splitMeshRegions.C

/*---------------------------------------------------------------------------*\
  =========                 |
  \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
   \\    /   O peration     |
    \\  /    A nd           | Copyright (C) 2011 OpenFOAM Foundation
     \\/     M anipulation  |
-------------------------------------------------------------------------------
License
    This file is part of OpenFOAM.

    OpenFOAM is free software: you can redistribute it and/or modify it
    under the terms of the GNU General Public License as published by
    the Free Software Foundation, either version 3 of the License, or
    (at your option) any later version.

    OpenFOAM is distributed in the hope that it will be useful, but WITHOUT
    ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
    FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
    for more details.

    You should have received a copy of the GNU General Public License
    along with OpenFOAM.  If not, see <http://www.gnu.org/licenses/>.

Application
    splitMeshRegions

Description
    Splits mesh into multiple regions.

    Each region is defined as a domain whose cells can all be reached by
    cell-face-cell walking without crossing
    - boundary faces
    - additional faces from faceset (-blockedFaces faceSet).
    - any face inbetween differing cellZones (-cellZones)

    Output is:
    - volScalarField with regions as different scalars (-detectOnly)
            or
    - mesh with multiple regions and mapped patches. These patches
      either cover the whole interface between two region (default) or
      only part according to faceZones (-useFaceZones)
            or
    - mesh with cells put into cellZones (-makeCellZones)

    Note:
    - cellZonesOnly does not do a walk and uses the cellZones only. Use
    this if you don't mind having disconnected domains in a single region.
    This option requires all cells to be in one (and one only) cellZone.

    - cellZonesFileOnly behaves like -cellZonesOnly but reads the cellZones
    from the specified file. This allows one to explicitly specify the region
    distribution and still have multiple cellZones per region.

    - useCellZonesOnly does not do a walk and uses the cellZones only. Use
    this if you don't mind having disconnected domains in a single region.
    This option requires all cells to be in one (and one only) cellZone.

    - Should work in parallel.
    cellZones can differ on either side of processor boundaries in which case
    the faces get moved from processor patch to mapped patch. Not
    very well tested.

    - If a cell zone gets split into more than one region it can detect
    the largest matching region (-sloppyCellZones). This will accept any
    region that covers more than 50% of the zone. It has to be a subset
    so cannot have any cells in any other zone.

    - If explicitly a single region has been selected (-largestOnly or
      -insidePoint) its region name will be either
        - name of a cellZone it matches to or
        - "largestOnly" respectively "insidePoint" or
        - polyMesh::defaultRegion if additionally -overwrite
          (so it will overwrite the input mesh!)

    - writes maps like decomposePar back to original mesh:
        - pointRegionAddressing : for every point in this region the point in
        the original mesh
        - cellRegionAddressing  :   ,,      cell                ,,  cell    ,,
        - faceRegionAddressing  :   ,,      face                ,,  face in
        the original mesh + 'turning index'. For a face in the same orientation
        this is the original facelabel+1, for a turned face this is -facelabel-1
\*---------------------------------------------------------------------------*/

#include "SortableList.H"
#include "argList.H"
#include "regionSplit.H"
#include "fvMeshSubset.H"
#include "IOobjectList.H"
#include "volFields.H"
#include "faceSet.H"
#include "cellSet.H"
#include "polyTopoChange.H"
#include "removeCells.H"
#include "EdgeMap.H"
#include "syncTools.H"
#include "ReadFields.H"
#include "mappedWallPolyPatch.H"
#include "zeroGradientFvPatchFields.H"

using namespace Foam;

// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

template<class GeoField>
void addPatchFields(fvMesh& mesh, const word& patchFieldType)
{
    HashTable<const GeoField*> flds
    (
        mesh.objectRegistry::lookupClass<GeoField>()
    );

    forAllConstIter(typename HashTable<const GeoField*>, flds, iter)
    {
        const GeoField& fld = *iter();

        typename GeoField::GeometricBoundaryField& bfld =
            const_cast<typename GeoField::GeometricBoundaryField&>
            (
                fld.boundaryField()
            );

        label sz = bfld.size();
        bfld.setSize(sz+1);
        bfld.set
        (
            sz,
            GeoField::PatchFieldType::New
            (
                patchFieldType,
                mesh.boundary()[sz],
                fld.dimensionedInternalField()
            )
        );
    }
}


// Remove last patch field
template<class GeoField>
void trimPatchFields(fvMesh& mesh, const label nPatches)
{
    HashTable<const GeoField*> flds
    (
        mesh.objectRegistry::lookupClass<GeoField>()
    );

    forAllConstIter(typename HashTable<const GeoField*>, flds, iter)
    {
        const GeoField& fld = *iter();

        const_cast<typename GeoField::GeometricBoundaryField&>
        (
            fld.boundaryField()
        ).setSize(nPatches);
    }
}


// Reorder patch field
template<class GeoField>
void reorderPatchFields(fvMesh& mesh, const labelList& oldToNew)
{
    HashTable<const GeoField*> flds
    (
        mesh.objectRegistry::lookupClass<GeoField>()
    );

    forAllConstIter(typename HashTable<const GeoField*>, flds, iter)
    {
        const GeoField& fld = *iter();

        typename GeoField::GeometricBoundaryField& bfld =
            const_cast<typename GeoField::GeometricBoundaryField&>
            (
                fld.boundaryField()
            );

        bfld.reorder(oldToNew);
    }
}


// Adds patch if not yet there. Returns patchID.
label addPatch(fvMesh& mesh, const polyPatch& patch)
{
    polyBoundaryMesh& polyPatches =
        const_cast<polyBoundaryMesh&>(mesh.boundaryMesh());

    label patchI = polyPatches.findPatchID(patch.name());
    if (patchI != -1)
    {
        if (polyPatches[patchI].type() == patch.type())
        {
            // Already there
            return patchI;
        }
        else
        {
            FatalErrorIn("addPatch(fvMesh&, const polyPatch*)")
                << "Already have patch " << patch.name()
                << " but of type " << patch.type()
                << exit(FatalError);
        }
    }


    label insertPatchI = polyPatches.size();
    label startFaceI = mesh.nFaces();

    forAll(polyPatches, patchI)
    {
        const polyPatch& pp = polyPatches[patchI];

        if (isA<processorPolyPatch>(pp))
        {
            insertPatchI = patchI;
            startFaceI = pp.start();
            break;
        }
    }


    // Below is all quite a hack. Feel free to change once there is a better
    // mechanism to insert and reorder patches.

    // Clear local fields and e.g. polyMesh parallelInfo.
    mesh.clearOut();

    label sz = polyPatches.size();

    fvBoundaryMesh& fvPatches = const_cast<fvBoundaryMesh&>(mesh.boundary());

    // Add polyPatch at the end
    polyPatches.setSize(sz+1);
    polyPatches.set
    (
        sz,
        patch.clone
        (
            polyPatches,
            insertPatchI,   //index
            0,              //size
            startFaceI      //start
        )
    );
    fvPatches.setSize(sz+1);
    fvPatches.set
    (
        sz,
        fvPatch::New
        (
            polyPatches[sz],  // point to newly added polyPatch
            mesh.boundary()
        )
    );

    addPatchFields<volScalarField>
    (
        mesh,
        calculatedFvPatchField<scalar>::typeName
    );
    addPatchFields<volVectorField>
    (
        mesh,
        calculatedFvPatchField<vector>::typeName
    );
    addPatchFields<volSphericalTensorField>
    (
        mesh,
        calculatedFvPatchField<sphericalTensor>::typeName
    );
    addPatchFields<volSymmTensorField>
    (
        mesh,
        calculatedFvPatchField<symmTensor>::typeName
    );
    addPatchFields<volTensorField>
    (
        mesh,
        calculatedFvPatchField<tensor>::typeName
    );

    // Surface fields

    addPatchFields<surfaceScalarField>
    (
        mesh,
        calculatedFvPatchField<scalar>::typeName
    );
    addPatchFields<surfaceVectorField>
    (
        mesh,
        calculatedFvPatchField<vector>::typeName
    );
    addPatchFields<surfaceSphericalTensorField>
    (
        mesh,
        calculatedFvPatchField<sphericalTensor>::typeName
    );
    addPatchFields<surfaceSymmTensorField>
    (
        mesh,
        calculatedFvPatchField<symmTensor>::typeName
    );
    addPatchFields<surfaceTensorField>
    (
        mesh,
        calculatedFvPatchField<tensor>::typeName
    );

    // Create reordering list
    // patches before insert position stay as is
    labelList oldToNew(sz+1);
    for (label i = 0; i < insertPatchI; i++)
    {
        oldToNew[i] = i;
    }
    // patches after insert position move one up
    for (label i = insertPatchI; i < sz; i++)
    {
        oldToNew[i] = i+1;
    }
    // appended patch gets moved to insert position
    oldToNew[sz] = insertPatchI;

    // Shuffle into place
    polyPatches.reorder(oldToNew);
    fvPatches.reorder(oldToNew);

    reorderPatchFields<volScalarField>(mesh, oldToNew);
    reorderPatchFields<volVectorField>(mesh, oldToNew);
    reorderPatchFields<volSphericalTensorField>(mesh, oldToNew);
    reorderPatchFields<volSymmTensorField>(mesh, oldToNew);
    reorderPatchFields<volTensorField>(mesh, oldToNew);
    reorderPatchFields<surfaceScalarField>(mesh, oldToNew);
    reorderPatchFields<surfaceVectorField>(mesh, oldToNew);
    reorderPatchFields<surfaceSphericalTensorField>(mesh, oldToNew);
    reorderPatchFields<surfaceSymmTensorField>(mesh, oldToNew);
    reorderPatchFields<surfaceTensorField>(mesh, oldToNew);

    return insertPatchI;
}


// Reorder and delete patches.
void reorderPatches
(
    fvMesh& mesh,
    const labelList& oldToNew,
    const label nNewPatches
)
{
    polyBoundaryMesh& polyPatches =
        const_cast<polyBoundaryMesh&>(mesh.boundaryMesh());
    fvBoundaryMesh& fvPatches = const_cast<fvBoundaryMesh&>(mesh.boundary());

    // Shuffle into place
    polyPatches.reorder(oldToNew);
    fvPatches.reorder(oldToNew);

    reorderPatchFields<volScalarField>(mesh, oldToNew);
    reorderPatchFields<volVectorField>(mesh, oldToNew);
    reorderPatchFields<volSphericalTensorField>(mesh, oldToNew);
    reorderPatchFields<volSymmTensorField>(mesh, oldToNew);
    reorderPatchFields<volTensorField>(mesh, oldToNew);
    reorderPatchFields<surfaceScalarField>(mesh, oldToNew);
    reorderPatchFields<surfaceVectorField>(mesh, oldToNew);
    reorderPatchFields<surfaceSphericalTensorField>(mesh, oldToNew);
    reorderPatchFields<surfaceSymmTensorField>(mesh, oldToNew);
    reorderPatchFields<surfaceTensorField>(mesh, oldToNew);

    // Remove last.
    polyPatches.setSize(nNewPatches);
    fvPatches.setSize(nNewPatches);
    trimPatchFields<volScalarField>(mesh, nNewPatches);
    trimPatchFields<volVectorField>(mesh, nNewPatches);
    trimPatchFields<volSphericalTensorField>(mesh, nNewPatches);
    trimPatchFields<volSymmTensorField>(mesh, nNewPatches);
    trimPatchFields<volTensorField>(mesh, nNewPatches);
    trimPatchFields<surfaceScalarField>(mesh, nNewPatches);
    trimPatchFields<surfaceVectorField>(mesh, nNewPatches);
    trimPatchFields<surfaceSphericalTensorField>(mesh, nNewPatches);
    trimPatchFields<surfaceSymmTensorField>(mesh, nNewPatches);
    trimPatchFields<surfaceTensorField>(mesh, nNewPatches);
}


template<class GeoField>
void subsetVolFields
(
    const fvMesh& mesh,
    const fvMesh& subMesh,
    const labelList& cellMap,
    const labelList& faceMap,
    const labelHashSet& addedPatches
)
{
    const labelList patchMap(identity(mesh.boundaryMesh().size()));

    HashTable<const GeoField*> fields
    (
        mesh.objectRegistry::lookupClass<GeoField>()
    );
    forAllConstIter(typename HashTable<const GeoField*>, fields, iter)
    {
        const GeoField& fld = *iter();

        Info<< "Mapping field " << fld.name() << endl;

        tmp<GeoField> tSubFld
        (
            fvMeshSubset::interpolate
            (
                fld,
                subMesh,
                patchMap,
                cellMap,
                faceMap
            )
        );

        // Hack: set value to 0 for introduced patches (since don't
        //       get initialised.
        forAll(tSubFld().boundaryField(), patchI)
        {
            if (addedPatches.found(patchI))
            {
                tSubFld().boundaryField()[patchI] ==
                    pTraits<typename GeoField::value_type>::zero;
            }
        }

        // Store on subMesh
        GeoField* subFld = tSubFld.ptr();
        subFld->rename(fld.name());
        subFld->writeOpt() = IOobject::AUTO_WRITE;
        subFld->store();
    }
}


template<class GeoField>
void subsetSurfaceFields
(
    const fvMesh& mesh,
    const fvMesh& subMesh,
    const labelList& faceMap,
    const labelHashSet& addedPatches
)
{
    const labelList patchMap(identity(mesh.boundaryMesh().size()));

    HashTable<const GeoField*> fields
    (
        mesh.objectRegistry::lookupClass<GeoField>()
    );
    forAllConstIter(typename HashTable<const GeoField*>, fields, iter)
    {
        const GeoField& fld = *iter();

        Info<< "Mapping field " << fld.name() << endl;

        tmp<GeoField> tSubFld
        (
            fvMeshSubset::interpolate
            (
                fld,
                subMesh,
                patchMap,
                faceMap
            )
        );

        // Hack: set value to 0 for introduced patches (since don't
        //       get initialised.
        forAll(tSubFld().boundaryField(), patchI)
        {
            if (addedPatches.found(patchI))
            {
                tSubFld().boundaryField()[patchI] ==
                    pTraits<typename GeoField::value_type>::zero;
            }
        }

        // Store on subMesh
        GeoField* subFld = tSubFld.ptr();
        subFld->rename(fld.name());
        subFld->writeOpt() = IOobject::AUTO_WRITE;
        subFld->store();
    }
}

// Select all cells not in the region
labelList getNonRegionCells(const labelList& cellRegion, const label regionI)
{
    DynamicList<label> nonRegionCells(cellRegion.size());
    forAll(cellRegion, cellI)
    {
        if (cellRegion[cellI] != regionI)
        {
            nonRegionCells.append(cellI);
        }
    }
    return nonRegionCells.shrink();
}


void addToInterface
(
    const polyMesh& mesh,
    const label zoneID,
    const label ownRegion,
    const label neiRegion,
    EdgeMap<Map<label> >& regionsToSize
)
{
    edge interface
    (
        min(ownRegion, neiRegion),
        max(ownRegion, neiRegion)
    );

    EdgeMap<Map<label> >::iterator iter = regionsToSize.find
    (
        interface
    );

    if (iter != regionsToSize.end())
    {
        // Check if zone present
        Map<label>::iterator zoneFnd = iter().find(zoneID);
        if (zoneFnd != iter().end())
        {
            // Found zone. Increment count.
            zoneFnd()++;
        }
        else
        {
            // New or no zone. Insert with count 1.
            iter().insert(zoneID, 1);
        }
    }
    else
    {
        // Create new interface of size 1.
        Map<label> zoneToSize;
        zoneToSize.insert(zoneID, 1);
        regionsToSize.insert(interface, zoneToSize);
    }
}


// Get region-region interface name and sizes.
// Returns interfaces as straight list for looping in identical order.
void getInterfaceSizes
(
    const polyMesh& mesh,
    const bool useFaceZones,
    const labelList& cellRegion,
    const wordList& regionNames,

    edgeList& interfaces,
    List<Pair<word> >& interfaceNames,
    labelList& interfaceSizes,
    labelList& faceToInterface
)
{
    // From region-region to faceZone (or -1) to number of faces.

    EdgeMap<Map<label> > regionsToSize;


    // Internal faces
    // ~~~~~~~~~~~~~~

    forAll(mesh.faceNeighbour(), faceI)
    {
        label ownRegion = cellRegion[mesh.faceOwner()[faceI]];
        label neiRegion = cellRegion[mesh.faceNeighbour()[faceI]];

        if (ownRegion != neiRegion)
        {
            addToInterface
            (
                mesh,
                (useFaceZones ? mesh.faceZones().whichZone(faceI) : -1),
                ownRegion,
                neiRegion,
                regionsToSize
            );
        }
    }

    // Boundary faces
    // ~~~~~~~~~~~~~~

    // Neighbour cellRegion.
    labelList coupledRegion(mesh.nFaces()-mesh.nInternalFaces());

    forAll(coupledRegion, i)
    {
        label cellI = mesh.faceOwner()[i+mesh.nInternalFaces()];
        coupledRegion[i] = cellRegion[cellI];
    }
    syncTools::swapBoundaryFaceList(mesh, coupledRegion);

    forAll(coupledRegion, i)
    {
        label faceI = i+mesh.nInternalFaces();
        label ownRegion = cellRegion[mesh.faceOwner()[faceI]];
        label neiRegion = coupledRegion[i];

        if (ownRegion != neiRegion)
        {
            addToInterface
            (
                mesh,
                (useFaceZones ? mesh.faceZones().whichZone(faceI) : -1),
                ownRegion,
                neiRegion,
                regionsToSize
            );
        }
    }


    if (Pstream::parRun())
    {
        if (Pstream::master())
        {
            // Receive and add to my sizes
            for
            (
                int slave=Pstream::firstSlave();
                slave<=Pstream::lastSlave();
                slave++
            )
            {
                IPstream fromSlave(Pstream::blocking, slave);

                EdgeMap<Map<label> > slaveSizes(fromSlave);

                forAllConstIter(EdgeMap<Map<label> >, slaveSizes, slaveIter)
                {
                    EdgeMap<Map<label> >::iterator masterIter =
                        regionsToSize.find(slaveIter.key());

                    if (masterIter != regionsToSize.end())
                    {
                        // Same inter-region
                        const Map<label>& slaveInfo = slaveIter();
                        Map<label>& masterInfo = masterIter();

                        forAllConstIter(Map<label>, slaveInfo, iter)
                        {
                            label zoneID = iter.key();
                            label slaveSize = iter();

                            Map<label>::iterator zoneFnd = masterInfo.find
                            (
                                zoneID
                            );
                            if (zoneFnd != masterInfo.end())
                            {
                                zoneFnd() += slaveSize;
                            }
                            else
                            {
                                masterInfo.insert(zoneID, slaveSize);
                            }
                        }
                    }
                    else
                    {
                        regionsToSize.insert(slaveIter.key(), slaveIter());
                    }
                }
            }
        }
        else
        {
            // Send to master
            {
                OPstream toMaster(Pstream::blocking, Pstream::masterNo());
                toMaster << regionsToSize;
            }
        }
    }

    // Rework

    Pstream::scatter(regionsToSize);



    // Now we have the global sizes of all inter-regions.
    // Invert this on master and distribute.
    label nInterfaces = 0;
    forAllConstIter(EdgeMap<Map<label> >, regionsToSize, iter)
    {
        const Map<label>& info = iter();
        nInterfaces += info.size();
    }

    interfaces.setSize(nInterfaces);
    interfaceNames.setSize(nInterfaces);
    interfaceSizes.setSize(nInterfaces);
    EdgeMap<Map<label> > regionsToInterface(nInterfaces);

    nInterfaces = 0;
    forAllConstIter(EdgeMap<Map<label> >, regionsToSize, iter)
    {
        const edge& e = iter.key();
        const word& name0 = regionNames[e[0]];
        const word& name1 = regionNames[e[1]];

        const Map<label>& info = iter();
        forAllConstIter(Map<label>, info, infoIter)
        {
            interfaces[nInterfaces] = iter.key();
            label zoneID = infoIter.key();
            if (zoneID == -1)
            {
                interfaceNames[nInterfaces] = Pair<word>
                (
                    name0 + "_to_" + name1,
                    name1 + "_to_" + name0
                );
            }
            else
            {
                const word& zoneName = mesh.faceZones()[zoneID].name();
                interfaceNames[nInterfaces] = Pair<word>
                (
                    zoneName + "_" + name0 + "_to_" + name1,
                    zoneName + "_" + name1 + "_to_" + name0
                );
            }
            interfaceSizes[nInterfaces] = infoIter();

            Map<label> zoneAndInterface;
            zoneAndInterface.insert(zoneID, nInterfaces);
            regionsToInterface.insert(e, zoneAndInterface);

            nInterfaces++;
        }
    }


    // Now all processor have consistent interface information

    Pstream::scatter(interfaces);
    Pstream::scatter(interfaceNames);
    Pstream::scatter(interfaceSizes);
    Pstream::scatter(regionsToInterface);

    // Mark all inter-region faces.
    faceToInterface.setSize(mesh.nFaces(), -1);

    forAll(mesh.faceNeighbour(), faceI)
    {
        label ownRegion = cellRegion[mesh.faceOwner()[faceI]];
        label neiRegion = cellRegion[mesh.faceNeighbour()[faceI]];

        if (ownRegion != neiRegion)
        {
            label zoneID = -1;
            if (useFaceZones)
            {
                zoneID = mesh.faceZones().whichZone(faceI);
            }

            edge interface
            (
                min(ownRegion, neiRegion),
                max(ownRegion, neiRegion)
            );

            faceToInterface[faceI] = regionsToInterface[interface][zoneID];
        }
    }
    forAll(coupledRegion, i)
    {
        label faceI = i+mesh.nInternalFaces();
        label ownRegion = cellRegion[mesh.faceOwner()[faceI]];
        label neiRegion = coupledRegion[i];

        if (ownRegion != neiRegion)
        {
            label zoneID = -1;
            if (useFaceZones)
            {
                zoneID = mesh.faceZones().whichZone(faceI);
            }

            edge interface
            (
                min(ownRegion, neiRegion),
                max(ownRegion, neiRegion)
            );

            faceToInterface[faceI] = regionsToInterface[interface][zoneID];
        }
    }
}


// Create mesh for region.
autoPtr<mapPolyMesh> createRegionMesh
(
    const fvMesh& mesh,
    // Region info
    const labelList& cellRegion,
    const label regionI,
    const word& regionName,
    // Interface info
    const labelList& interfacePatches,
    const labelList& faceToInterface,

    autoPtr<fvMesh>& newMesh
)
{
    // Create dummy system/fv*
    {
        IOobject io
        (
            "fvSchemes",
            mesh.time().system(),
            regionName,
            mesh,
            IOobject::NO_READ,
            IOobject::NO_WRITE,
            false
        );

        Info<< "Testing:" << io.objectPath() << endl;

        if (!io.headerOk())
        // if (!exists(io.objectPath()))
        {
            Info<< "Writing dummy " << regionName/io.name() << endl;
            dictionary dummyDict;
            dictionary divDict;
            dummyDict.add("divSchemes", divDict);
            dictionary gradDict;
            dummyDict.add("gradSchemes", gradDict);
            dictionary laplDict;
            dummyDict.add("laplacianSchemes", laplDict);

            IOdictionary(io, dummyDict).regIOobject::write();
        }
    }
    {
        IOobject io
        (
            "fvSolution",
            mesh.time().system(),
            regionName,
            mesh,
            IOobject::NO_READ,
            IOobject::NO_WRITE,
            false
        );

        if (!io.headerOk())
        //if (!exists(io.objectPath()))
        {
            Info<< "Writing dummy " << regionName/io.name() << endl;
            dictionary dummyDict;
            IOdictionary(io, dummyDict).regIOobject::write();
        }
    }


    // Neighbour cellRegion.
    labelList coupledRegion(mesh.nFaces()-mesh.nInternalFaces());

    forAll(coupledRegion, i)
    {
        label cellI = mesh.faceOwner()[i+mesh.nInternalFaces()];
        coupledRegion[i] = cellRegion[cellI];
    }
    syncTools::swapBoundaryFaceList(mesh, coupledRegion);


    // Topology change container. Start off from existing mesh.
    polyTopoChange meshMod(mesh);

    // Cell remover engine
    removeCells cellRemover(mesh);

    // Select all but region cells
    labelList cellsToRemove(getNonRegionCells(cellRegion, regionI));

    // Find out which faces will get exposed. Note that this
    // gets faces in mesh face order. So both regions will get same
    // face in same order (important!)
    labelList exposedFaces = cellRemover.getExposedFaces(cellsToRemove);

    labelList exposedPatchIDs(exposedFaces.size());
    forAll(exposedFaces, i)
    {
        label faceI = exposedFaces[i];
        label interfaceI = faceToInterface[faceI];

        label ownRegion = cellRegion[mesh.faceOwner()[faceI]];
        label neiRegion = -1;

        if (mesh.isInternalFace(faceI))
        {
            neiRegion = cellRegion[mesh.faceNeighbour()[faceI]];
        }
        else
        {
            neiRegion = coupledRegion[faceI-mesh.nInternalFaces()];
        }


        // Check which side is being kept - determines which of the two
        // patches will be used.

        label otherRegion = -1;

        if (ownRegion == regionI && neiRegion != regionI)
        {
            otherRegion = neiRegion;
        }
        else if (ownRegion != regionI && neiRegion == regionI)
        {
            otherRegion = ownRegion;
        }
        else
        {
            FatalErrorIn("createRegionMesh(..)")
                << "Exposed face:" << faceI
                << " fc:" << mesh.faceCentres()[faceI]
                << " has owner region " << ownRegion
                << " and neighbour region " << neiRegion
                << " when handling region:" << regionI
                << exit(FatalError);
        }

        // Find the patch.
        if (regionI < otherRegion)
        {
            exposedPatchIDs[i] = interfacePatches[interfaceI];
        }
        else
        {
            exposedPatchIDs[i] = interfacePatches[interfaceI]+1;
        }
    }

    // Remove faces
    cellRemover.setRefinement
    (
        cellsToRemove,
        exposedFaces,
        exposedPatchIDs,
        meshMod
    );

    autoPtr<mapPolyMesh> map = meshMod.makeMesh
    (
        newMesh,
        IOobject
        (
            regionName,
            mesh.time().timeName(),
            mesh.time(),
            IOobject::NO_READ,
            IOobject::AUTO_WRITE
        ),
        mesh
    );

    return map;
}


void createAndWriteRegion
(
    const fvMesh& mesh,
    const labelList& cellRegion,
    const wordList& regionNames,
    const labelList& faceToInterface,
    const labelList& interfacePatches,
    const label regionI,
    const word& newMeshInstance
)
{
    Info<< "Creating mesh for region " << regionI
        << ' ' << regionNames[regionI] << endl;

    autoPtr<fvMesh> newMesh;
    autoPtr<mapPolyMesh> map = createRegionMesh
    (
        mesh,
        cellRegion,
        regionI,