Guangqi: A 2D Radiation Hydrodynamic Code with Realistic Equations of State

Abstract We present Guangqi , a new 2D, finite-volume radiation hydrodynamics code designed for high-performance astrophysical simulations. The code simultaneously resolves the hydrodynamic equations for complex equations of state (EoS) and implicit radiation transport under the flux-limited-diffusion approximation. Written in Fortran and parallelized via the Message Passing Interface, Guangqi supports analytic hydrogen and helium EoS under the assumption of local thermal and chemical equilibrium. The framework is compatible with both Cartesian and spherical-polar geometries—utilizing nonuniform grid spacing—and incorporates static mesh refinement and adaptive mesh refinement to optimize computational efficiency. To address the inherent challenges of angular momentum conservation in spherical-polar coordinates, we implement a robust and consistent “passive scalar angular momentum algorithm.” Domain decomposition is managed through both Z-order and Hilbert space-filling curves to ensure scalability. The code has been rigorously verified against a suite of standard benchmarks and newly designed test cases specifically intended to diagnose the nonlinear coupling between gas dynamics, intricate EoS, radiation transport, and angular momentum conservation.