HPC Coupling of Multiphysics Codes
Coupling multiphysics codes on high-performance computers requires moving data between parallel programs efficiently while keeping the coupled solution stable.
Parallel codes talking to each other
Fusion multiphysics couples codes that each run in parallel across many processors and often use different meshes and decompositions. HPC coupling is the discipline of connecting these parallel programs so they exchange data efficiently and produce a stable coupled solution, without one code idling while another runs or drowning in communication overhead.
The naive approach, writing files and reading them back, does not scale. Production coupling uses in-memory data exchange and coupling libraries that move fields directly between the running codes.
Data redistribution and interpolation
When two codes use different grids, coupled fields must be interpolated from one mesh to the other conservatively, so that transferred quantities like power and particles are not created or lost at the interface. When they use different parallel decompositions, the data must also be redistributed among processors, an all-to-all communication pattern that coupling libraries optimize.
Coupling architectures
Codes may be coupled tightly, sharing an address space and stepping together, or loosely, running as separate executables exchanging data at intervals. Loose coupling is more flexible and fault-tolerant; tight coupling can be more stable for stiff interactions. The choice interacts with the splitting scheme used.
Design relevance
The largest Hyperion breeder studies, coupling global turbulence, edge, and heating physics, run at scale only because HPC coupling keeps the parallel codes fed and balanced. Efficient coupling is what makes end-to-end simulation of the design feasible before construction begins Q2 2027.
- Connects parallel codes efficiently
- Conservative interpolation between meshes
- Redistributes data across decompositions
- Tight vs loose coupling architectures