Twoflux

Twoflux is a term used in radiative transfer to denote a two-flux, or two-stream, approximation that models radiant energy within a participating medium by splitting it into an upward and a downward flux. By reducing the angular dependence of radiation to two opposing directions, the method yields tractable equations for net flux, heating rates, and energy balance. It is commonly employed where full angular discretization is computationally expensive, such as atmospheric modeling, solar thermal systems, and astrophysical contexts.

The two-flux approach is one of several two-stream methods. It originated in transport theory and has been developed in parallel with diffusion and Eddington approximations. Variants differ in closure relations and boundary treatments, but all share the aim of a simple yet robust description of radiative transfer in layered or semi-infinite media.

Twoflux, the software library, implements two-flux solvers for radiative transport and related energy-transport problems. It provides modular solvers for steady and transient problems, multi-band spectra, and coupling with hydrodynamics or computational fluid dynamics codes. The library supports C++ and Python interfaces, parallel execution, and common mesh and boundary conditions. It is distributed under an open-source license and is designed to integrate with existing simulation workflows and data formats.

Twoflux is used by researchers in climate science, combustion and furnace design, and solar energy engineering. Development is community-driven, with documentation, tutorials, and example models available on the project site.