comparison with the overall finite difference method. The linking of a design sensitivity capability for shape variables in MSC/NASTRAN with an optimizer would give designers a powerful, automated tool to carry out practical optimization design of real life, complicated structures
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