From ff8518a89a2bf41e2e22f3c0eb3461ac320fd1f6 Mon Sep 17 00:00:00 2001
From: mattijs <mattijs>
Date: Thu, 24 Feb 2022 09:41:21 +0000
Subject: [PATCH] ENH: snappyHexMesh: express small regions. Fixes #2379

Extend selection of small cells to use absolute cell count
---
 etc/caseDicts/annotated/snappyHexMeshDict             |  2 ++
 .../refinementParameters/refinementParameters.C       |  1 +
 .../refinementParameters/refinementParameters.H       |  9 +++++++++
 .../snappyHexMeshDriver/snappyRefineDriver.C          | 11 ++++++-----
 4 files changed, 18 insertions(+), 5 deletions(-)

diff --git a/etc/caseDicts/annotated/snappyHexMeshDict b/etc/caseDicts/annotated/snappyHexMeshDict
index ea9fd6165c..5903d9a792 100644
--- a/etc/caseDicts/annotated/snappyHexMeshDict
+++ b/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.
diff --git a/src/mesh/snappyHexMesh/snappyHexMeshDriver/refinementParameters/refinementParameters.C b/src/mesh/snappyHexMesh/snappyHexMeshDriver/refinementParameters/refinementParameters.C
index f774db14ce..211b42411e 100644
--- a/src/mesh/snappyHexMesh/snappyHexMeshDriver/refinementParameters/refinementParameters.C
+++ b/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;
diff --git a/src/mesh/snappyHexMesh/snappyHexMeshDriver/refinementParameters/refinementParameters.H b/src/mesh/snappyHexMesh/snappyHexMeshDriver/refinementParameters/refinementParameters.H
index 068d1fe95a..cd7c4082ae 100644
--- a/src/mesh/snappyHexMesh/snappyHexMeshDriver/refinementParameters/refinementParameters.H
+++ b/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
 
diff --git a/src/mesh/snappyHexMesh/snappyHexMeshDriver/snappyRefineDriver.C b/src/mesh/snappyHexMesh/snappyHexMeshDriver/snappyRefineDriver.C
index 9d5d4411c1..b8ac1ac7f5 100644
--- a/src/mesh/snappyHexMesh/snappyHexMeshDriver/snappyRefineDriver.C
+++ b/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;
         }
     }