BUG: overset: support GAMG. Fixes #2559

Note that there is no support for using GAMG with mass-correction.

src/OpenFOAM/matrices/lduMatrix/solvers/GAMG/GAMGAgglomerations/pairGAMGAgglomeration/pairGAMGAgglomerate.C

@@ -36,6 +36,18 @@ void Foam::pairGAMGAgglomeration::agglomerate
     const scalarField& faceWeights
 )
 {
+    const label nFaces = mesh.lduAddr().lowerAddr().size();
+    if (faceWeights.size() != nFaces)
+    {
+        FatalErrorInFunction
+            << "Supplied number of face weights " << faceWeights.size()
+            << " does not correspond to the number of faces " << nFaces << endl
+            << "This may be because of using a geometry-based"
+            << " agglomeration method instead of a matrix-based one"
+            << exit(FatalError);
+    }
+
+
     // Start geometric agglomeration from the given faceWeights
     scalarField* faceWeightsPtr = const_cast<scalarField*>(&faceWeights);
 

src/overset/oversetCoupledPolyPatch/oversetGAMGInterface/oversetGAMGInterface.C

@@ -5,7 +5,7 @@
     \\  /    A nd           | www.openfoam.com
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
-    Copyright (C) 2021 OpenCFD Ltd.
+    Copyright (C) 2021-2022 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -61,7 +61,53 @@ Foam::oversetGAMGInterface::oversetGAMGInterface
         index,
         coarseInterfaces
     )
-{}
+{
+    // Problem:
+    // - we only want the oversetGAMGInterfaceField
+    // - but this needs a oversetGAMGInterface of at least size 1
+    //   (since faceRestrictAddressing cannot have -1 or so in it - it
+    //    can only have >= 0 i.e. we cannot agglomerate into nothing)
+    // - note that the end result is not used - the (current) corresponding
+    //   oversetGAMGInterfaceField has dummy updateInterfaceMatrix.
+
+    // Construct face agglomeration from cell agglomeration. Code same as
+    // in e.g. cyclicAMIGAMGInterface.
+    {
+        // From coarse face to cell
+        DynamicList<label> dynFaceCells(localRestrictAddressing.size());
+
+        // From face to coarse face
+        DynamicList<label> dynFaceRestrictAddressing
+        (
+            localRestrictAddressing.size()
+        );
+
+        Map<label> masterToCoarseFace(localRestrictAddressing.size());
+
+        for (const label curMaster : localRestrictAddressing)
+        {
+            const auto iter = masterToCoarseFace.cfind(curMaster);
+
+            if (iter.found())
+            {
+                // Already have coarse face
+                dynFaceRestrictAddressing.append(iter.val());
+            }
+            else
+            {
+                // New coarse face
+                const label coarseI = dynFaceCells.size();
+
+                dynFaceRestrictAddressing.append(coarseI);
+                dynFaceCells.append(curMaster);
+                masterToCoarseFace.insert(curMaster, coarseI);
+            }
+        }
+
+        faceCells_.transfer(dynFaceCells);
+        faceRestrictAddressing_.transfer(dynFaceRestrictAddressing);
+    }
+}
 
 
 // * * * * * * * * * * * * * * * * Destructor  * * * * * * * * * * * * * * * //

tutorials/basic/overLaplacianDyMFoam/heatTransferMassConservation1D/system/fvSolution

@@ -18,10 +18,18 @@ solvers
 {
     T
     {
-        solver          PBiCGStab;
-        preconditioner  DILU;
         tolerance       1e-10;
         relTol          0;
+
+        solver          PBiCGStab;
+        preconditioner  DILU;
+
+        //// Note: GAMG not supported with massCorrection switched on
+        //solver          GAMG;
+        //smoother        GaussSeidel;
+        //nCellsInCoarsestLevel   1;
+        //// Important: overset adds faces so use matrix-based agglomerator
+        //agglomerator        algebraicPair;
     }
 
     cellDisplacement