CASTRO: A New Compressible Astrophysical Solver. III. Multigroup Radiation Hydrodynamics

TL;DR

This paper introduces a multigroup radiation hydrodynamics formulation and a numerical solver implemented in CASTRO, capable of accurately modeling radiation-fluid interactions with adaptive mesh refinement for astrophysical simulations.

Contribution

It presents a novel multigroup radiation solver using a split approach with explicit and implicit schemes, integrated into the CASTRO code for astrophysical applications.

Findings

The solver accurately models radiation and fluid coupling.

It handles both neutrino and photon radiation.

The adaptive mesh refinement enhances computational efficiency.

Abstract

We present a formulation for multigroup radiation hydrodynamics that is correct to order $O(v/c)$ using the comoving-frame approach and the flux-limited diffusion approximation. We describe a numerical algorithm for solving the system, implemented in the compressible astrophysics code, CASTRO. CASTRO uses an Eulerian grid with block-structured adaptive mesh refinement based on a nested hierarchy of logically-rectangular variable-sized grids with simultaneous refinement in both space and time. In our multigroup radiation solver, the system is split into three parts, one part that couples the radiation and fluid in a hyperbolic subsystem, another part that advects the radiation in frequency space, and a parabolic part that evolves radiation diffusion and source-sink terms. The hyperbolic subsystem and the frequency space advection are solved explicitly with high-order Godunov schemes, whereas the parabolic part is solved implicitly with a first-order backward Euler method. Our multigroup radiation solver works for both neutrino and photon radiation.