Two solvers on one problem¶

Some problems are best split in two: a solid and a fluid, a hot part and a cold part, a region where one method is good and a region where another is. openPASO can give each half to a different solver and make them agree.

How it works¶

The problem is split along an interface, the boundary the two halves share.
Each half is a small script for its own solver, called a participant. Every participant follows one simple rule: it reads what the other side sent from imports.json, solves once, and writes its own interface values to exports.json.
openPASO's driver runs the two participants in turn. One side receives a value (for example a temperature), the other a flux (for example a heat flow).
This repeats until both sides agree at the interface to within a tolerance.
The whole thing is repeated on finer meshes (couple_levels), so the accuracy of the coupled answer can be measured as well.

Because each side only talks through those two small files, any pair of solvers can be coupled, also solvers that were never designed to work together.

What openPASO checks¶

that each participant follows the file contract, before it runs;
that the two sides actually agree, measured from their own exported values, not from an internal number that can look converged while the sides still disagree;
that the interface carried something: a coupling that exchanged nothing converges immediately and looks perfect, so openPASO names it;
that what leaves one side arrives at the other (verify_interface_flux);
that the error falls at the right rate as the meshes get finer.

preCICE¶

preCICE is a separate, widely used coupling library. openPASO can use it instead of its own driver through the couple_precice tool.