ENH: snappyHexMesh: express small regions. Fixes #2379

Extend selection of small cells to use absolute cell count

etc/caseDicts/annotated/snappyHexMeshDict

@@ -480,6 +480,8 @@ castellatedMeshControls
     //           when generating meshes with faceZones as baffles or boundary
     //           faces.
     //minCellFraction 0.001;
+    // Optional: same but in absolute number of cells. Default is 0.
+    //nMinCells 100;
 }
 
 // Settings for the snapping.

src/mesh/snappyHexMesh/snappyHexMeshDriver/refinementParameters/refinementParameters.C

@@ -95,6 +95,7 @@ Foam::refinementParameters::refinementParameters
     nErodeCellZone_(dict.getOrDefault<label>("nCellZoneErodeIter", 0)),
     nFilterIter_(dict.getOrDefault<label>("nFilterIter", 2)),
     minCellFraction_(dict.getOrDefault<scalar>("minCellFraction", 0)),
+    nMinCells_(dict.getOrDefault<label>("nMinCells", 0)),
     dryRun_(dryRun)
 {
     point locationInMesh;

src/mesh/snappyHexMesh/snappyHexMeshDriver/refinementParameters/refinementParameters.H

@@ -115,6 +115,8 @@ class refinementParameters
 
         const scalar minCellFraction_;
 
+        const label nMinCells_;
+
         const bool dryRun_;
 
 
@@ -244,6 +246,13 @@ public:
                 return minCellFraction_;
             }
 
+            //- When are disconnected regions small. Absolute number of cells
+            //  in a zone or background. Default 0.
+            label nMinCells() const
+            {
+                return nMinCells_;
+            }
+
 
         // Other
 

src/mesh/snappyHexMesh/snappyHexMeshDriver/snappyRefineDriver.C

@@ -6,7 +6,7 @@
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
     Copyright (C) 2011-2015 OpenFOAM Foundation
-    Copyright (C) 2015-2021 OpenCFD Ltd.
+    Copyright (C) 2015-2022 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -3209,21 +3209,22 @@ void Foam::snappyRefineDriver::deleteSmallRegions
     forAll(nCellsPerRegion, regioni)
     {
         const label zonei = regionToZone[regioni];
+        label& nRegionCells = nCellsPerRegion[regioni];
 
         if
         (
-            nCellsPerRegion[regioni]
-          < refineParams.minCellFraction()*nCellsPerZone[zonei]
+            nRegionCells < refineParams.minCellFraction()*nCellsPerZone[zonei]
+         || nRegionCells < refineParams.nMinCells()
         )
         {
             Info<< "Deleting region " << regioni
-                << " (size " << nCellsPerRegion[regioni]
+                << " (size " << nRegionCells
                 << ") of zone size " << nCellsPerZone[zonei]
                 << endl;
 
             // Mark region to be deleted. 0 size (= global) should never
             // occur.
-            nCellsPerRegion[regioni] = 0;
+            nRegionCells = 0;
         }
     }