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The development and implementation of GEAR-RT, a radiative transfer solverusing the M1 closure in the open source code SWIFT, is presented, and validatedusing standard tests for radiative transfer. GEAR-RT is modeled after RAMSES-RT(Rosdahl et al. 2013) with some key differences. Firstly, while RAMSES-RT usesFinite Volume methods and an Adaptive Mesh Refinement (AMR) strategy, GEAR-RTemploys particles as discretization elements and solves the equations using aFinite Volume Particle Method (FVPM). Secondly, GEAR-RT makes use of thetask-based parallelization strategy of SWIFT, which allows for optimized loadbalancing, increased cache efficiency, asynchronous communications, and adomain decomposition based on work rather than on data. GEAR-RT is able toperform sub-cycles of radiative transfer steps w.r.t. a single hydrodynamicsstep. Radiation requires much smaller time step sizes than hydrodynamics, andsub-cycling permits calculations which are not strictly necessary to beskipped. Indeed, in a test case with gravity, hydrodynamics, and radiativetransfer, the sub-cycling is able to reduce the runtime of a simulation by over90%. Allowing only a part of the involved physics to be sub-cycled is acontrived matter when task-based parallelism is involved, and is an entirelynovel feature in SWIFT.  Since GEAR-RT uses a FVPM, a detailed introduction into Finite Volume methodsand Finite Volume Particle Methods is presented. In astrophysical literature,two FVPM methods are written about: Hopkins (2015) have implemented one intheir GIZMO code, while the one mentioned in Ivanova et al. (2013) isn't usedto date. In this work, I test an implementation of the Ivanova et al. (2013)version, and conclude that in its current form, it is not suitable for use withparticles which are co-moving with the fluid, which in turn is an essentialfeature for cosmological simulations.

cornell university

GEAR-RT: Towards Exa-Scale Moment Based Radiative Transfer For  Cosmological Simulations Using Task-Based Parallelism And Dynamic Sub-Cycling  with SWIFT