ENH: allow invariant surfaces for sampling (#1418)

- an advanced feature, for example when sampling on a static patch
  while some motion occurs elsewhere. [use with caution]

- If the sampled surface dictionary is modified during run-time, the
  ensight file indexing for the geometry will become out of sync.
  This is addressed in a subsequent commit.

src/sampling/sampledSurface/sampledSurface/sampledSurface.C

@@ -95,6 +95,7 @@ Foam::sampledSurface::sampledSurface(const word& name, std::nullptr_t)
     name_(name),
     mesh_(NullObjectRef<polyMesh>()),
     enabled_(true),
+    invariant_(false),
     interpolate_(false),
     area_(-1)
 {}
@@ -110,6 +111,7 @@ Foam::sampledSurface::sampledSurface
     name_(name),
     mesh_(mesh),
     enabled_(true),
+    invariant_(false),
     interpolate_(interpolate),
     area_(-1)
 {}
@@ -125,6 +127,7 @@ Foam::sampledSurface::sampledSurface
     name_(dict.lookupOrDefault<word>("name", name)),
     mesh_(mesh),
     enabled_(dict.lookupOrDefault("enabled", true)),
+    invariant_(dict.lookupOrDefault("invariant", false)),
     interpolate_(dict.lookupOrDefault("interpolate", false)),
     area_(-1)
 {}

src/sampling/sampledSurface/sampledSurface/sampledSurface.H

@@ -54,8 +54,15 @@ Description
         name        | Alternative name                      | no  |
         enabled     | Enable/disable the surface?           | no  | yes
         interpolate | Sample to nodes instead of faces      | no  | false
+        invariant   | Invariant with geometry change (use with caution!) | no  | false
     \endtable
 
+Note
+    The invariant switch is an advanced feature to declare that the surface is
+    unaffected by changes in the general mesh geometry.  For example, if sampling
+    on a static patch while some other motion occurs elsewhere. If used improperly,
+    there is a significant possibility for problems (caveat emptor).
+
 SourceFiles
     sampledSurface.C
     sampledSurfaceTemplates.C
@@ -110,6 +117,9 @@ private:
         //- Should surface sampling be enabled?
         bool enabled_;
 
+        //- Geometry is invariant (never changes)
+        bool invariant_;
+
         //- Interpolate information to the nodes?
         bool interpolate_;
 
@@ -281,7 +291,13 @@ public:
             return enabled_;
         }
 
-        //- interpolation to nodes requested for surface
+        //- Surface is invariant with geometry change (caution)
+        bool invariant() const
+        {
+            return invariant_;
+        }
+
+        //- Interpolation to nodes requested for surface
         bool interpolate() const
         {
             return interpolate_;

src/sampling/sampledSurface/sampledSurfaces/sampledSurfaces.C

@@ -480,7 +480,7 @@ bool Foam::sampledSurfaces::read(const dictionary& dict)
     }
 
     // Ensure all surfaces and merge information are expired
-    expire();
+    expire(true);
 
     return true;
 }
@@ -657,6 +657,7 @@ void Foam::sampledSurfaces::readUpdate(const polyMesh::readUpdateState state)
 {
     if (state != polyMesh::UNCHANGED)
     {
+        // May want to use force expiration here
         expire();
     }
 }
@@ -676,7 +677,7 @@ bool Foam::sampledSurfaces::needsUpdate() const
 }
 
 
-bool Foam::sampledSurfaces::expire()
+bool Foam::sampledSurfaces::expire(const bool force)
 {
     // Dimension as fraction of mesh bounding box
     const scalar mergeDim = mergeTol_ * mesh_.bounds().mag();
@@ -687,6 +688,11 @@ bool Foam::sampledSurfaces::expire()
     {
         sampledSurface& s = (*this)[surfi];
 
+        if (s.invariant() && !force)
+        {
+            // 'Invariant' - does not change when geometry does
+            continue;
+        }
         if (s.expire())
         {
             ++nChanged;

src/sampling/sampledSurface/sampledSurfaces/sampledSurfaces.H

@@ -299,7 +299,9 @@ class sampledSurfaces
 
         //- Mark the surfaces as needing an update.
         //  Return false if all surfaces were already marked as expired.
-        virtual bool expire();
+        //  Optionally force expire, even if a surface has been marked as
+        //  invariant.
+        virtual bool expire(const bool force=false);
 
         //- Update the surfaces as required.
         //  Return false if no surfaces required an update.

tutorials/incompressible/pimpleFoam/RAS/propeller/system/surfaces

@@ -15,10 +15,10 @@ surfaces
     surfaceFormat   vtk;
     fields          (p U Q);
 
-    interpolationScheme cellPoint;
+    // interpolationScheme cellPoint;  //<- default
 
     surfaces
-    (
+    {
         zNormal
         {
             type            cuttingPlane;
@@ -44,8 +44,10 @@ surfaces
             type            patch;
             patches         ("propeller.*");
             interpolate     true;
+            invariant       true;  // Unaffected by mesh motion
+            surfaceFormat   ensight;
         }
-    );
+    }
 }