The Sweep Method for radiative Transfer in Arepo

TL;DR

The paper introduces Sweep, a radiative transfer code for Arepo that efficiently handles large-scale cosmological simulations with many sources by solving the transfer equation under steady state assumptions using a task-parallel approach.

Contribution

It presents a novel discrete ordinates method for radiative transfer in unstructured grids, optimized for parallel computation and large cosmological simulations.

Findings

Successfully tested on physical scenarios like HII region expansion and shadow formation.

Achieves computational cost scaling with grid size, independent of source number.

Demonstrates high parallel efficiency in large-scale simulations.

Abstract

We introduce the radiative transfer code Sweep for the cosmological simulation suite Arepo. Sweep is a discrete ordinates method in which the radiative transfer equation is solved under the infinite speed of light, steady state assumption by a transport sweep across the entire computational grid. Since Arepo is based on an adaptive, unstructured grid, the dependency graph induced by the sweep dependencies of the grid cells is non-trivial. In order to solve the topological sorting problem in a distributed manner, we employ a task-based-parallelism approach. The main advantage of the sweep method is that the computational cost scales only with the size of the grid, and is independent of the number of sources or the distribution of sources in the computational domain, which is an advantage for radiative transfer in cosmological simulations, where there are large numbers of sparsely distributed sources. We successfully apply the code to a number of physical tests such as the expansion of HII regions, the formation of shadows behind dense objects, the scattering of light, as well as its behavior in the presence of periodic boundary conditions. In addition, we measure its computational performance with a focus on highly parallel, large-scale simulations.