Wednesday, June 5, 2013

1306.0645 (Michael L. Norman et al.)

Direct Numerical Simulation of Reionization in Large Cosmological Volumes I: Numerical Methods and Tests    [PDF]

Michael L. Norman, Daniel R. Reynolds, Geoffrey C. So, Robsert P. Harkness
We describe an extension of the {\em Enzo} code to enable the direct numerical simulation of inhomogeneous reionization in large cosmological volumes. By direct we mean all dynamical, radiative, and chemical properties are solved self-consistently on the same mesh, as opposed to a postprocessing approach which coarse-grains the radiative transfer. We do, however, employ a simple subgrid model for star formation, which we calibrate to observations. The numerical method presented is a modification of an earlier method presented in Reynolds et al. Radiation transport is done in the grey flux-limited diffusion (FLD) approximation, which is solved by implicit time integration split off from the gas energy and ionization equations, which are solved separately. This results in a faster and more robust scheme for cosmological applications compared to the earlier method. The FLD equation is solved using the {\em hypre} optimally scalable geometric multigrid solver from LLNL. By treating the ionizing radiation as a grid field as opposed to rays, our method is scalable with respect to the number of ionizing sources, limited only by the parallel scaling properties of the radiation solver. We test the speed and accuracy of our approach on a number of standard verification and validation tests. We show that the well-known inability of FLD to cast a shadow behind opaque clouds has little effect on the photoevaporation timescale of the cloud, or the evolution of ionized volume fraction in a reionization simulation validation test. We illustrate its application to the problem of inhomogeneous reionization in a 20 Mpc comoving box resolved with 800**3 Eulerian grid cells and dark matter particles.
View original:

No comments:

Post a Comment