TR3D:
by
Doris Folini and
Rolf Walder

Page contentsAbstractTransfer part Rate equation part Visualization History References Versions 
AbstractTR3D is part of the AMAZE code package which provides codes for astrophysical simulations.TR3D solves the 3D, optically thick, nonrelativistic, stationary, frequency decoupled, and unpolarized NLTE radiative transfer problem for moving media for a given density, temperature, and velocity distribution and given radiation sources. In its current form, TR3D is suited for mildly optically thick problems. The optical thickness over one spatial grid cell should not exceed a value of about 0.3 to 0.5. TR3D can be adapted to yet larger optical depths, but so far this has not yet been done. The solution of the optically thick NLTE radiative transfer problem in 3D generally needs quite a lot of memory. TR3D, in essence, requires you to store Transfer partIn TR3D the radiative transfer equation for nonrelativistic, stationary, frequency decoupled, and unpolarized radiative transfer is solved by a mean intensity approach.First, the radiative transfer equation has to be discretized. TR3D uses an equidistant Cartesian mesh and first order upwind finite differences for the advection term. To account for the direction dependence of the specific intensities the unite sphere in 3D space has to be discretized, discrete ordinates are introduced. This ordinate discretization is equidistant in longitude as well as in latitude, except that some ordinates are omitted towards the poles. Once discretized, we can proceed to the actual heart of TR3D, the mean intensity approach. The basic idea for this approach stems from Turek. The point is to rewrite the discretized transfer equation in such a way that it becomes an equation for the discretized mean intensity J rather than for the directed (or specific) intensities I. One ends up with a set of linear system of equations, AJ=E, one such set per discrete frequency to be treated. Here J is a vector containing the mean intensity for one particular frequency at each grid point, E is a vector containing, in essence, emission terms with the exception of scattering, and A is a rather full, nonsymmetric, definite matrix, which is given only implicitly. This system of equations we solve using a BiCGStab algorithm with diagonal preconditioning. Note that the above rewriting of the transfer equation requires an inversion of the matrix Thm. Such an inversion is efficiently possible as, for each discrete ordinate direction separately, the spatial grid can be renumbered such that the matrix Thm becomes lower triangular. Rate equation partThe rate equations or statistical equilibrium equations, along with some conservations laws, describe the level populations of the different atoms and ions. Actually a nonlinear system of ordinary differential equations, they are treated in TR3D as a nonlinear system of equations only. This corresponds to the assumption that the time scales governing the rate equations are much faster than any flow time scale.With the atomic processes considered within TR3D this system of equations has block structure and is nonlinear in the electron density only. Usually, the system is badly conditioned and the entries of the solution vector, the atomic level populations, easily differ by ten orders of magnitude. The system is solved by iteration, using the values of the previous iteration step to get rid of the nonlinearities in the present iteration step. For the solution of the then linearized system of equations Housholder transformations are used. Optically thick lines are taken into account using Sobolev coefficients, adapted to 3D. VisualizationVisualization of the output of TR3D mainly relies on AVS/Express. Dr. Jean Favre of the Swiss Center of Scientific Computing, CSCS, developed for our purpose some specific modules for use in AVS/Express, which allow the visualization of population numbers and radiation fields. Some rudimentary graphics based on IDL exists as well.History
ReferencesR. Walder and D. FoliniAMAZE: A code package to compute 3D magnetic flows, 3D NLTE radiative transfer, and synthetic spectra in Thermal and Ionization Aspects of Flows from Hot Stars: Observations and Theory, ASP Conference Series 204, p. 281284, 2000 (Available as a 47 KB gzipped psfile) D. Folini, R. Walder, Michael Psarros, and Alexandre Desboeufs A new method for 3D radiative transfer with adaptive grids in Stellar atmosphere modeling, PASP Conference Series, to appear 2003 (Available as a 68 KB gzipped psfile) D. Folini and R. Walder 3D radiative transfer under conditions of non local thermodynamic equilibrium: A contribution to the numerical solution in Hyperbolic Problems: Theory, Numerics, Applications; Editors: M.Fey and R.Jeltsch; p. 305314, 1999 (Available as a 95 KB gzipped psfile) D. Folini Computational approaches to multidimensional radiative transfer and the physics of radiative colliding flows PhD Thesis, ETH No. 12606, 1998 (Available as a 3.3 MB gzipped psfile) Available versionsTR3D: betaversion 
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Last Update: October 14, 2002 