We have described a framework for solving three-dimensional radiative transfer
problems in scattering dominated environments. The method uses a non-local
operator splitting technique to solve the scattering problem. The formal
solution is based on a long characteristic piece-wise parabolic procedure. For
strongly scattering dominated test cases (sphere in a box) we find good
convergence with non-local
operators, as well as minimal numerical
diffusion with the long characteristics method and adequate resolution. A
simple MPI parallelization gives excellent speedups on parallel clusters.
In subsequent work we will implement a domain decomposition method to
allow much larger spatial grids. Presently, we have implemented
the method for static media, it can be used without significant changes to
solve problems in the Eulerian-frame for media with low velocity fields.
The distribution of matter over the voxels is,
in the general 3D case, arbitrary. We chose a spherical test case
to be able to compare the results our 1D code.
In Figs. 19 and 20 we compare
the results for the
test case using a simple implementation
of the short characteristics method and the long characteristics method used
in this paper. The test grid contains
voxel and
angular points.
The high diffusivity of the SC method is evident. Other authors
(Auer et al., 1994; Steiner, 1991; Trujillo Bueno & Fabiani Bendicho, 1995; Vath, 1994; van Noort et al., 2002; Fabiani Bendicho et al., 1997) have used SC methods in
multi-dimensional radiative transport problems. Short characteristics
techniques are faster but require special considerations to reduce numerical
diffusion (Auer, 2003).
We may further look into the SC method in later papers.
We have generalized the operator splitting to include larger bandwidth
operators. They lead to faster convergence although they do require more memory
and ultimately more computing time. Nevertheless, they will be useful for
highly complex problems and we have developed a highly flexible approach to the
construction of the
operator so that the bandwidth may be set for each
spatial point individually as the problem and computational resources require.
We have designed an especially general and flexible framework for 3D radiative transfer problems with scattering. In future papers of this series we will describe its extension to line transfer problems, multi-level NLTE calculations, and differentially moving flows.