fvcAverage and fvcReconstruct: Return zero for 0-D cases

fvcAverage and fvcReconstruct both do divisions or inverses of surface
summed fields. A single-cell zero-dimension case, has no genuine faces
on which to sum, so surface sums are identically zero. This change
detects this situation and returns a zero value instead of failing due
to a divide by zero.

This allows the multiphase test cases to be reduced to just one cell.

src/finiteVolume/finiteVolume/fvc/fvcAverage.C

@@ -2,7 +2,7 @@
   =========                 |
   \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
    \\    /   O peration     |
-    \\  /    A nd           | Copyright (C) 2011-2016 OpenFOAM Foundation
+    \\  /    A nd           | Copyright (C) 2011-2018 OpenFOAM Foundation
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
 License
@@ -62,9 +62,15 @@ average
                 IOobject::NO_WRITE
             ),
             mesh,
-            ssf.dimensions()
+            dimensioned<Type>("0", ssf.dimensions(), Zero)
         )
     );
+
+    if (!mesh.nGeometricD())
+    {
+        return taverage;
+    }
+
     GeometricField<Type, fvPatchField, volMesh>& av = taverage.ref();
 
     av.primitiveFieldRef() =

src/finiteVolume/finiteVolume/fvc/fvcReconstruct.C

@@ -2,7 +2,7 @@
   =========                 |
   \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
    \\    /   O peration     |
-    \\  /    A nd           | Copyright (C) 2011-2016 OpenFOAM Foundation
+    \\  /    A nd           | Copyright (C) 2011-2018 OpenFOAM Foundation
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
 License
@@ -73,11 +73,19 @@ reconstruct
                 IOobject::NO_READ,
                 IOobject::NO_WRITE
             ),
-            inv(surfaceSum(SfHat*mesh.Sf()))&surfaceSum(SfHat*ssf),
+            mesh,
+            dimensioned<GradType>("0", ssf.dimensions()/dimArea, Zero),
             extrapolatedCalculatedFvPatchField<GradType>::typeName
         )
     );
 
+    if (!mesh.nGeometricD())
+    {
+        return treconField;
+    }
+
+    treconField.ref() = inv(surfaceSum(SfHat*mesh.Sf()))&surfaceSum(SfHat*ssf),
+
     treconField.ref().correctBoundaryConditions();
 
     return treconField;

test/multiphase/reactingMultiphaseEulerFoam/populationBalanceModeling/binaryBreakup/system/blockMeshDict

@@ -30,7 +30,7 @@ vertices
 
 blocks
 (
-    hex (0 1 2 3 4 5 6 7) (1 2 1) simpleGrading (1 1 1)
+    hex (0 1 2 3 4 5 6 7) (1 1 1) simpleGrading (1 1 1)
 );
 
 edges

test/multiphase/reactingMultiphaseEulerFoam/populationBalanceModeling/breakup/system/blockMeshDict

@@ -30,7 +30,7 @@ vertices
 
 blocks
 (
-    hex (0 1 2 3 4 5 6 7) (1 2 1) simpleGrading (1 1 1)
+    hex (0 1 2 3 4 5 6 7) (1 1 1) simpleGrading (1 1 1)
 );
 
 edges

test/multiphase/reactingMultiphaseEulerFoam/populationBalanceModeling/coalescence/system/blockMeshDict

@@ -30,7 +30,7 @@ vertices
 
 blocks
 (
-    hex (0 1 2 3 4 5 6 7) (1 2 1) simpleGrading (1 1 1)
+    hex (0 1 2 3 4 5 6 7) (1 1 1) simpleGrading (1 1 1)
 );
 
 edges

test/multiphase/reactingMultiphaseEulerFoam/populationBalanceModeling/simultaneousCoalescenceAndBreakup/system/blockMeshDict

@@ -30,7 +30,7 @@ vertices
 
 blocks
 (
-    hex (0 1 2 3 4 5 6 7) (1 2 1) simpleGrading (1 1 1)
+    hex (0 1 2 3 4 5 6 7) (1 1 1) simpleGrading (1 1 1)
 );
 
 edges

test/multiphase/reactingTwoPhaseEulerFoam/populationBalanceModeling/drift/system/blockMeshDict

@@ -30,7 +30,7 @@ vertices
 
 blocks
 (
-    hex (0 1 2 3 4 5 6 7) (1 2 1) simpleGrading (1 1 1)
+    hex (0 1 2 3 4 5 6 7) (1 1 1) simpleGrading (1 1 1)
 );
 
 edges

test/multiphase/reactingTwoPhaseEulerFoam/populationBalanceModeling/negativeDrift/system/blockMeshDict

@@ -30,7 +30,7 @@ vertices
 
 blocks
 (
-    hex (0 1 2 3 4 5 6 7) (1 2 1) simpleGrading (1 1 1)
+    hex (0 1 2 3 4 5 6 7) (1 1 1) simpleGrading (1 1 1)
 );
 
 edges