Handling cases where there are repeated eigenvalues for a face, i.e. a

square.  The subdeterminants in the eigenvector calculation in
tensor.C ran off the bottom of SMALL.  Handling by identifying the
longest edge as the collapseAxis.

Adding function to return the index of the longest edge for a face.

src/mesh/conformalVoronoiMesh/conformalVoronoiMesh/conformalVoronoiMesh.H

@@ -549,6 +549,9 @@ private:
             label maxFC
         ) const;
 
+        // Identify the index of the longest edge on the face
+        label longestEdge(const face& f, const pointField& pts) const;
+
         //- Identify the face labels of the deferred collapse faces
         void deferredCollapseFaceSet
         (

src/mesh/conformalVoronoiMesh/conformalVoronoiMesh/conformalVoronoiMeshCalcDualMesh.C

@@ -865,6 +865,8 @@ Foam::conformalVoronoiMesh::collapseFace
         maxFC
     );
 
+    labelList facePts(f);
+
     const vector fC = f.centre(pts);
 
     vector fN = f.normal(pts);
@@ -898,47 +900,37 @@ Foam::conformalVoronoiMesh::collapseFace
 
     vector collapseAxis = vector::zero;
 
-    scalar aspectRatio = 1;
+    scalar aspectRatio = 1.0;
 
     if (detJ < 1e-5)
     {
-        const edgeList& eds = f.edges();
+        collapseAxis = f.edges()[longestEdge(f, pts)].vec(pts);
-
-        label longestEdgeI = -1;
-
-        scalar longestEdgeLength = -SMALL;
-
-        forAll(eds, edI)
-        {
-            scalar edgeLength = eds[edI].mag(pts);
-
-            if (edgeLength > longestEdgeLength)
-            {
-                longestEdgeI = edI;
-
-                longestEdgeLength = edgeLength;
-            }
-        }
-
-        collapseAxis = eds[longestEdgeI].vec(pts);
-
-        if (mag(collapseAxis) < VSMALL)
-        {
-            Info<< "if (mag(collapseAxis) < VSMALL) " << collapseAxis << endl;
-        }
 
         collapseAxis /= mag(collapseAxis);
 
         // Empirical correlation for high aspect ratio faces
 
         aspectRatio = sqrt(0.35/detJ);
-
-        // Info<< "# Longest edge determined collapseAxis" << endl;
     }
     else
     {
         vector eVals = eigenValues(J);
 
+        if (mag(eVals.y() - eVals.x()) < 100*SMALL)
+        {
+            // First two eigenvalues are the same: i.e. a square face
+
+            // Cannot necessarily determine linearly independent
+            // eigenvectors, or any at all, use longest edge direction.
+
+            collapseAxis = f.edges()[longestEdge(f, pts)].vec(pts);
+
+            collapseAxis /= mag(collapseAxis);
+
+            aspectRatio = 1.0;
+        }
+        else
+        {
             // The maximum eigenvalue (z()) must be the direction of the
             // normal, as it has the greatest value.  The minimum eigenvalue
             // is the dominant collapse axis for high aspect ratio faces.
@@ -952,6 +944,7 @@ Foam::conformalVoronoiMesh::collapseFace
 
             aspectRatio = sqrt(eVals.y()/max(eVals.x(), SMALL));
         }
+    }
 
     if (magSqr(collapseAxis) < VSMALL)
     {
@@ -962,6 +955,30 @@ Foam::conformalVoronoiMesh::collapseFace
             << "No collapse axis found for face, not collapsing."
             << endl;
 
+        // Output face and collapse axis for visualisation
+
+        Info<< "# Aspect ratio = " << aspectRatio << nl
+            << "# inertia = " << J << nl
+            << "# determinant = " << detJ << nl
+            << "# eigenvalues = " << eigenValues(J) << nl
+            << "# collapseAxis = " << collapseAxis << nl
+            << "# facePts = " << facePts << nl
+            << endl;
+
+        forAll(f, fPtI)
+        {
+            meshTools::writeOBJ(Info, pts[f[fPtI]]);
+        }
+
+        Info<< "f";
+
+        forAll(f, fPtI)
+        {
+            Info << " " << fPtI + 1;
+        }
+
+        Info<< endl;
+
         return fcmNone;
     }
 
@@ -980,8 +997,6 @@ Foam::conformalVoronoiMesh::collapseFace
     // Sort the projected distances and the corresponding vertex
     // indices along the collapse axis
 
-    labelList facePts(f);
-
     labelList oldToNew;
 
     sortedOrder(d, oldToNew);
@@ -1214,6 +1229,34 @@ Foam::conformalVoronoiMesh::collapseFace
 }
 
 
+Foam::label Foam::conformalVoronoiMesh::longestEdge
+(
+    const face& f,
+    const pointField& pts
+) const
+{
+    const edgeList& eds = f.edges();
+
+    label longestEdgeI = -1;
+
+    scalar longestEdgeLength = -SMALL;
+
+    forAll(eds, edI)
+    {
+        scalar edgeLength = eds[edI].mag(pts);
+
+        if (edgeLength > longestEdgeLength)
+        {
+            longestEdgeI = edI;
+
+            longestEdgeLength = edgeLength;
+        }
+    }
+
+    return longestEdgeI;
+}
+
+
 void Foam::conformalVoronoiMesh::deferredCollapseFaceSet
 (
     labelList& owner,