From 9c13057b801aef1baf7c6313e330971c13acfb9f Mon Sep 17 00:00:00 2001
From: mattijs <mattijs>
Date: Wed, 20 Aug 2025 15:13:09 +0100
Subject: [PATCH] ENH: createPatch: do not remove added patches. Fixes #2726

---
 .../manipulation/createPatch/createPatch.C    | 12 +++--
 .../fvMesh/fvMeshTools/fvMeshTools.C          | 44 +++++++++++++++----
 .../fvMesh/fvMeshTools/fvMeshTools.H          |  8 ++++
 3 files changed, 52 insertions(+), 12 deletions(-)

diff --git a/applications/utilities/mesh/manipulation/createPatch/createPatch.C b/applications/utilities/mesh/manipulation/createPatch/createPatch.C
index cdad1c1fc8..045a2b7b42 100644
--- a/applications/utilities/mesh/manipulation/createPatch/createPatch.C
+++ b/applications/utilities/mesh/manipulation/createPatch/createPatch.C
@@ -42,14 +42,14 @@ Description
 
 \*---------------------------------------------------------------------------*/
 
-#include "cyclicPolyPatch.H"
-#include "syncTools.H"
 #include "argList.H"
 #include "Time.H"
 #include "OFstream.H"
 #include "meshTools.H"
 #include "faceSet.H"
 #include "IOPtrList.H"
+#include "cyclicPolyPatch.H"
+#include "syncTools.H"
 #include "polyTopoChange.H"
 #include "polyModifyFace.H"
 #include "polyAddFace.H"
@@ -942,6 +942,9 @@ int main(int argc, char *argv[])
     }
 
 
+    // Maintain list of added patches so we exclude them from filtering
+    // later on
+    List<DynamicList<word>> allAddedPatches(meshes.size());
 
     // Loop over all regions
 
@@ -1036,6 +1039,7 @@ int main(int argc, char *argv[])
                                 fvPatchFieldBase::calculatedType(),
                                 true
                             );
+                            allAddedPatches[meshi].append(ppPtr->name());
                         }
                     }
                 }
@@ -1087,6 +1091,7 @@ int main(int argc, char *argv[])
                                 fvPatchFieldBase::calculatedType(),
                                 true
                             );
+                            allAddedPatches[meshi].append(ppPtr->name());
                         }
                     }
                 }
@@ -1116,6 +1121,7 @@ int main(int argc, char *argv[])
                         fvPatchFieldBase::calculatedType(),
                         true
                     );
+                    allAddedPatches[meshi].append(ppPtr->name());
                 }
             }
         }
@@ -1453,7 +1459,7 @@ int main(int argc, char *argv[])
         Info<< "Removing patches with no faces in them." << nl << endl;
         const wordList oldPatchNames(mesh.boundaryMesh().names());
         const wordList oldPatchTypes(mesh.boundaryMesh().types());
-        fvMeshTools::removeEmptyPatches(mesh, true);
+        fvMeshTools::removeEmptyPatches(mesh, allAddedPatches[meshi], true);
         forAll(oldPatchNames, patchi)
         {
             const word& pName = oldPatchNames[patchi];
diff --git a/src/finiteVolume/fvMesh/fvMeshTools/fvMeshTools.C b/src/finiteVolume/fvMesh/fvMeshTools/fvMeshTools.C
index b647efca8c..aa794581a8 100644
--- a/src/finiteVolume/fvMesh/fvMeshTools/fvMeshTools.C
+++ b/src/finiteVolume/fvMesh/fvMeshTools/fvMeshTools.C
@@ -369,6 +369,7 @@ void Foam::fvMeshTools::reorderPatches
 Foam::labelList Foam::fvMeshTools::removeEmptyPatches
 (
     fvMesh& mesh,
+    const wordList& keepPatches,
     const bool validBoundary
 )
 {
@@ -386,16 +387,24 @@ Foam::labelList Foam::fvMeshTools::removeEmptyPatches
 
         if (!isA<processorPolyPatch>(pp))
         {
-            label nFaces = pp.size();
-            if (validBoundary)
-            {
-                reduce(nFaces, sumOp<label>());
-            }
-
-            if (nFaces > 0)
+            if (keepPatches.found(pp.name()))
             {
                 newToOld[newI] = patchI;
-                oldToNew[patchI] = newI++;
+                oldToNew[patchI] = newI++;                
+            }
+            else
+            {
+                label nFaces = pp.size();
+                if (validBoundary)
+                {
+                    reduce(nFaces, sumOp<label>());
+                }
+
+                if (nFaces > 0)
+                {
+                    newToOld[newI] = patchI;
+                    oldToNew[patchI] = newI++;
+                }
             }
         }
     }
@@ -405,7 +414,14 @@ Foam::labelList Foam::fvMeshTools::removeEmptyPatches
     {
         const polyPatch& pp = pbm[patchI];
 
-        if (isA<processorPolyPatch>(pp) && pp.size())
+        if
+        (
+            isA<processorPolyPatch>(pp)
+         && (
+                pp.size()
+             || keepPatches.found(pp.name())
+            )
+        )
         {
             newToOld[newI] = patchI;
             oldToNew[patchI] = newI++;
@@ -429,6 +445,16 @@ Foam::labelList Foam::fvMeshTools::removeEmptyPatches
 }
 
 
+Foam::labelList Foam::fvMeshTools::removeEmptyPatches
+(
+    fvMesh& mesh,
+    const bool validBoundary
+)
+{
+    return removeEmptyPatches(mesh, wordList::null(), validBoundary);
+}
+
+
 void Foam::fvMeshTools::setBasicGeometry(fvMesh& mesh)
 {
     // Set the fvGeometryScheme to basic since it does not require
diff --git a/src/finiteVolume/fvMesh/fvMeshTools/fvMeshTools.H b/src/finiteVolume/fvMesh/fvMeshTools/fvMeshTools.H
index 0a85225664..347e7f212b 100644
--- a/src/finiteVolume/fvMesh/fvMeshTools/fvMeshTools.H
+++ b/src/finiteVolume/fvMesh/fvMeshTools/fvMeshTools.H
@@ -156,6 +156,14 @@ public:
     //  to old patches
     static labelList removeEmptyPatches(fvMesh&, const bool validBoundary);
 
+    //- Remove zero sized patches unless explicitly forced to preserve.
+    //  See above.
+    static labelList removeEmptyPatches
+    (
+        fvMesh&,
+        const wordList& keepPatches,
+        const bool validBoundary
+    );
 
     //- Set the fvGeometryScheme to basic (to avoid parallel communication)
     static void setBasicGeometry(fvMesh& mesh);