`decomposeParMonolithic`

Coupled multi-region decomposition utility for monolithic FSI cases.

Purpose

decomposeParMonolithic partitions multiple volume regions as one graph and then runs the standard decomposePar utility internally with method manual so that a single command produces the normal OpenFOAM processor directory tree for each region.

This is useful for monolithic FSI cases where one region is much smaller than the other. Instead of forcing every rank to own both fluid and solid cells, the combined decomposition can assign some ranks only fluid cells, some only solid cells, and some both.

The name reflects this target monolithic FSI workflow, but the utility is not tied to a specific solver class. The generated per-region decomposition can be used by other multi-region solvers if they support processor ranks with zero cells in some regions.

Current workflow

The utility:

Reads system/decomposeParDict by default, or the file supplied with -decomposeParDict.
Reads the coupled region list and interface definitions from the monolithicCoeffs sub-dictionary.
Loads all listed fvMesh regions.
Builds one combined graph from:
- each region's internal cell adjacency
- extra cross-region edges across the declared FSI interface pairs
Runs the selected OpenFOAM decomposition method once on the combined graph.
Splits the result back into one cellDecomposition list per region.
By default, runs decomposePar internally for each region using method manual.

Dictionary format

Example:

numberOfSubdomains 4;

method scotch;

monolithicCoeffs
{
    regions (fluid solid);

    regionWeights
    {
        fluid 4;
        solid 3;
    }

    interfaces
    (
        {
            regionA fluid;
            patchA  flag;
            regionB solid;
            patchB  interface;
        }
    );
}

Entries

numberOfSubdomains Total MPI ranks for the coupled decomposition.
method Standard OpenFOAM decomposition method, for example scotch.
monolithicCoeffs.regions Volume regions to decompose together.
monolithicCoeffs.regionWeights Optional scalar weight per region. The current implementation applies these as per-cell weights in the combined graph.
monolithicCoeffs.interfaces List of cross-region interfaces used to connect the region graphs.

Region selection

Region selection is currently dictionary-driven only:

the coupled region list is read from monolithicCoeffs.regions
command-line -region and -regions overrides are not currently supported

Interface assumptions in v1

The current implementation is intentionally narrow:

it expects conformal interface patches
it requires the paired patches to have the same number of faces
it matches the two patches by nearest face centre using an O(n^2) search
it does not read or interpret a separate mapping mode from the dictionary

In practice, this utility is best suited to direct-map style interfaces such as the conformal flag/interface pair in foilInWind.

If multiple interface face pairs connect the same two owner cells, the duplicate cell-cell graph edges are collapsed before decomposition.

Command-line options

decomposeParMonolithic
decomposeParMonolithic -decomposeParDict system/decomposeParDict.monolithic
decomposeParMonolithic -cellDist
decomposeParMonolithic -decompose-only
decomposeParMonolithic -force
decomposeParMonolithic -copy-zero
decomposeParMonolithic -no-fields
  

Supported options

-decomposeParDict <file> Use an alternative decompose dictionary.
-cellDist Also write cellDist fields for visualisation.
-decompose-only Only write cellDecomposition files and stop before the internal decomposePar calls.
-force Pass -force to the internal decomposePar calls.
-copy-zero Pass -copyZero to the internal decomposePar calls.
-no-fields Pass -no-fields to the internal decomposePar calls.

`-decompose-only` mode

-decompose-only writes:

constant/<region>/cellDecomposition

for each region and prints the equivalent manual decomposePar workflow.

If you complete the decomposition manually afterwards, ensure that your manual decomposeParDict uses the same numberOfSubdomains as the monolithic decomposition that produced the cellDecomposition files.

Output

During the run, the utility prints:

total cells per region
matched interface face count per coupled interface
per-rank cell counts per region
zero-cell rank counts per region

These statistics are useful for checking whether the coupled decomposition is doing what you intended.

decomposeParMonolithic