From 6c3f2f4df1e83b6819bd6fda969f8bb7cec748c4 Mon Sep 17 00:00:00 2001
From: Mattijs Janssens <m.janssens@opencfd.co.uk>
Date: Mon, 25 Mar 2019 16:49:50 +0000
Subject: [PATCH] Snappy hex mesh proximity check

---
 src/mesh/snappyHexMesh/Make/files             |   1 +
 src/mesh/snappyHexMesh/Make/options           |   1 -
 .../meshRefinement/meshRefinement.C           | 216 +++++-
 .../meshRefinement/meshRefinement.H           | 108 ++-
 .../meshRefinement/meshRefinementBlock.C      | 437 +++++++++++
 .../meshRefinementProblemCells.C              |  90 ---
 .../meshRefinement/meshRefinementRefine.C     |  61 +-
 .../refinementSurfaces/refinementSurfaces.C   |  52 +-
 .../refinementSurfaces/refinementSurfaces.H   |  29 +-
 .../shellSurfaces/shellSurfaces.H             |  12 +
 .../refinementParameters.C                    |   3 +-
 .../refinementParameters.H                    |  14 +-
 .../snappyHexMeshDriver/snappyRefineDriver.C  |  90 ++-
 .../snappyHexMeshDriver/snappyRefineDriver.H  |   7 +
 .../triSurfaceMesh/triSurfaceMesh.C           |   2 +-
 .../0/polyMesh/sets/cellsToRemove             |  21 +
 .../snappyHexMesh/opposite_walls/Allclean     |   7 +
 .../mesh/snappyHexMesh/opposite_walls/Allrun  |  15 +
 .../constant/triSurface/gap0.stl              | 702 ++++++++++++++++++
 .../constant/triSurface/gap1.stl              | 702 ++++++++++++++++++
 .../constant/triSurface/gap_transformed0.stl  | 485 ++++++++++++
 .../triSurface/gap_transformed0_with_hook.stl | 702 ++++++++++++++++++
 .../constant/triSurface/gap_transformed1.stl  | 702 ++++++++++++++++++
 .../opposite_walls/system/blockMeshDict       |  98 +++
 .../opposite_walls/system/controlDict         |  49 ++
 .../opposite_walls/system/decomposeParDict    |  30 +
 .../opposite_walls/system/fvSchemes           |  57 ++
 .../opposite_walls/system/fvSolution          |  69 ++
 .../opposite_walls/system/meshQualityDict     |  21 +
 .../opposite_walls/system/snappyHexMeshDict   | 326 ++++++++
 30 files changed, 4974 insertions(+), 135 deletions(-)
 create mode 100644 src/mesh/snappyHexMesh/meshRefinement/meshRefinementBlock.C
 create mode 100644 tutorials/mesh/snappyHexMesh/opposite_walls/0/polyMesh/sets/cellsToRemove
 create mode 100755 tutorials/mesh/snappyHexMesh/opposite_walls/Allclean
 create mode 100755 tutorials/mesh/snappyHexMesh/opposite_walls/Allrun
 create mode 100644 tutorials/mesh/snappyHexMesh/opposite_walls/constant/triSurface/gap0.stl
 create mode 100644 tutorials/mesh/snappyHexMesh/opposite_walls/constant/triSurface/gap1.stl
 create mode 100644 tutorials/mesh/snappyHexMesh/opposite_walls/constant/triSurface/gap_transformed0.stl
 create mode 100644 tutorials/mesh/snappyHexMesh/opposite_walls/constant/triSurface/gap_transformed0_with_hook.stl
 create mode 100644 tutorials/mesh/snappyHexMesh/opposite_walls/constant/triSurface/gap_transformed1.stl
 create mode 100644 tutorials/mesh/snappyHexMesh/opposite_walls/system/blockMeshDict
 create mode 100644 tutorials/mesh/snappyHexMesh/opposite_walls/system/controlDict
 create mode 100644 tutorials/mesh/snappyHexMesh/opposite_walls/system/decomposeParDict
 create mode 100644 tutorials/mesh/snappyHexMesh/opposite_walls/system/fvSchemes
 create mode 100644 tutorials/mesh/snappyHexMesh/opposite_walls/system/fvSolution
 create mode 100644 tutorials/mesh/snappyHexMesh/opposite_walls/system/meshQualityDict
 create mode 100644 tutorials/mesh/snappyHexMesh/opposite_walls/system/snappyHexMeshDict

diff --git a/src/mesh/snappyHexMesh/Make/files b/src/mesh/snappyHexMesh/Make/files
index 00c46b066e..0bdadb9848 100644
--- a/src/mesh/snappyHexMesh/Make/files
+++ b/src/mesh/snappyHexMesh/Make/files
@@ -14,6 +14,7 @@ meshRefinement/meshRefinementMerge.C
 meshRefinement/meshRefinementProblemCells.C
 meshRefinement/meshRefinementRefine.C
 meshRefinement/meshRefinementGapRefine.C
+meshRefinement/meshRefinementBlock.C
 meshRefinement/patchFaceOrientation.C
 
 refinementFeatures/refinementFeatures.C
diff --git a/src/mesh/snappyHexMesh/Make/options b/src/mesh/snappyHexMesh/Make/options
index 2cf98acf6b..f76521cbc2 100644
--- a/src/mesh/snappyHexMesh/Make/options
+++ b/src/mesh/snappyHexMesh/Make/options
@@ -1,5 +1,4 @@
 EXE_INC = \
-    /* -g -DFULLDEBUG -O0 */ \
     -I$(LIB_SRC)/parallel/decompose/decompositionMethods/lnInclude \
     -I$(LIB_SRC)/dynamicMesh/lnInclude \
     -I$(LIB_SRC)/finiteVolume/lnInclude \
diff --git a/src/mesh/snappyHexMesh/meshRefinement/meshRefinement.C b/src/mesh/snappyHexMesh/meshRefinement/meshRefinement.C
index 9717e6e553..1f1839bac4 100644
--- a/src/mesh/snappyHexMesh/meshRefinement/meshRefinement.C
+++ b/src/mesh/snappyHexMesh/meshRefinement/meshRefinement.C
@@ -58,6 +58,8 @@ License
 #include "fvMeshTools.H"
 #include "motionSmoother.H"
 #include "faceSet.H"
+#include "topoDistanceData.H"
+#include "FaceCellWave.H"
 
 // Leak path
 #include "shortestPathSet.H"
@@ -358,6 +360,218 @@ void Foam::meshRefinement::updateIntersections(const labelList& changedFaces)
 }
 
 
+Foam::labelList Foam::meshRefinement::nearestPatch
+(
+    const labelList& adaptPatchIDs
+) const
+{
+    // Determine nearest patch for all mesh faces. Used when removing cells
+    // to give some reasonable patch to exposed faces.
+
+    const polyBoundaryMesh& patches = mesh_.boundaryMesh();
+
+    labelList nearestAdaptPatch;
+
+    if (adaptPatchIDs.size())
+    {
+        nearestAdaptPatch.setSize(mesh_.nFaces(), adaptPatchIDs[0]);
+
+
+        // Count number of faces in adaptPatchIDs
+        label nFaces = 0;
+        forAll(adaptPatchIDs, i)
+        {
+            const polyPatch& pp = patches[adaptPatchIDs[i]];
+            nFaces += pp.size();
+        }
+
+        // Field on cells and faces.
+        List<topoDistanceData> cellData(mesh_.nCells());
+        List<topoDistanceData> faceData(mesh_.nFaces());
+
+        // Start of changes
+        labelList patchFaces(nFaces);
+        List<topoDistanceData> patchData(nFaces);
+        nFaces = 0;
+        forAll(adaptPatchIDs, i)
+        {
+            label patchi = adaptPatchIDs[i];
+            const polyPatch& pp = patches[patchi];
+
+            forAll(pp, i)
+            {
+                patchFaces[nFaces] = pp.start()+i;
+                patchData[nFaces] = topoDistanceData(patchi, 0);
+                nFaces++;
+            }
+        }
+
+        // Propagate information inwards
+        FaceCellWave<topoDistanceData> deltaCalc
+        (
+            mesh_,
+            patchFaces,
+            patchData,
+            faceData,
+            cellData,
+            mesh_.globalData().nTotalCells()+1
+        );
+
+        // And extract
+
+        bool haveWarned = false;
+        forAll(faceData, facei)
+        {
+            if (!faceData[facei].valid(deltaCalc.data()))
+            {
+                if (!haveWarned)
+                {
+                    WarningInFunction
+                        << "Did not visit some faces, e.g. face " << facei
+                        << " at " << mesh_.faceCentres()[facei] << endl
+                        << "Assigning these faces to patch "
+                        << adaptPatchIDs[0]
+                        << endl;
+                    haveWarned = true;
+                }
+            }
+            else
+            {
+                nearestAdaptPatch[facei] = faceData[facei].data();
+            }
+        }
+    }
+    else
+    {
+        // Use patch 0
+        nearestAdaptPatch.setSize(mesh_.nFaces(), 0);
+    }
+
+    return nearestAdaptPatch;
+}
+
+
+Foam::labelList Foam::meshRefinement::nearestIntersection
+(
+    const labelList& surfacesToTest,
+    const label defaultRegion
+) const
+{
+    // Determine nearest intersection for all mesh faces. Used when removing
+    // cells to give some reasonable patch to exposed faces. Use this
+    // function instead of nearestPatch if you don't have patches yet.
+
+
+    // Swap neighbouring cell centres and cell level
+    labelList neiLevel(mesh_.nFaces()-mesh_.nInternalFaces());
+    pointField neiCc(mesh_.nFaces()-mesh_.nInternalFaces());
+    calcNeighbourData(neiLevel, neiCc);
+
+
+    // Collect segments
+    // ~~~~~~~~~~~~~~~~
+
+    const labelList testFaces(intersectedFaces());
+
+    pointField start(testFaces.size());
+    pointField end(testFaces.size());
+    labelList minLevel(testFaces.size());
+
+    calcCellCellRays
+    (
+        neiCc,
+        neiLevel,
+        testFaces,
+        start,
+        end,
+        minLevel
+    );
+
+    // Do tests in one go
+    labelList surface1;
+    List<pointIndexHit> hit1;
+    labelList region1;
+    labelList surface2;
+    List<pointIndexHit> hit2;
+    labelList region2;
+    surfaces_.findNearestIntersection
+    (
+        surfacesToTest,
+        start,
+        end,
+
+        surface1,
+        hit1,
+        region1,
+        surface2,
+        hit2,
+        region2
+    );
+
+    labelList nearestRegion(mesh_.nFaces(), defaultRegion);
+
+    // Field on cells and faces.
+    List<topoDistanceData> cellData(mesh_.nCells());
+    List<topoDistanceData> faceData(mesh_.nFaces());
+
+    // Start walking from all intersected faces
+    DynamicList<label> patchFaces(start.size());
+    DynamicList<topoDistanceData> patchData(start.size());
+    forAll(start, i)
+    {
+        label facei = testFaces[i];
+        if (surface1[i] != -1)
+        {
+            patchFaces.append(facei);
+            label regioni = surfaces_.globalRegion(surface1[i], region1[i]);
+            patchData.append(topoDistanceData(regioni, 0));
+        }
+        else if (surface2[i] != -1)
+        {
+            patchFaces.append(facei);
+            label regioni = surfaces_.globalRegion(surface2[i], region2[i]);
+            patchData.append(topoDistanceData(regioni, 0));
+        }
+    }
+
+    // Propagate information inwards
+    FaceCellWave<topoDistanceData> deltaCalc
+    (
+        mesh_,
+        patchFaces,
+        patchData,
+        faceData,
+        cellData,
+        mesh_.globalData().nTotalCells()+1
+    );
+
+    // And extract
+
+    bool haveWarned = false;
+    forAll(faceData, facei)
+    {
+        if (!faceData[facei].valid(deltaCalc.data()))
+        {
+            if (!haveWarned)
+            {
+                WarningInFunction
+                    << "Did not visit some faces, e.g. face " << facei
+                    << " at " << mesh_.faceCentres()[facei] << endl
+                    << "Assigning these faces to global region "
+                    << defaultRegion << endl;
+                haveWarned = true;
+            }
+        }
+        else
+        {
+            nearestRegion[facei] = faceData[facei].data();
+        }
+    }
+
+    return nearestRegion;
+}
+
+
 void Foam::meshRefinement::testSyncPointList
 (
     const string& msg,
@@ -3147,7 +3361,7 @@ void Foam::meshRefinement::write
         dumpRefinementLevel();
     }
 
-    if (debugFlags & OBJINTERSECTIONS && prefix.size())
+    if ((debugFlags & OBJINTERSECTIONS) && prefix.size())
     {
         dumpIntersections(prefix);
     }
diff --git a/src/mesh/snappyHexMesh/meshRefinement/meshRefinement.H b/src/mesh/snappyHexMesh/meshRefinement/meshRefinement.H
index 1db4c4120b..56067d2b9f 100644
--- a/src/mesh/snappyHexMesh/meshRefinement/meshRefinement.H
+++ b/src/mesh/snappyHexMesh/meshRefinement/meshRefinement.H
@@ -2,7 +2,7 @@
   =========                 |
   \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
    \\    /   O peration     |
-    \\  /    A nd           | Copyright (C) 2015-2017 OpenCFD Ltd.
+    \\  /    A nd           | Copyright (C) 2015-2019 OpenCFD Ltd.
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
                             | Copyright (C) 2011-2017 OpenFOAM Foundation
@@ -313,7 +313,7 @@ private:
             ) const;
 
             //- Collect faces that are intersected and whose neighbours aren't
-            //  yet marked  for refinement.
+            //  yet marked for refinement.
             labelList getRefineCandidateFaces
             (
                 const labelList& refineCell
@@ -329,6 +329,43 @@ private:
                 label& nRefine
             ) const;
 
+            //- Collect cells intersected by the surface that are candidates
+            //  for gap checking. Used inside markSurfaceGapRefinement
+            void collectGapCandidates
+            (
+                const shellSurfaces& shells,
+                const labelList& testFaces,
+                const labelList& neiLevel,
+                const pointField& neiCc,
+                labelList& cellToCompact,
+                labelList& bFaceToCompact,
+                List<FixedList<label, 3>>& shellGapInfo,
+                List<volumeType>& shellGapMode
+            ) const;
+            void collectGapCells
+            (
+                const scalar planarCos,
+
+                const List<FixedList<label, 3>>& extendedGapLevel,
+                const List<volumeType>& extendedGapMode,
+                const labelList& testFaces,
+                const pointField& start,
+                const pointField& end,
+
+                const labelList& cellToCompact,
+                const labelList& bFaceToCompact,
+                const List<FixedList<label, 3>>& shellGapInfo,
+                const List<volumeType>& shellGapMode,
+
+                const label nAllowRefine,
+                const labelList& neiLevel,
+                const pointField& neiCc,
+
+                labelList& refineCell,
+                label& nRefine
+            ) const;
+
+
             //- Mark cells intersected by the surface if they are inside
             //  close gaps
             label markSurfaceGapRefinement
@@ -472,6 +509,11 @@ private:
             label markProximityRefinement
             (
                 const scalar curvature,
+
+                // Per region the min and max cell level
+                const labelList& surfaceMinLevel,
+                const labelList& surfaceMaxLevel,
+
                 const label nAllowRefine,
                 const labelList& neiLevel,
                 const pointField& neiCc,
@@ -608,6 +650,15 @@ private:
             //  adaptPatchIDs[0]
             labelList nearestPatch(const labelList& adaptPatchIDs) const;
 
+            //- Returns list with for every face the label of the nearest
+            //  (global) region. Any unreached face (disconnected mesh?) becomes
+            //  defaultRegion
+            labelList nearestIntersection
+            (
+                const labelList& surfacesToTest,
+                const label defaultRegion
+            ) const;
+
             //- Returns list with for every internal face -1 or the patch
             //  they should be baffled into.
             labelList markFacesOnProblemCellsGeometric
@@ -1005,11 +1056,13 @@ public:
             //- Is local topology a small gap normal to the test vector
             bool isNormalGap
             (
-                const scalar,
-                const vector&,
-                const vector&,
-                const vector&,
-                const vector&
+                const scalar planarCos,
+                const label level0,
+                const vector& point0,
+                const vector& normal0,
+                const label level1,
+                const vector& point1,
+                const vector& normal1
             ) const;
 
             //- Calculate list of cells to refine.
@@ -1032,6 +1085,44 @@ public:
                 const label maxLocalCells
             ) const;
 
+
+            // Blocking cells
+
+                //- Mark faces on interface between set and rest
+                //  (and same cell level)
+                void markOutsideFaces
+                (
+                    const labelList& cellLevel,
+                    const labelList& neiLevel,
+                    const labelList& refineCell,
+                    bitSet& isOutsideFace
+                ) const;
+
+                //- Count number of faces on cell that are in set
+                label countFaceDirs
+                (
+                    const bitSet& isOutsideFace,
+                    const label celli
+                ) const;
+
+                //- Add one layer of cells to set
+                void growSet
+                (
+                    const labelList& neiLevel,
+                    const bitSet& isOutsideFace,
+                    labelList& refineCell,
+                    label& nRefine
+                ) const;
+
+                //- Detect gapRefinement cells and remove them
+                autoPtr<mapPolyMesh> removeGapCells
+                (
+                    const scalar planarAngle,
+                    const labelList& minSurfaceLevel,
+                    const labelList& globalToMasterPatch,
+                    const label growIter
+                );
+
             //- Refine some cells
             autoPtr<mapPolyMesh> refine(const labelList& cellsToRefine);
 
@@ -1561,6 +1652,9 @@ public:
             );
 };
 
+template<>
+inline bool contiguous<FixedList<label, 3>>()               {return true;}
+
 
 // * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
 
diff --git a/src/mesh/snappyHexMesh/meshRefinement/meshRefinementBlock.C b/src/mesh/snappyHexMesh/meshRefinement/meshRefinementBlock.C
new file mode 100644
index 0000000000..7b20766fdb
--- /dev/null
+++ b/src/mesh/snappyHexMesh/meshRefinement/meshRefinementBlock.C
@@ -0,0 +1,437 @@
+/*---------------------------------------------------------------------------*\
+  =========                 |
+  \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
+   \\    /   O peration     |
+    \\  /    A nd           | Copyright (C) 2018-2019 OpenCFD Ltd.
+     \\/     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/>.
+
+\*---------------------------------------------------------------------------*/
+
+#include "meshRefinement.H"
+#include "fvMesh.H"
+#include "Time.H"
+#include "refinementSurfaces.H"
+#include "removeCells.H"
+#include "unitConversion.H"
+#include "bitSet.H"
+
+// * * * * * * * * * * * * * Private Member Functions  * * * * * * * * * * * //
+
+//Foam::label Foam::meshRefinement::markFakeGapRefinement
+//(
+//    const scalar planarCos,
+//
+//    const label nAllowRefine,
+//    const labelList& neiLevel,
+//    const pointField& neiCc,
+//
+//    labelList& refineCell,
+//    label& nRefine
+//) const
+//{
+//    label oldNRefine = nRefine;
+//
+//
+//    // Collect candidate faces (i.e. intersecting any surface and
+//    // owner/neighbour not yet refined.
+//    const labelList testFaces(getRefineCandidateFaces(refineCell));
+//
+//    // Collect segments
+//    pointField start(testFaces.size());
+//    pointField end(testFaces.size());
+//    labelList minLevel(testFaces.size());
+//
+//    calcCellCellRays
+//    (
+//        neiCc,
+//        neiLevel,
+//        testFaces,
+//        start,
+//        end,
+//        minLevel
+//    );
+//
+//
+//    // Re-use the gap shooting methods. This needs:
+//    //  - shell gapLevel : faked. Set to 0,labelMax
+//    //  - surface gapLevel : faked by overwriting
+//
+//
+//    List<FixedList<label, 3>>& surfGapLevel = const_cast
+//    <
+//        List<FixedList<label, 3>>&
+//    >(surfaces_.extendedGapLevel());
+//
+//    List<volumeType>& surfGapMode = const_cast
+//    <
+//        List<volumeType>&
+//    >(surfaces_.extendedGapMode());
+//
+//    const List<FixedList<label, 3>> surfOldLevel(surfGapLevel);
+//    const List<volumeType> surfOldMode(surfGapMode);
+//
+//    // Set the extended gap levels
+//    forAll(surfaces_.gapLevel(), regioni)
+//    {
+//        surfGapLevel[regioni] = FixedList<label, 3>
+//        ({
+//            3,
+//           -1,
+//            surfaces_.gapLevel()[regioni]+1
+//        });
+//    }
+//    surfGapMode = volumeType::MIXED;
+//
+//Pout<< "gapLevel was:" << surfOldLevel << endl;
+//Pout<< "gapLevel now:" << surfGapLevel << endl;
+//Pout<< "gapMode was:" << surfOldMode << endl;
+//Pout<< "gapMode now:" << surfGapMode << endl;
+//Pout<< "nRefine was:" << oldNRefine << endl;
+//
+//
+//
+//    List<List<FixedList<label, 3>>>& shellGapLevel = const_cast
+//    <
+//        List<List<FixedList<label, 3>>>&
+//    >(shells_.extendedGapLevel());
+//
+//    List<List<volumeType>>& shellGapMode = const_cast
+//    <
+//        List<List<volumeType>>&
+//    >(shells_.extendedGapMode());
+//
+//    const List<List<FixedList<label, 3>>> shellOldLevel(shellGapLevel);
+//    const List<List<volumeType>> shellOldMode(shellGapMode);
+//
+//    // Set the extended gap levels
+//    forAll(shellGapLevel, shelli)
+//    {
+//        shellGapLevel[shelli] =  FixedList<label, 3>({3, -1, labelMax});
+//        shellGapMode[shelli] = volumeType::MIXED;
+//    }
+//Pout<< "shellLevel was:" << shellOldLevel << endl;
+//Pout<< "shellLevel now:" << shellGapLevel << endl;
+//
+//    const label nAdditionalRefined = markSurfaceGapRefinement
+//    (
+//        planarCos,
+//
+//        nAllowRefine,
+//        neiLevel,
+//        neiCc,
+//
+//        refineCell,
+//        nRefine
+//    );
+//
+//Pout<< "nRefine now:" << nRefine << endl;
+//
+//    // Restore
+//    surfGapLevel = surfOldLevel;
+//    surfGapMode = surfOldMode;
+//    shellGapLevel = shellOldLevel;
+//    shellGapMode = shellOldMode;
+//
+//    return nAdditionalRefined;
+//}
+
+
+void Foam::meshRefinement::markOutsideFaces
+(
+    const labelList& cellLevel,
+    const labelList& neiLevel,
+    const labelList& refineCell,
+    bitSet& isOutsideFace
+) const
+{
+    // Get faces:
+    // - on outside of cell set
+    // - inbetween same cell level (i.e. quads)
+
+    isOutsideFace.setSize(mesh_.nFaces());
+    isOutsideFace = Zero;
+
+    forAll(mesh_.faceNeighbour(), facei)
+    {
+        label own = mesh_.faceOwner()[facei];
+        label nei = mesh_.faceNeighbour()[facei];
+        if
+        (
+            (cellLevel[own] == cellLevel[nei])
+         && (
+                (refineCell[own] != -1)
+             != (refineCell[nei] != -1)
+            )
+        )
+        {
+            isOutsideFace.set(facei);
+        }
+    }
+    {
+
+        const label nBnd = mesh_.nFaces()-mesh_.nInternalFaces();
+
+        labelList neiRefineCell(nBnd);
+        syncTools::swapBoundaryCellList(mesh_, refineCell, neiRefineCell);
+        for (label bFacei = 0; bFacei < nBnd; ++bFacei)
+        {
+            label facei = mesh_.nInternalFaces()+bFacei;
+            label own = mesh_.faceOwner()[facei];
+
+            if
+            (
+                (cellLevel[own] == neiLevel[bFacei])
+             && (
+                    (refineCell[own] != -1)
+                 != (neiRefineCell[bFacei] != -1)
+                )
+            )
+            {
+                isOutsideFace.set(facei);
+            }
+        }
+    }
+}
+
+
+Foam::label Foam::meshRefinement::countFaceDirs
+(
+    const bitSet& isOutsideFace,
+    const label celli
+) const
+{
+    const cell& cFaces = mesh_.cells()[celli];
+    const vectorField& faceAreas = mesh_.faceAreas();
+
+    Vector<bool> haveDirs(vector::uniform(false));
+
+    forAll(cFaces, cFacei)
+    {
+        const label facei = cFaces[cFacei];
+
+        if (isOutsideFace[facei])
+        {
+            const vector& n = faceAreas[facei];
+            scalar magSqrN = magSqr(n);
+
+            if (magSqrN > ROOTVSMALL)
+            {
+                for
+                (
+                    direction dir = 0;
+                    dir < pTraits<vector>::nComponents;
+                    dir++
+                )
+                {
+                    if (Foam::sqr(n[dir]) > 0.99*magSqrN)
+                    {
+                        haveDirs[dir] = true;
+                        break;
+                    }
+                }
+            }
+        }
+    }
+
+    label nDirs = 0;
+    forAll(haveDirs, dir)
+    {
+        if (haveDirs[dir])
+        {
+            nDirs++;
+        }
+    }
+    return nDirs;
+}
+
+
+void Foam::meshRefinement::growSet
+(
+    const labelList& neiLevel,
+    const bitSet& isOutsideFace,
+    labelList& refineCell,
+    label& nRefine
+) const
+{
+    // Get cells with three or more outside faces
+    const cellList& cells = mesh_.cells();
+    forAll(cells, celli)
+    {
+        if (refineCell[celli] == -1)
+        {
+            if (countFaceDirs(isOutsideFace, celli) == 3)
+            {
+                // Mark cell with any value
+                refineCell[celli] = 0;
+                nRefine++;
+            }
+        }
+    }
+}
+
+
+Foam::autoPtr<Foam::mapPolyMesh> Foam::meshRefinement::removeGapCells
+(
+    const scalar planarAngle,
+    const labelList& minSurfaceLevel,
+    const labelList& globalToMasterPatch,
+    const label growIter
+)
+{
+    // Swap neighbouring cell centres and cell level
+    labelList neiLevel(mesh_.nFaces()-mesh_.nInternalFaces());
+    pointField neiCc(mesh_.nFaces()-mesh_.nInternalFaces());
+    calcNeighbourData(neiLevel, neiCc);
+
+    labelList refineCell(mesh_.nCells(), -1);
+    label nRefine = 0;
+    markProximityRefinement
+    (
+        Foam::cos(degToRad(planarAngle)),
+
+        minSurfaceLevel,                                // surface min level
+        labelList(minSurfaceLevel.size(), labelMax),    // surfaceGapLevel
+
+        labelMax/Pstream::nProcs(), //nAllowRefine,
+        neiLevel,
+        neiCc,
+
+        refineCell,
+        nRefine
+    );
+
+
+    //// Mark big-gap refinement
+    //markFakeGapRefinement
+    //(
+    //    Foam::cos(degToRad(planarAngle)),
+    //
+    //    labelMax/Pstream::nProcs(), //nAllowRefine,
+    //    neiLevel,
+    //    neiCc,
+    //
+    //    refineCell,
+    //    nRefine
+    //);
+
+
+    Info<< "Marked for blocking due to close opposite surfaces         : "
+        << returnReduce(nRefine, sumOp<label>()) << " cells." << endl;
+
+    // Remove outliers, i.e. cells with all points exposed
+    if (growIter)
+    {
+        labelList oldRefineCell(refineCell);
+
+        // Pass1: extend the set to fill in gaps
+        bitSet isOutsideFace;
+        for (label iter = 0; iter < growIter; iter++)
+        {
+            // Get outside faces
+            markOutsideFaces
+            (
+                meshCutter_.cellLevel(),
+                neiLevel,
+                refineCell,
+                isOutsideFace
+            );
+            // Extend with cells with three outside faces
+            growSet(neiLevel, isOutsideFace, refineCell, nRefine);
+        }
+
+
+        // Pass2: erode back to original set if pass1 didn't help
+        for (label iter = 0; iter < growIter; iter++)
+        {
+            // Get outside faces. Ignore cell level.
+            markOutsideFaces
+            (
+                labelList(mesh_.nCells(), 0),
+                labelList(neiLevel.size(), 0),
+                refineCell,
+                isOutsideFace
+            );
+
+            // Unmark cells with three or more outside faces
+            for (label celli = 0; celli < mesh_.nCells(); celli++)
+            {
+                if (refineCell[celli] != -1 && oldRefineCell[celli] == -1)
+                {
+                    if (countFaceDirs(isOutsideFace, celli) >= 3)
+                    {
+                        refineCell[celli] = -1;
+                        --nRefine;
+                    }
+                }
+            }
+        }
+
+        Info<< "Marked for blocking after filtering                        : "
+            << returnReduce(nRefine, sumOp<label>()) << " cells." << endl;
+    }
+
+
+    // Determine patch for every mesh face
+    const PtrList<surfaceZonesInfo>& surfZones = surfaces_.surfZones();
+    labelList unnamedSurfaces(surfaceZonesInfo::getUnnamedSurfaces(surfZones));
+    const label defaultRegion(surfaces_.globalRegion(unnamedSurfaces[0], 0));
+
+    const labelList nearestRegion
+    (
+        nearestIntersection
+        (
+            unnamedSurfaces,
+            defaultRegion
+        )
+    );
+
+    // Pack
+    labelList cellsToRemove(nRefine);
+    nRefine = 0;
+
+    forAll(refineCell, cellI)
+    {
+        if (refineCell[cellI] != -1)
+        {
+            cellsToRemove[nRefine++] = cellI;
+        }
+    }
+
+    // Remove cells
+    removeCells cellRemover(mesh_);
+    labelList exposedFaces(cellRemover.getExposedFaces(cellsToRemove));
+
+    labelList exposedPatches(exposedFaces.size());
+    forAll(exposedFaces, i)
+    {
+        label facei = exposedFaces[i];
+        exposedPatches[i] = globalToMasterPatch[nearestRegion[facei]];
+    }
+
+    return doRemoveCells
+    (
+        cellsToRemove,
+        exposedFaces,
+        exposedPatches,
+        cellRemover
+    );
+}
+
+
+// ************************************************************************* //
diff --git a/src/mesh/snappyHexMesh/meshRefinement/meshRefinementProblemCells.C b/src/mesh/snappyHexMesh/meshRefinement/meshRefinementProblemCells.C
index 2d66b8eb78..000ae8de28 100644
--- a/src/mesh/snappyHexMesh/meshRefinement/meshRefinementProblemCells.C
+++ b/src/mesh/snappyHexMesh/meshRefinement/meshRefinementProblemCells.C
@@ -42,8 +42,6 @@ License
 
 #include "snapParameters.H"
 #include "motionSmoother.H"
-#include "topoDistanceData.H"
-#include "FaceCellWave.H"
 
 // * * * * * * * * * * * * * Private Member Functions  * * * * * * * * * * * //
 
@@ -375,94 +373,6 @@ bool Foam::meshRefinement::isCollapsedCell
 }
 
 
-Foam::labelList Foam::meshRefinement::nearestPatch
-(
-    const labelList& adaptPatchIDs
-) const
-{
-    const polyBoundaryMesh& patches = mesh_.boundaryMesh();
-
-    labelList nearestAdaptPatch;
-
-    if (adaptPatchIDs.size())
-    {
-        nearestAdaptPatch.setSize(mesh_.nFaces(), adaptPatchIDs[0]);
-
-
-        // Count number of faces in adaptPatchIDs
-        label nFaces = 0;
-        forAll(adaptPatchIDs, i)
-        {
-            const polyPatch& pp = patches[adaptPatchIDs[i]];
-            nFaces += pp.size();
-        }
-
-        // Field on cells and faces.
-        List<topoDistanceData> cellData(mesh_.nCells());
-        List<topoDistanceData> faceData(mesh_.nFaces());
-
-        // Start of changes
-        labelList patchFaces(nFaces);
-        List<topoDistanceData> patchData(nFaces);
-        nFaces = 0;
-        forAll(adaptPatchIDs, i)
-        {
-            label patchi = adaptPatchIDs[i];
-            const polyPatch& pp = patches[patchi];
-
-            forAll(pp, i)
-            {
-                patchFaces[nFaces] = pp.start()+i;
-                patchData[nFaces] = topoDistanceData(patchi, 0);
-                nFaces++;
-            }
-        }
-
-        // Propagate information inwards
-        FaceCellWave<topoDistanceData> deltaCalc
-        (
-            mesh_,
-            patchFaces,
-            patchData,
-            faceData,
-            cellData,
-            mesh_.globalData().nTotalCells()+1
-        );
-
-        // And extract
-
-        bool haveWarned = false;
-        forAll(faceData, facei)
-        {
-            if (!faceData[facei].valid(deltaCalc.data()))
-            {
-                if (!haveWarned)
-                {
-                    WarningInFunction
-                        << "Did not visit some faces, e.g. face " << facei
-                        << " at " << mesh_.faceCentres()[facei] << endl
-                        << "Assigning  these cells to patch "
-                        << adaptPatchIDs[0]
-                        << endl;
-                    haveWarned = true;
-                }
-            }
-            else
-            {
-                nearestAdaptPatch[facei] = faceData[facei].data();
-            }
-        }
-    }
-    else
-    {
-        // Use patch 0
-        nearestAdaptPatch.setSize(mesh_.nFaces(), 0);
-    }
-
-    return nearestAdaptPatch;
-}
-
-
 // Returns list with for every internal face -1 or the patch they should
 // be baffled into. Gets run after createBaffles so all the unzoned surface
 // intersections have already been turned into baffles. (Note: zoned surfaces
diff --git a/src/mesh/snappyHexMesh/meshRefinement/meshRefinementRefine.C b/src/mesh/snappyHexMesh/meshRefinement/meshRefinementRefine.C
index 60f41bd9f0..f5dfad986b 100644
--- a/src/mesh/snappyHexMesh/meshRefinement/meshRefinementRefine.C
+++ b/src/mesh/snappyHexMesh/meshRefinement/meshRefinementRefine.C
@@ -1191,13 +1191,14 @@ Foam::label Foam::meshRefinement::markSurfaceCurvatureRefinement
         // Per segment the list of levels of the surfaces
         labelListList surfaceLevel;
 
-        surfaces_.findAllHigherIntersections
+        surfaces_.findAllIntersections
         (
             start,
             end,
             minLevel,           // max level of surface has to be bigger
                                 // than min level of neighbouring cells
 
+            labelList(surfaces_.maxLevel().size(), 0),  // min level
             surfaces_.maxLevel(),
 
             surfaceNormal,
@@ -1572,13 +1573,17 @@ bool Foam::meshRefinement::isGap
 bool Foam::meshRefinement::isNormalGap
 (
     const scalar planarCos,
+    const label level0,
     const vector& point0,
     const vector& normal0,
 
+    const label level1,
     const vector& point1,
     const vector& normal1
 ) const
 {
+    //const scalar edge0Len = meshCutter_.level0EdgeLength();
+
     //- Hits differ and angles are oppositeish and
     //  hits have a normal distance
     vector d = point1-point0;
@@ -1588,24 +1593,31 @@ bool Foam::meshRefinement::isNormalGap
     {
         scalar cosAngle = (normal0 & normal1);
 
-        vector avg = Zero;
+        vector avgNormal = Zero;
         if (cosAngle < (-1+planarCos))
         {
             // Opposite normals
-            avg = 0.5*(normal0-normal1);
+            avgNormal = 0.5*(normal0-normal1);
         }
         else if (cosAngle > (1-planarCos))
         {
-            avg = 0.5*(normal0+normal1);
+            avgNormal = 0.5*(normal0+normal1);
         }
 
-        if (avg != vector::zero)
+        if (avgNormal != vector::zero)
         {
-            avg /= mag(avg);
-            d /= magD;
+            avgNormal /= mag(avgNormal);
+
+            //scalar normalDist = mag(d&avgNormal);
+            //const scalar avgCellSize = edge0Len/pow(2.0, 0.5*(level0+level1));
+            //if (normalDist > 1*avgCellSize)
+            //{
+            //    Pout<< "** skipping large distance " << endl;
+            //    return false;
+            //}
 
             // Check average normal with respect to intersection locations
-            if (mag(avg&d) > Foam::cos(degToRad(45.0)))
+            if (mag(avgNormal&d/magD) > Foam::cos(degToRad(45.0)))
             {
                 return true;
             }
@@ -1628,7 +1640,7 @@ bool Foam::meshRefinement::checkProximity
     const label cellI,
 
     label& cellMaxLevel,        // cached max surface level for this cell
-    vector& cellMaxLocation,    // cached surface normal for this cell
+    vector& cellMaxLocation,    // cached surface location for this cell
     vector& cellMaxNormal,      // cached surface normal for this cell
 
     labelList& refineCell,
@@ -1657,8 +1669,10 @@ bool Foam::meshRefinement::checkProximity
             bool closeSurfaces = isNormalGap
             (
                 planarCos,
+                cellLevel[cellI],   //cellMaxLevel,
                 cellMaxLocation,
                 cellMaxNormal,
+                cellLevel[cellI],   //surfaceLevel,
                 surfaceLocation,
                 surfaceNormal
             );
@@ -1702,6 +1716,10 @@ bool Foam::meshRefinement::checkProximity
 Foam::label Foam::meshRefinement::markProximityRefinement
 (
     const scalar planarCos,
+
+    const labelList& surfaceMinLevel,
+    const labelList& surfaceMaxLevel,
+
     const label nAllowRefine,
     const labelList& neiLevel,
     const pointField& neiCc,
@@ -1723,7 +1741,7 @@ Foam::label Foam::meshRefinement::markProximityRefinement
 
     // Collect candidate faces (i.e. intersecting any surface and
     // owner/neighbour not yet refined.
-    labelList testFaces(getRefineCandidateFaces(refineCell));
+    const labelList testFaces(getRefineCandidateFaces(refineCell));
 
     // Collect segments
     pointField start(testFaces.size());
@@ -1755,14 +1773,15 @@ Foam::label Foam::meshRefinement::markProximityRefinement
         // Per segment the list of levels of the surfaces
         labelListList surfaceLevel;
 
-        surfaces_.findAllHigherIntersections
+        surfaces_.findAllIntersections
         (
             start,
             end,
             minLevel,           // gap level of surface has to be bigger
                                 // than min level of neighbouring cells
 
-            surfaces_.gapLevel(),
+            surfaceMinLevel,
+            surfaceMaxLevel,
 
             surfaceLocation,
             surfaceNormal,
@@ -1777,7 +1796,7 @@ Foam::label Foam::meshRefinement::markProximityRefinement
         //OBJstream str
         //(
         //    mesh_.time().path()
-        //  / "findAllHigherIntersections_"
+        //  / "findAllIntersections_"
         //  + mesh_.time().timeName()
         //  + ".obj"
         //);
@@ -1785,7 +1804,7 @@ Foam::label Foam::meshRefinement::markProximityRefinement
         //OBJstream str2
         //(
         //    mesh_.time().path()
-        //  / "findAllHigherIntersections2_"
+        //  / "findAllIntersections2_"
         //  + mesh_.time().timeName()
         //  + ".obj"
         //);
@@ -1884,8 +1903,10 @@ Foam::label Foam::meshRefinement::markProximityRefinement
                 isNormalGap
                 (
                     planarCos,
+                    cellLevel[own], //cellMaxLevel[own],
                     cellMaxLocation[own],
                     cellMaxNormal[own],
+                    cellLevel[nei], //cellMaxLevel[nei],
                     cellMaxLocation[nei],
                     cellMaxNormal[nei]
                 )
@@ -1954,8 +1975,10 @@ Foam::label Foam::meshRefinement::markProximityRefinement
                 isNormalGap
                 (
                     planarCos,
+                    cellLevel[own], //cellMaxLevel[own],
                     cellMaxLocation[own],
                     cellMaxNormal[own],
+                    neiLevel[bFaceI], //neiBndMaxLevel[bFaceI],
                     neiBndMaxLocation[bFaceI],
                     neiBndMaxNormal[bFaceI]
                 )
@@ -2217,19 +2240,25 @@ Foam::labelList Foam::meshRefinement::refineCandidates
         // Refinement based on gap (only neighbouring cells)
         // ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
+        const labelList& surfaceGapLevel = surfaces_.gapLevel();
+
         if
         (
             gapRefinement
          && (planarCos >= -1 && planarCos <= 1)
-         && (max(surfaces_.gapLevel()) > -1)
+         && (max(surfaceGapLevel) > -1)
         )
         {
-            Info<< "Specified gap level : " << max(surfaces_.gapLevel())
+            Info<< "Specified gap level : " << max(surfaceGapLevel)
                 << ", planar angle " << planarAngle << endl;
 
             label nGap = markProximityRefinement
             (
                 planarCos,
+
+                labelList(surfaceGapLevel.size(), -1),  // surface min level
+                surfaceGapLevel,                        // surface max level
+
                 nAllowRefine,
                 neiLevel,
                 neiCc,
diff --git a/src/mesh/snappyHexMesh/refinementSurfaces/refinementSurfaces.C b/src/mesh/snappyHexMesh/refinementSurfaces/refinementSurfaces.C
index a80cdbad90..98496210fd 100644
--- a/src/mesh/snappyHexMesh/refinementSurfaces/refinementSurfaces.C
+++ b/src/mesh/snappyHexMesh/refinementSurfaces/refinementSurfaces.C
@@ -160,6 +160,7 @@ Foam::refinementSurfaces::refinementSurfaces
     surfZones_.setSize(surfI);
     regionOffset_.setSize(surfI);
 
+    // Per surface data
     labelList globalMinLevel(surfI, Zero);
     labelList globalMaxLevel(surfI, Zero);
     labelList globalLevelIncr(surfI, Zero);
@@ -174,6 +175,10 @@ Foam::refinementSurfaces::refinementSurfaces
 
     scalarField globalAngle(surfI, -GREAT);
     PtrList<dictionary> globalPatchInfo(surfI);
+
+    labelList globalBlockLevel(surfI, labelMax);
+
+    // Per surface, per region data
     List<Map<label>> regionMinLevel(surfI);
     List<Map<label>> regionMaxLevel(surfI);
     List<Map<label>> regionLevelIncr(surfI);
@@ -181,7 +186,7 @@ Foam::refinementSurfaces::refinementSurfaces
     List<Map<volumeType>> regionGapMode(surfI);
     List<Map<scalar>> regionAngle(surfI);
     List<Map<autoPtr<dictionary>>> regionPatchInfo(surfI);
-
+    List<Map<label>> regionBlockLevel(surfI);
 
     wordHashSet unmatchedKeys(surfacesDict.toc());
 
@@ -282,6 +287,8 @@ Foam::refinementSurfaces::refinementSurfaces
                 );
             }
             dict.readIfPresent("perpendicularAngle", globalAngle[surfI]);
+            dict.readIfPresent("blockLevel", globalBlockLevel[surfI]);
+
 
             if (dict.found("regions"))
             {
@@ -392,6 +399,12 @@ Foam::refinementSurfaces::refinementSurfaces
                                 regionDict.subDict("patchInfo").clone()
                             );
                         }
+
+                        label l;
+                        if (regionDict.readIfPresent<label>("blockLevel", l))
+                        {
+                            regionBlockLevel[surfI].insert(regionI, l);
+                        }
                     }
                 }
             }
@@ -432,6 +445,8 @@ Foam::refinementSurfaces::refinementSurfaces
     perpendicularAngle_.setSize(nRegions);
     perpendicularAngle_ = -GREAT;
     patchInfo_.setSize(nRegions);
+    blockLevel_.setSize(nRegions);
+    blockLevel_ = labelMax;
 
 
     forAll(globalMinLevel, surfI)
@@ -458,6 +473,7 @@ Foam::refinementSurfaces::refinementSurfaces
                     globalPatchInfo[surfI].clone()
                 );
             }
+            blockLevel_[globalRegionI] = globalBlockLevel[surfI];
         }
 
         // Overwrite with region specific information
@@ -490,6 +506,12 @@ Foam::refinementSurfaces::refinementSurfaces
 
             patchInfo_.set(globalRegionI, dict.clone());
         }
+
+        forAllConstIter(Map<label>, regionBlockLevel[surfI], iter)
+        {
+            label globalRegionI = regionOffset_[surfI] + iter.key();
+            blockLevel_[globalRegionI] = regionBlockLevel[surfI][iter.key()];
+        }
     }
 }
 
@@ -903,13 +925,14 @@ void Foam::refinementSurfaces::findHigherIntersection
 }
 
 
-void Foam::refinementSurfaces::findAllHigherIntersections
+void Foam::refinementSurfaces::findAllIntersections
 (
     const pointField& start,
     const pointField& end,
     const labelList& currentLevel,   // current cell refinement level
 
-    const labelList& globalRegionLevel,
+    const labelList& globalMinLevel,
+    const labelList& globalMaxLevel,
 
     List<vectorList>& surfaceNormal,
     labelListList& surfaceLevel
@@ -976,7 +999,11 @@ void Foam::refinementSurfaces::findAllHigherIntersections
             label region = globalRegion(surfI, surfRegion[i]);
             label pointI = pointMap[i];
 
-            if (globalRegionLevel[region] > currentLevel[pointI])
+            if
+            (
+                currentLevel[pointI] >= globalMinLevel[region]
+             && currentLevel[pointI] < globalMaxLevel[region]
+            )
             {
                 // Append to pointI info
                 label sz = surfaceNormal[pointI].size();
@@ -984,20 +1011,21 @@ void Foam::refinementSurfaces::findAllHigherIntersections
                 surfaceNormal[pointI][sz] = surfNormal[i];
 
                 surfaceLevel[pointI].setSize(sz+1);
-                surfaceLevel[pointI][sz] = globalRegionLevel[region];
+                surfaceLevel[pointI][sz] = globalMaxLevel[region];
             }
         }
     }
 }
 
 
-void Foam::refinementSurfaces::findAllHigherIntersections
+void Foam::refinementSurfaces::findAllIntersections
 (
     const pointField& start,
     const pointField& end,
     const labelList& currentLevel,   // current cell refinement level
 
-    const labelList& globalRegionLevel,
+    const labelList& globalMinLevel,
+    const labelList& globalMaxLevel,
 
     List<pointList>& surfaceLocation,
     List<vectorList>& surfaceNormal,
@@ -1063,7 +1091,11 @@ void Foam::refinementSurfaces::findAllHigherIntersections
             label region = globalRegion(surfI, surfRegion[i]);
             label pointI = pointMap[i];
 
-            if (globalRegionLevel[region] > currentLevel[pointI])
+            if
+            (
+                currentLevel[pointI] >= globalMinLevel[region]
+             && currentLevel[pointI] < globalMaxLevel[region]
+            )
             {
                 // Append to pointI info
                 label sz = surfaceNormal[pointI].size();
@@ -1074,7 +1106,9 @@ void Foam::refinementSurfaces::findAllHigherIntersections
                 surfaceNormal[pointI][sz] = surfNormal[i];
 
                 surfaceLevel[pointI].setSize(sz+1);
-                surfaceLevel[pointI][sz] = globalRegionLevel[region];
+                // Level should just be higher than provided point level.
+                // Actual value is not important.
+                surfaceLevel[pointI][sz] = globalMaxLevel[region];
             }
         }
     }
diff --git a/src/mesh/snappyHexMesh/refinementSurfaces/refinementSurfaces.H b/src/mesh/snappyHexMesh/refinementSurfaces/refinementSurfaces.H
index 26f695ef5c..c438b0c6a8 100644
--- a/src/mesh/snappyHexMesh/refinementSurfaces/refinementSurfaces.H
+++ b/src/mesh/snappyHexMesh/refinementSurfaces/refinementSurfaces.H
@@ -88,6 +88,10 @@ class refinementSurfaces
         //- From global region number to small-gap level
         labelList gapLevel_;
 
+        //- From global region number to cell level at which blockage detection
+        //  needs to apply
+        labelList blockLevel_;
+
         //- From global region number to small-gap level specification
         List<FixedList<label, 3>> extendedGapLevel_;
 
@@ -202,6 +206,13 @@ public:
                 return gapLevel_;
             }
 
+            //- From global region number to cell level at which blockage
+            //- detection is applied
+            const labelList& blockLevel() const
+            {
+                return blockLevel_;
+            }
+
             //- From global region number to specification of gap and its
             //  refinement: 3 labels specifying
             //  - minimum wanted number of cells in the gap
@@ -287,25 +298,31 @@ public:
                 labelList& surfaceLevel
             ) const;
 
-            //- Find all intersections of edge. Unsorted order.
-            void findAllHigherIntersections
+            //- Find all intersections of edge with any surface with applicable
+            //  min/max refinement level. Unsorted order.
+            void findAllIntersections
             (
                 const pointField& start,
                 const pointField& end,
                 const labelList& currentLevel,  // current cell refinement level
-                const labelList& globalRegionLevel, // level per surfregion
+
+                const labelList& globalMinLevel,    // level per surfregion
+                const labelList& globalMaxLevel,    // level per surfregion
 
                 List<vectorList>& surfaceNormal,
                 labelListList& surfaceLevel
             ) const;
 
-            //- Find all intersections of edge. Unsorted order.
-            void findAllHigherIntersections
+            //- Find all intersections of edge with any surface with applicable
+            //  min/max refinement level. Unsorted order.
+            void findAllIntersections
             (
                 const pointField& start,
                 const pointField& end,
                 const labelList& currentLevel,  // current cell refinement level
-                const labelList& globalRegionLevel, // level per surfregion
+
+                const labelList& globalMinLevel,    // level per surfregion
+                const labelList& globalMaxLevel,    // level per surfregion
 
                 List<pointList>& surfaceLocation,
                 List<vectorList>& surfaceNormal,
diff --git a/src/mesh/snappyHexMesh/shellSurfaces/shellSurfaces.H b/src/mesh/snappyHexMesh/shellSurfaces/shellSurfaces.H
index 26a8a263d1..c5763abf99 100644
--- a/src/mesh/snappyHexMesh/shellSurfaces/shellSurfaces.H
+++ b/src/mesh/snappyHexMesh/shellSurfaces/shellSurfaces.H
@@ -193,6 +193,18 @@ public:
                 return shells_;
             }
 
+            //- Per shell, per region the small-gap level specification
+            const List<List<FixedList<label, 3>>>& extendedGapLevel() const
+            {
+                return extendedGapLevel_;
+            }
+
+            //- Per shell, per region the small-gap level specification
+            const List<List<volumeType>>& extendedGapMode() const
+            {
+                return extendedGapMode_;
+            }
+
         // Query
 
             //- Highest shell level
diff --git a/src/mesh/snappyHexMesh/snappyHexMeshDriver/refinementParameters/refinementParameters.C b/src/mesh/snappyHexMesh/snappyHexMeshDriver/refinementParameters/refinementParameters.C
index c36516f918..47b4edcf48 100644
--- a/src/mesh/snappyHexMesh/snappyHexMeshDriver/refinementParameters/refinementParameters.C
+++ b/src/mesh/snappyHexMesh/snappyHexMeshDriver/refinementParameters/refinementParameters.C
@@ -2,7 +2,7 @@
   =========                 |
   \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
    \\    /   O peration     |
-    \\  /    A nd           | Copyright (C) 2015-2018 OpenCFD Ltd.
+    \\  /    A nd           | Copyright (C) 2015-2019 OpenCFD Ltd.
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
                             | Copyright (C) 2011-2015 OpenFOAM Foundation
@@ -92,6 +92,7 @@ Foam::refinementParameters::refinementParameters
         dict.lookupOrDefault<Switch>("interfaceRefine", true)
     ),
     nErodeCellZone_(dict.lookupOrDefault<label>("nCellZoneErodeIter", 0)),
+    nFilterIter_(dict.lookupOrDefault<label>("nFilterIter", 2)),
     dryRun_(dryRun)
 {
     point locationInMesh;
diff --git a/src/mesh/snappyHexMesh/snappyHexMeshDriver/refinementParameters/refinementParameters.H b/src/mesh/snappyHexMesh/snappyHexMeshDriver/refinementParameters/refinementParameters.H
index 7c4f095348..3c81a2f31a 100644
--- a/src/mesh/snappyHexMesh/snappyHexMeshDriver/refinementParameters/refinementParameters.H
+++ b/src/mesh/snappyHexMesh/snappyHexMeshDriver/refinementParameters/refinementParameters.H
@@ -2,7 +2,7 @@
   =========                 |
   \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
    \\    /   O peration     |
-    \\  /    A nd           | Copyright (C) 2015 OpenCFD Ltd.
+    \\  /    A nd           | Copyright (C) 2015-2019 OpenCFD Ltd.
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
                             | Copyright (C) 2011-2014 OpenFOAM Foundation
@@ -90,7 +90,7 @@ class refinementParameters
             wordList zonesInMesh_;
 
             //- Information on how to handle faces on faceZones
-            dictionary faceZoneControls_;
+            const dictionary faceZoneControls_;
 
 
         //- FaceZone faces allowed which have owner and neighbour in same
@@ -110,6 +110,8 @@ class refinementParameters
 
         const label nErodeCellZone_;
 
+        const label nFilterIter_;
+
         const bool dryRun_;
 
 
@@ -219,11 +221,19 @@ public:
                 return interfaceRefine_;
             }
 
+            //- Number of iterations of erosion to get a consistent cellZone
             label nErodeCellZone() const
             {
                 return nErodeCellZone_;
             }
 
+            //- Number of iterations of infill/erosion to get a smoother
+            //  blockage
+            label nFilterIter() const
+            {
+                return nFilterIter_;
+            }
+
 
         // Other
 
diff --git a/src/mesh/snappyHexMesh/snappyHexMeshDriver/snappyRefineDriver.C b/src/mesh/snappyHexMesh/snappyHexMeshDriver/snappyRefineDriver.C
index 5a2957b113..75ba0a0f15 100644
--- a/src/mesh/snappyHexMesh/snappyHexMeshDriver/snappyRefineDriver.C
+++ b/src/mesh/snappyHexMesh/snappyHexMeshDriver/snappyRefineDriver.C
@@ -549,6 +549,22 @@ Foam::label Foam::snappyRefineDriver::gapOnlyRefine
 
         if (debug&meshRefinement::MESH)
         {
+            Pout<< "Writing current mesh to time "
+                << meshRefiner_.timeName() << endl;
+            meshRefiner_.write
+            (
+                meshRefinement::debugType(debug),
+                meshRefinement::writeType
+                (
+                    meshRefinement::writeLevel()
+                  | meshRefinement::WRITEMESH
+                ),
+                mesh.time().path()/meshRefiner_.timeName()
+            );
+            Pout<< "Dumped mesh in = "
+                << mesh.time().cpuTimeIncrement() << " s\n" << nl << endl;
+
+
             Pout<< "Dumping " << candidateCells.size()
                 << " cells to cellSet candidateCellsFromGap." << endl;
             cellSet c(mesh, "candidateCellsFromGap", candidateCells);
@@ -716,6 +732,56 @@ Foam::label Foam::snappyRefineDriver::gapOnlyRefine
 }
 
 
+Foam::label Foam::snappyRefineDriver::surfaceProximityBlock
+(
+    const refinementParameters& refineParams,
+    const label maxIter
+)
+{
+    if (refineParams.minRefineCells() == -1)
+    {
+        // Special setting to be able to restart shm on meshes with inconsistent
+        // cellLevel/pointLevel
+        return 0;
+    }
+
+    fvMesh& mesh = meshRefiner_.mesh();
+
+    if (min(meshRefiner_.surfaces().blockLevel()) == labelMax)
+    {
+        return 0;
+    }
+
+    label iter = 0;
+
+    for (iter = 0; iter < maxIter; iter++)
+    {
+        Info<< nl
+            << "Gap blocking iteration " << iter << nl
+            << "------------------------" << nl
+            << endl;
+
+
+        // Determine cells to block
+        // ~~~~~~~~~~~~~~~~~~~~~~~~
+
+        meshRefiner_.removeGapCells
+        (
+            refineParams.planarAngle(),
+            meshRefiner_.surfaces().blockLevel(),
+            globalToMasterPatch_,
+            refineParams.nFilterIter()
+        );
+
+        if (debug)
+        {
+            const_cast<Time&>(mesh.time())++;
+        }
+    }
+    return iter;
+}
+
+
 Foam::label Foam::snappyRefineDriver::bigGapOnlyRefine
 (
     const refinementParameters& refineParams,
@@ -787,6 +853,21 @@ Foam::label Foam::snappyRefineDriver::bigGapOnlyRefine
 
         if (debug&meshRefinement::MESH)
         {
+            Pout<< "Writing current mesh to time "
+                << meshRefiner_.timeName() << endl;
+            meshRefiner_.write
+            (
+                meshRefinement::debugType(debug),
+                meshRefinement::writeType
+                (
+                    meshRefinement::writeLevel()
+                  | meshRefinement::WRITEMESH
+                ),
+                mesh.time().path()/meshRefiner_.timeName()
+            );
+            Pout<< "Dumped mesh in = "
+                << mesh.time().cpuTimeIncrement() << " s\n" << nl << endl;
+
             Pout<< "Dumping " << candidateCells.size()
                 << " cells to cellSet candidateCellsFromBigGap." << endl;
             cellSet c(mesh, "candidateCellsFromBigGap", candidateCells);
@@ -1421,7 +1502,7 @@ void Foam::snappyRefineDriver::removeInsideCells
     );
 
     // Fix any additional (e.g. locationsOutsideMesh). Note: probably not
-    // nessecary.
+    // necessary.
     meshRefiner_.splitMesh
     (
         nBufferLayers,                  // nBufferLayers
@@ -2859,6 +2940,13 @@ void Foam::snappyRefineDriver::doRefine
         100     // maxIter
     );
 
+    // Remove cells inbetween two surfaces
+    surfaceProximityBlock
+    (
+        refineParams,
+        1  //100     // maxIter
+    );
+
     // Remove cells (a certain distance) beyond surface intersections
     removeInsideCells
     (
diff --git a/src/mesh/snappyHexMesh/snappyHexMeshDriver/snappyRefineDriver.H b/src/mesh/snappyHexMesh/snappyHexMeshDriver/snappyRefineDriver.H
index ab4d6a06e4..eeec7ed89a 100644
--- a/src/mesh/snappyHexMesh/snappyHexMeshDriver/snappyRefineDriver.H
+++ b/src/mesh/snappyHexMesh/snappyHexMeshDriver/snappyRefineDriver.H
@@ -134,6 +134,13 @@ class snappyRefineDriver
             const label maxIter
         );
 
+        //- Block all cells in small gaps
+        label surfaceProximityBlock
+        (
+            const refinementParameters& refineParams,
+            const label maxIter
+        );
+
         //- Refine all cells in large gaps
         label bigGapOnlyRefine
         (
diff --git a/src/meshTools/searchableSurfaces/triSurfaceMesh/triSurfaceMesh.C b/src/meshTools/searchableSurfaces/triSurfaceMesh/triSurfaceMesh.C
index e0677dd0e0..0a0a09597e 100644
--- a/src/meshTools/searchableSurfaces/triSurfaceMesh/triSurfaceMesh.C
+++ b/src/meshTools/searchableSurfaces/triSurfaceMesh/triSurfaceMesh.C
@@ -106,7 +106,7 @@ Foam::fileName Foam::triSurfaceMesh::checkFile
     {
         const fileName rawFName(fName);
 
-        fName = relativeFilePath(io, fName, isGlobal);
+        fName = relativeFilePath(io, rawFName, isGlobal);
 
         if (!exists(fName))
         {
diff --git a/tutorials/mesh/snappyHexMesh/opposite_walls/0/polyMesh/sets/cellsToRemove b/tutorials/mesh/snappyHexMesh/opposite_walls/0/polyMesh/sets/cellsToRemove
new file mode 100644
index 0000000000..d60446d02a
--- /dev/null
+++ b/tutorials/mesh/snappyHexMesh/opposite_walls/0/polyMesh/sets/cellsToRemove
@@ -0,0 +1,21 @@
+/*--------------------------------*- C++ -*----------------------------------*\
+| =========                 |                                                 |
+| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
+|  \\    /   O peration     | Version:  v1806                                 |
+|   \\  /    A nd           | Web:      www.OpenFOAM.com                      |
+|    \\/     M anipulation  |                                                 |
+\*---------------------------------------------------------------------------*/
+FoamFile
+{
+    version     2.0;
+    format      binary;
+    class       cellSet;
+    arch        "LSB;label=32;scalar=64";
+    location    "0/polyMesh/sets";
+    object      cellsToRemove;
+}
+// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
+
+0()
+
+// ************************************************************************* //
diff --git a/tutorials/mesh/snappyHexMesh/opposite_walls/Allclean b/tutorials/mesh/snappyHexMesh/opposite_walls/Allclean
new file mode 100755
index 0000000000..be7ddb0688
--- /dev/null
+++ b/tutorials/mesh/snappyHexMesh/opposite_walls/Allclean
@@ -0,0 +1,7 @@
+#!/bin/sh
+cd ${0%/*} || exit 1                        # Run from this directory
+. $WM_PROJECT_DIR/bin/tools/CleanFunctions  # Tutorial clean functions
+
+cleanCase
+
+#------------------------------------------------------------------------------
diff --git a/tutorials/mesh/snappyHexMesh/opposite_walls/Allrun b/tutorials/mesh/snappyHexMesh/opposite_walls/Allrun
new file mode 100755
index 0000000000..b97b2ffde1
--- /dev/null
+++ b/tutorials/mesh/snappyHexMesh/opposite_walls/Allrun
@@ -0,0 +1,15 @@
+#!/bin/sh
+cd ${0%/*} || exit 1                        # Run from this directory
+. $WM_PROJECT_DIR/bin/tools/RunFunctions    # Tutorial run functions
+
+runApplication blockMesh
+
+runApplication snappyHexMesh
+
+#runApplication decomposePar
+#
+#runParallel snappyHexMesh -overwrite
+#
+#runApplication reconstructParMesh -constant
+
+#------------------------------------------------------------------------------
diff --git a/tutorials/mesh/snappyHexMesh/opposite_walls/constant/triSurface/gap0.stl b/tutorials/mesh/snappyHexMesh/opposite_walls/constant/triSurface/gap0.stl
new file mode 100644
index 0000000000..7e8153b9ff
--- /dev/null
+++ b/tutorials/mesh/snappyHexMesh/opposite_walls/constant/triSurface/gap0.stl
@@ -0,0 +1,702 @@
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diff --git a/tutorials/mesh/snappyHexMesh/opposite_walls/constant/triSurface/gap1.stl b/tutorials/mesh/snappyHexMesh/opposite_walls/constant/triSurface/gap1.stl
new file mode 100644
index 0000000000..c3552e532d
--- /dev/null
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diff --git a/tutorials/mesh/snappyHexMesh/opposite_walls/constant/triSurface/gap_transformed0.stl b/tutorials/mesh/snappyHexMesh/opposite_walls/constant/triSurface/gap_transformed0.stl
new file mode 100644
index 0000000000..8d52d72f5e
--- /dev/null
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diff --git a/tutorials/mesh/snappyHexMesh/opposite_walls/constant/triSurface/gap_transformed0_with_hook.stl b/tutorials/mesh/snappyHexMesh/opposite_walls/constant/triSurface/gap_transformed0_with_hook.stl
new file mode 100644
index 0000000000..93b3359a4c
--- /dev/null
+++ b/tutorials/mesh/snappyHexMesh/opposite_walls/constant/triSurface/gap_transformed0_with_hook.stl
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diff --git a/tutorials/mesh/snappyHexMesh/opposite_walls/constant/triSurface/gap_transformed1.stl b/tutorials/mesh/snappyHexMesh/opposite_walls/constant/triSurface/gap_transformed1.stl
new file mode 100644
index 0000000000..a478b610a3
--- /dev/null
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+ endfacet
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diff --git a/tutorials/mesh/snappyHexMesh/opposite_walls/system/blockMeshDict b/tutorials/mesh/snappyHexMesh/opposite_walls/system/blockMeshDict
new file mode 100644
index 0000000000..b7b28115b9
--- /dev/null
+++ b/tutorials/mesh/snappyHexMesh/opposite_walls/system/blockMeshDict
@@ -0,0 +1,98 @@
+/*--------------------------------*- C++ -*----------------------------------*\
+| =========                 |                                                 |
+| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
+|  \\    /   O peration     | Version:  v1806                                 |
+|   \\  /    A nd           | Web:      www.OpenFOAM.com                      |
+|    \\/     M anipulation  |                                                 |
+\*---------------------------------------------------------------------------*/
+FoamFile
+{
+    version     2.0;
+    format      ascii;
+    class       dictionary;
+    object      blockMeshDict;
+}
+
+// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
+
+scale   1;
+
+vertices
+(
+    (-1   -3  -1)
+    ( 5   -3  -1)
+    ( 5    3  -1)
+    (-1    3  -1)
+    (-1   -3   5)
+    ( 5   -3   5)
+    ( 5    3   5)
+    (-1    3   5)
+);
+
+blocks
+(
+    hex (0 1 2 3 4 5 6 7) (6 6 6) simpleGrading (1 1 1)
+);
+
+edges
+(
+);
+
+boundary
+(
+    maxY
+    {
+        type patch;
+        faces
+        (
+            (3 7 6 2)
+        );
+    }
+
+    minX
+    {
+        type patch;
+        faces
+        (
+            (0 4 7 3)
+        );
+    }
+
+    maxX
+    {
+        type patch;
+        faces
+        (
+            (2 6 5 1)
+        );
+    }
+
+    minY
+    {
+        type patch;
+        faces
+        (
+            (1 5 4 0)
+        );
+    }
+
+    minZ
+    {
+        type patch;
+        faces
+        (
+            (0 3 2 1)
+        );
+    }
+
+    maxZ
+    {
+        type patch;
+        faces
+        (
+            (4 5 6 7)
+        );
+    }
+);
+
+// ************************************************************************* //
diff --git a/tutorials/mesh/snappyHexMesh/opposite_walls/system/controlDict b/tutorials/mesh/snappyHexMesh/opposite_walls/system/controlDict
new file mode 100644
index 0000000000..b9a88e8955
--- /dev/null
+++ b/tutorials/mesh/snappyHexMesh/opposite_walls/system/controlDict
@@ -0,0 +1,49 @@
+/*--------------------------------*- C++ -*----------------------------------*\
+| =========                 |                                                 |
+| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
+|  \\    /   O peration     | Version:  v1806 s                               |
+|   \\  /    A nd           | Web:      www.OpenFOAM.com                      |
+|    \\/     M anipulation  |                                                 |
+\*---------------------------------------------------------------------------*/
+FoamFile
+{
+    version     2.0;
+    format      ascii;
+    class       dictionary;
+    location    "system";
+    object      controlDict;
+}
+// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
+
+application     snappyHexMesh;
+
+startFrom       startTime;
+
+startTime       0;
+
+stopAt          endTime;
+
+endTime         2000;
+
+deltaT          1;
+
+writeControl    timeStep;
+
+writeInterval   100;
+
+purgeWrite      0;
+
+writeFormat     binary;
+
+writePrecision  6;
+
+writeCompression off;
+
+timeFormat      general;
+
+timePrecision   6;
+
+runTimeModifiable true;
+
+
+// ************************************************************************* //
diff --git a/tutorials/mesh/snappyHexMesh/opposite_walls/system/decomposeParDict b/tutorials/mesh/snappyHexMesh/opposite_walls/system/decomposeParDict
new file mode 100644
index 0000000000..edfa32d242
--- /dev/null
+++ b/tutorials/mesh/snappyHexMesh/opposite_walls/system/decomposeParDict
@@ -0,0 +1,30 @@
+/*--------------------------------*- C++ -*----------------------------------*\
+| =========                 |                                                 |
+| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
+|  \\    /   O peration     | Version:  v1806                                 |
+|   \\  /    A nd           | Web:      www.OpenFOAM.com                      |
+|    \\/     M anipulation  |                                                 |
+\*---------------------------------------------------------------------------*/
+FoamFile
+{
+    version     2.0;
+    format      ascii;
+    class       dictionary;
+    object      decomposeParDict;
+}
+// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
+
+numberOfSubdomains 8;
+
+method          hierarchical;
+
+coeffs
+{
+    n           (2 2 2);
+    //delta       0.001; // default=0.001
+    //order       xyz;   // default=xzy
+    dataFile    "cellDecomposition";
+}
+
+
+// ************************************************************************* //
diff --git a/tutorials/mesh/snappyHexMesh/opposite_walls/system/fvSchemes b/tutorials/mesh/snappyHexMesh/opposite_walls/system/fvSchemes
new file mode 100644
index 0000000000..8ef0651ea1
--- /dev/null
+++ b/tutorials/mesh/snappyHexMesh/opposite_walls/system/fvSchemes
@@ -0,0 +1,57 @@
+/*--------------------------------*- C++ -*----------------------------------*\
+| =========                 |                                                 |
+| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
+|  \\    /   O peration     | Version:  v1806                                 |
+|   \\  /    A nd           | Web:      www.OpenFOAM.com                      |
+|    \\/     M anipulation  |                                                 |
+\*---------------------------------------------------------------------------*/
+FoamFile
+{
+    version     2.0;
+    format      ascii;
+    class       dictionary;
+    location    "system";
+    object      fvSchemes;
+}
+// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
+
+ddtSchemes
+{
+    default         steadyState;
+}
+
+gradSchemes
+{
+    default         Gauss linear;
+}
+
+divSchemes
+{
+    default         none;
+
+    div(phi,U)      bounded Gauss upwind;
+    div(phi,T)      bounded Gauss upwind;
+    div(phi,k)      bounded Gauss upwind;
+    div(phi,epsilon) bounded Gauss upwind;
+    div(phi,R)      bounded Gauss upwind;
+    div(R)          Gauss linear;
+    div((nuEff*dev(T(grad(U))))) Gauss linear;
+}
+
+laplacianSchemes
+{
+    default         Gauss linear limited corrected 0.333;
+}
+
+interpolationSchemes
+{
+    default         linear;
+}
+
+snGradSchemes
+{
+    default         limited corrected 0.333;
+}
+
+
+// ************************************************************************* //
diff --git a/tutorials/mesh/snappyHexMesh/opposite_walls/system/fvSolution b/tutorials/mesh/snappyHexMesh/opposite_walls/system/fvSolution
new file mode 100644
index 0000000000..b8ea24dd90
--- /dev/null
+++ b/tutorials/mesh/snappyHexMesh/opposite_walls/system/fvSolution
@@ -0,0 +1,69 @@
+/*--------------------------------*- C++ -*----------------------------------*\
+| =========                 |                                                 |
+| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
+|  \\    /   O peration     | Version:  v1806                                 |
+|   \\  /    A nd           | Web:      www.OpenFOAM.com                      |
+|    \\/     M anipulation  |                                                 |
+\*---------------------------------------------------------------------------*/
+FoamFile
+{
+    version     2.0;
+    format      ascii;
+    class       dictionary;
+    location    "system";
+    object      fvSolution;
+}
+// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
+
+solvers
+{
+    p_rgh
+    {
+        solver          PCG;
+        preconditioner  DIC;
+        tolerance       1e-08;
+        relTol          0.01;
+    }
+
+    "(U|T|k|epsilon)"
+    {
+        solver          PBiCGStab;
+        preconditioner  DILU;
+        tolerance       1e-07;
+        relTol          0.1;
+    }
+}
+
+SIMPLE
+{
+    nNonOrthogonalCorrectors 2;
+    pRefCell        0;
+    pRefValue       0;
+
+    residualControl
+    {
+        p_rgh           1e-2;
+        U               1e-4;
+        T               1e-3;
+
+        // possibly check turbulence fields
+        "(k|epsilon|omega)" 1e-3;
+    }
+}
+
+relaxationFactors
+{
+    fields
+    {
+        p_rgh           0.7;
+    }
+    equations
+    {
+        U               0.2;
+        T               0.5;
+        "(k|epsilon)"   0.7;
+    }
+}
+
+
+// ************************************************************************* //
diff --git a/tutorials/mesh/snappyHexMesh/opposite_walls/system/meshQualityDict b/tutorials/mesh/snappyHexMesh/opposite_walls/system/meshQualityDict
new file mode 100644
index 0000000000..f94681c006
--- /dev/null
+++ b/tutorials/mesh/snappyHexMesh/opposite_walls/system/meshQualityDict
@@ -0,0 +1,21 @@
+/*--------------------------------*- C++ -*----------------------------------*\
+| =========                 |                                                 |
+| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
+|  \\    /   O peration     | Version:  v1806                                 |
+|   \\  /    A nd           | Web:      www.OpenFOAM.com                      |
+|    \\/     M anipulation  |                                                 |
+\*---------------------------------------------------------------------------*/
+FoamFile
+{
+    version     2.0;
+    format      ascii;
+    class       dictionary;
+    object      meshQualityDict;
+}
+// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
+
+// Include defaults parameters from master dictionary
+#includeEtc "caseDicts/meshQualityDict"
+
+
+// ************************************************************************* //
diff --git a/tutorials/mesh/snappyHexMesh/opposite_walls/system/snappyHexMeshDict b/tutorials/mesh/snappyHexMesh/opposite_walls/system/snappyHexMeshDict
new file mode 100644
index 0000000000..66ef0e1f46
--- /dev/null
+++ b/tutorials/mesh/snappyHexMesh/opposite_walls/system/snappyHexMeshDict
@@ -0,0 +1,326 @@
+/*--------------------------------*- C++ -*----------------------------------*\
+| =========                 |                                                 |
+| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
+|  \\    /   O peration     | Version:  v1806                               |
+|   \\  /    A nd           | Web:      www.OpenFOAM.com                      |
+|    \\/     M anipulation  |                                                 |
+\*---------------------------------------------------------------------------*/
+FoamFile
+{
+    version     2.0;
+    format      ascii;
+    class       dictionary;
+    object      snappyHexMeshDict;
+}
+// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
+
+// Which of the steps to run
+castellatedMesh true;
+snap            true;
+addLayers       false;
+
+
+// Geometry. Definition of all surfaces. All surfaces are of class
+// searchableSurface.
+// Surfaces are used
+// - to specify refinement for any mesh cell intersecting it
+// - to specify refinement for any mesh cell inside/outside/near
+// - to 'snap' the mesh boundary to the surface
+geometry
+{
+    gap0
+    {
+        file "gap_transformed0.stl";
+        type triSurfaceMesh;
+    }
+    gap1
+    {
+        file "gap_transformed1.stl";
+        type triSurfaceMesh;
+    }
+    all
+    {
+        type searchableBox;
+        min (-1000 -1000 -1000);
+        max (1000 1000 1000);
+    }
+};
+
+
+// Settings for the castellatedMesh generation.
+castellatedMeshControls
+{
+
+    // Refinement parameters
+    // ~~~~~~~~~~~~~~~~~~~~~
+
+    // If local number of cells is >= maxLocalCells on any processor
+    // switches from from refinement followed by balancing
+    // (current method) to (weighted) balancing before refinement.
+    maxLocalCells 100000;
+
+    // Overall cell limit (approximately). Refinement will stop immediately
+    // upon reaching this number so a refinement level might not complete.
+    // Note that this is the number of cells before removing the part which
+    // is not 'visible' from the keepPoint. The final number of cells might
+    // actually be a lot less.
+    maxGlobalCells 2000000;
+
+    // The surface refinement loop might spend lots of iterations refining just a
+    // few cells. This setting will cause refinement to stop if <= minimumRefine
+    // are selected for refinement. Note: it will at least do one iteration
+    // (unless the number of cells to refine is 0)
+    minRefinementCells 0;
+
+    // Number of buffer layers between different levels.
+    // 1 means normal 2:1 refinement restriction, larger means slower
+    // refinement.
+    nCellsBetweenLevels 1;
+
+
+
+    // Explicit feature edge refinement
+    // ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+    // Specifies a level for any cell intersected by its edges.
+    // This is a featureEdgeMesh, read from constant/triSurface for now.
+    features
+    (
+    );
+
+
+
+    // Surface based refinement
+    // ~~~~~~~~~~~~~~~~~~~~~~~~
+
+    // Specifies two levels for every surface. The first is the minimum level,
+    // every cell intersecting a surface gets refined up to the minimum level.
+    // The second level is the maximum level. Cells that 'see' multiple
+    // intersections where the intersections make an
+    // angle > resolveFeatureAngle get refined up to the maximum level.
+
+    refinementSurfaces
+    {
+        "gap.*"
+        {
+            // Surface-wise min and max refinement level
+            level       (2 2);
+            // Additional refinement in tight gaps
+            //MEJ: only to trigger entering gap finding
+            //gapLevelIncrement 1;
+
+            //MEJ: from cell level 2 onwards start checking for opposite
+            //     surfaces
+            //gapLevel    (1 2 10);
+
+            //MEJ: from cell level 2 onwards start checking for opposite
+            //     surfaces
+            blockLevel  2;
+        }
+    }
+
+    // Resolve sharp angles
+    resolveFeatureAngle 60;
+
+
+    // Region-wise refinement
+    // ~~~~~~~~~~~~~~~~~~~~~~
+
+    // Specifies refinement level for cells in relation to a surface. One of
+    // three modes
+    // - distance. 'levels' specifies per distance to the surface the
+    //   wanted refinement level. The distances need to be specified in
+    //   descending order.
+    // - inside. 'levels' is only one entry and only the level is used. All
+    //   cells inside the surface get refined up to the level. The surface
+    //   needs to be closed for this to be possible.
+    // - outside. Same but cells outside.
+
+    refinementRegions
+    {
+        all
+        {
+            mode        inside;
+
+            // Dummy base level
+            levels      ((10000 0));
+
+            // If cells
+            // - have level 4..9
+            // - and are in a gap < 3 cell sizes across
+            // - with the gap on the inside ('inside'), outside ('outside')
+            //   or both ('mixed') of the surface
+            // refine them
+            gapLevel    (100 100 100);
+            gapMode     inside;
+        }
+    }
+
+
+    // Mesh selection
+    // ~~~~~~~~~~~~~~
+
+    // After refinement patches get added for all refinementSurfaces and
+    // all cells intersecting the surfaces get put into these patches. The
+    // section reachable from the locationInMesh is kept.
+    // NOTE: This point should never be on a face, always inside a cell, even
+    // after refinement.
+    locationInMesh (1.5 -0.6 2.1);
+
+
+    // Whether any faceZones (as specified in the refinementSurfaces)
+    // are only on the boundary of corresponding cellZones or also allow
+    // free-standing zone faces. Not used if there are no faceZones.
+    allowFreeStandingZoneFaces false;
+}
+
+
+
+// Settings for the snapping.
+snapControls
+{
+    //- Number of patch smoothing iterations before finding correspondence
+    //  to surface
+    nSmoothPatch 3;
+
+    //- Relative distance for points to be attracted by surface feature point
+    //  or edge. True distance is this factor times local
+    //  maximum edge length.
+    tolerance 2.0;
+
+    //- Number of mesh displacement relaxation iterations.
+    nSolveIter 30;
+
+    //- Maximum number of snapping relaxation iterations. Should stop
+    //  before upon reaching a correct mesh.
+    nRelaxIter 5;
+
+
+    // Feature snapping
+
+        //- Number of feature edge snapping iterations.
+        //  Leave out altogether to disable.
+        nFeatureSnapIter 10;
+
+        //- Detect (geometric) features by sampling the surface (default=false)
+        //implicitFeatureSnap true;
+
+        //- Use castellatedMeshControls::features (default = true)
+        explicitFeatureSnap true;
+}
+
+
+
+// Settings for the layer addition.
+addLayersControls
+{
+    // Are the thickness parameters below relative to the undistorted
+    // size of the refined cell outside layer (true) or absolute sizes (false).
+    relativeSizes true;
+
+    // Per final patch (so not geometry!) the layer information
+    layers
+    {
+    }
+
+    // Expansion factor for layer mesh
+    expansionRatio 1.0;
+
+    // Wanted thickness of final added cell layer. If multiple layers
+    // is the thickness of the layer furthest away from the wall.
+    // Relative to undistorted size of cell outside layer.
+    // is the thickness of the layer furthest away from the wall.
+    // See relativeSizes parameter.
+    finalLayerThickness 0.5;
+
+    // Minimum thickness of cell layer. If for any reason layer
+    // cannot be above minThickness do not add layer.
+    // Relative to undistorted size of cell outside layer.
+    // See relativeSizes parameter.
+    minThickness 0.25;
+
+    // If points get not extruded do nGrow layers of connected faces that are
+    // also not grown. This helps convergence of the layer addition process
+    // close to features.
+    // Note: changed(corrected) w.r.t 1.7.x! (didn't do anything in 1.7.x)
+    nGrow 0;
+
+
+    // Advanced settings
+
+    // When not to extrude surface. 0 is flat surface, 90 is when two faces
+    // are perpendicular
+    featureAngle 60;
+
+    // Maximum number of snapping relaxation iterations. Should stop
+    // before upon reaching a correct mesh.
+    nRelaxIter 5;
+
+    // Number of smoothing iterations of surface normals
+    nSmoothSurfaceNormals 1;
+
+    // Number of smoothing iterations of interior mesh movement direction
+    nSmoothNormals 3;
+
+    // Smooth layer thickness over surface patches
+    nSmoothThickness 10;
+
+    // Stop layer growth on highly warped cells
+    maxFaceThicknessRatio 0.5;
+
+    // Reduce layer growth where ratio thickness to medial
+    // distance is large
+    maxThicknessToMedialRatio 0.3;
+
+    // Angle used to pick up medial axis points
+    // Note: changed(corrected) w.r.t 16x! 90 degrees corresponds to 130 in 16x.
+    minMedianAxisAngle 90;
+
+    // Create buffer region for new layer terminations
+    nBufferCellsNoExtrude 0;
+
+
+    // Overall max number of layer addition iterations. The mesher will exit
+    // if it reaches this number of iterations; possibly with an illegal
+    // mesh.
+    nLayerIter 50;
+}
+
+
+
+// Generic mesh quality settings. At any undoable phase these determine
+// where to undo.
+meshQualityControls
+{
+    #include "meshQualityDict"
+
+    // Advanced
+
+    //- Number of error distribution iterations
+    nSmoothScale 4;
+    //- amount to scale back displacement at error points
+    errorReduction 0.75;
+}
+
+
+// Advanced
+
+// Merge tolerance. Is fraction of overall bounding box of initial mesh.
+// Note: the write tolerance needs to be higher than this.
+mergeTolerance 1e-6;
+
+
+//// Debug flags
+//debugFlags
+//(
+//    mesh            // write intermediate meshes
+//    attraction      // write attraction as .obj files
+//);
+//
+//// Write flags
+//writeFlags
+//(
+//    scalarLevels    // write volScalarField with cellLevel for postprocessing
+//);
+
+// ************************************************************************* //