72,402 research outputs found
Discontinuities in numerical radiative transfer
Observations and magnetohydrodynamic simulations of solar and stellar
atmospheres reveal an intermittent behavior or steep gradients in physical
parameters, such as magnetic field, temperature, and bulk velocities. The
numerical solution of the stationary radiative transfer equation is
particularly challenging in such situations, because standard numerical methods
may perform very inefficiently in the absence of local smoothness. However, a
rigorous investigation of the numerical treatment of the radiative transfer
equation in discontinuous media is still lacking. The aim of this work is to
expose the limitations of standard convergence analyses for this problem and to
identify the relevant issues. Moreover, specific numerical tests are performed.
These show that discontinuities in the atmospheric physical parameters
effectively induce first-order discontinuities in the radiative transfer
equation, reducing the accuracy of the solution and thwarting high-order
convergence. In addition, a survey of the existing numerical schemes for
discontinuous ordinary differential systems and interpolation techniques for
discontinuous discrete data is given, evaluating their applicability to the
radiative transfer problem
Time Dependent Monte Carlo Radiative Transfer Calculations For 3-Dimensional Supernova Spectra, Lightcurves, and Polarization
We discuss Monte-Carlo techniques for addressing the 3-dimensional
time-dependent radiative transfer problem in rapidly expanding supernova
atmospheres. The transfer code SEDONA has been developed to calculate the
lightcurves, spectra, and polarization of aspherical supernova models. From the
onset of free-expansion in the supernova ejecta, SEDONA solves the radiative
transfer problem self-consistently, including a detailed treatment of gamma-ray
transfer from radioactive decay and with a radiative equilibrium solution of
the temperature structure. Line fluorescence processes can also be treated
directly. No free parameters need be adjusted in the radiative transfer
calculation, providing a direct link between multi-dimensional hydrodynamical
explosion models and observations. We describe the computational techniques
applied in SEDONA, and verify the code by comparison to existing calculations.
We find that convergence of the Monte Carlo method is rapid and stable even for
complicated multi-dimensional configurations. We also investigate the accuracy
of a few commonly applied approximations in supernova transfer, namely the
stationarity approximation and the two-level atom expansion opacity formalism.Comment: 16 pages, ApJ accepte
A 3D radiative transfer framework: X. Arbitrary Velocity Fields in the Co-moving Frame
3-D astrophysical atmospheres will have random velocity fields. We seek to
combine the methods we have developed for solving the 1-D problem with
arbitrary flows to those that we have developed for solving the fully 3-D
relativistic radiative transfer problem in the case of monotonic flows. The
methods developed in the case of 3-D atmospheres with monotonic flows, solving
the fully relativistic problem along curves defined by an affine parameter, are
very flexible and can be extended to the case of arbitrary velocity fields in
3-D. Simultaneously, the techniques we developed for treating the 1-D problem
with arbitrary velocity fields are easily adapted to the 3-D problem. The
algorithm we present allows the solution of 3-D radiative transfer problems
that include arbitrary wavelength couplings. We use a quasi-analytic formal
solution of the radiative transfer equation that significantly improves the
overall computation speed. We show that the approximate lambda operator
developed in previous work gives good convergence, even neglecting wavelength
coupling. Ng acceleration also gives good results. We present tests that are of
similar resolution to what has been presented using Monte-Carlo techniques,
thus our methods will be applicable to problems outside of our test setup.
Additional domain decomposition parallelization strategies will be explored in
future work.Comment: 9 pages, 9 figures, A&A, in press, new version matches copy edited
version, definition restore
A 3D radiative transfer framework: XI. multi-level NLTE
Multi-level non-local thermodynamic equilibrium (NLTE) radiation transfer
calculations have become standard throughout the stellar atmospheres community
and are applied to all types of stars as well as dynamical systems such as
novae and supernovae. Even today spherically symmetric 1D calculations with
full physics are computationally intensive. We show that full NLTE calculations
can be done with fully 3 dimensional (3D) radiative transfer. With modern
computational techniques and current massive parallel computational resources,
full detailed solution of the multi-level NLTE problem coupled to the solution
of the radiative transfer scattering problem can be solved without sacrificing
the micro physics description. We extend the use of a rate operator developed
to solve the coupled NLTE problem in spherically symmetric 1D systems. In order
to spread memory among processors we have implemented the NLTE/3D module with a
hierarchical domain decomposition method that distributes the NLTE levels,
radiative rates, and rate operator data over a group of processes so that each
process only holds the data for a fraction of the voxels. Each process in a
group holds all the relevant data to participate in the solution of the 3DRT
problem so that the 3DRT solution is parallelized within a domain decomposition
group. We solve a spherically symmetric system in 3D spherical coordinates in
order to directly compare our well-tested 1D code to the 3D case. We compare
three levels of tests: a) a simple H+He test calculation, b) H+He+CNO+Mg, c)
H+He+Fe. The last test is computationally large and shows that realistic
astrophysical problems are solvable now, but they do require significant
computational resources. With presently available computational resources it is
possible to solve the full 3D multi-level problem with the same detailed
micro-physics as included in 1D modeling.Comment: 20 pages, 14 figures, A&A, in pres
Development of an infrared radiative heating model
Infrared radiative transfer solution algorithms used in global circulation models were assessed. Computation techniques applicable to the Ames circulation model are identified. Transmission properties of gaseous CO2, H2O, and O3 are gathered, and a computer program is developed, using the line parameter tape and Voight profile subroutine, which computes the transmission of CO2, H2O, and O3. A computer code designed to compute atmospheric cooling rates was developed
Analysis of the Discrete Theory of Radiative Transfer in the Coupled "Ocean-Atmosphere" System: Current Status, Problems and Development Prospects
In this paper, we analyze the current state of the discrete theory of radiative transfer. One-dimensional, three-dimensional and stochastic radiative transfer models are considered. It is shown that the discrete theory provides a unique solution to the one-dimensional radiative transfer equation. All approximate solution techniques based on the discrete ordinate formalism can be derived based on the synthetic iterations, the small-angle approximation, and the matrix operator method. The possible directions for the perspective development of radiative transfer are outlined
Multidimensional Modeling of Atmospheric Effects and Surface Heterogeneities on Remote Sensing
The overall goal of this project is to establish a modeling capability that allows a quantitative determination of atmospheric effects on remote sensing including the effects of surface heterogeneities. This includes an improved understanding of aerosol and haze effects in connection with structural, angular, and spatial surface heterogeneities. One important objective of the research is the possible identification of intrinsic surface or canopy characteristics that might be invariant to atmospheric perturbations so that they could be used for scene identification. Conversely, an equally important objective is to find a correction algorithm for atmospheric effects in satellite-sensed surface reflectances. The technical approach is centered around a systematic model and code development effort based on existing, highly advanced computer codes that were originally developed for nuclear radiation shielding applications. Computational techniques for the numerical solution of the radiative transfer equation are adapted on the basis of the discrete-ordinates finite-element method which proved highly successful for one and two-dimensional radiative transfer problems with fully resolved angular representation of the radiation field
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