ENH: displacementMotionSolver: additional hierarchy

- shm: have displacementMotionSolver as alternative mesh shrinker
  (instead of medialAxis).
- updated iglooWithFridges tutorial to use displacementLaplacian
- selectable interpolation from cells to points in the motion solvers
  using the 'interpolation' keyword:
    interpolation volPointInterpolation;    // default
    or
    interpolation patchCorrected (lowerWall upperWall);
- wrapped up mesh shrinkers (see above) for use as a displacementMotionSolver
  (i.e. the opposite of the displacementMotionSolver mesh shrinker)

tutorials/heatTransfer/buoyantBoussinesqSimpleFoam/iglooWithFridges/system/fvSchemes

@@ -41,6 +41,7 @@ divSchemes
 laplacianSchemes
 {
     default         Gauss linear limited corrected 0.333;
+    laplacian(diffusivity,cellDisplacement) Gauss linear corrected;
 }
 
 interpolationSchemes

tutorials/heatTransfer/buoyantBoussinesqSimpleFoam/iglooWithFridges/system/fvSolution

@@ -32,6 +32,12 @@ solvers
         tolerance       1e-07;
         relTol          0.1;
     }
+
+    cellDisplacement
+    {
+        $p_rgh;
+        relTol          0;
+    }
 }
 
 SIMPLE

tutorials/heatTransfer/buoyantBoussinesqSimpleFoam/iglooWithFridges/system/snappyHexMeshDict

@@ -294,7 +294,7 @@ addLayersControls
     // cannot be above minThickness do not add layer.
     // Relative to undistorted size of cell outside layer.
     // See relativeSizes parameter.
-    minThickness 0.25;
+    minThickness 0.05;
 
     // If points get not extruded do nGrow layers of connected faces that are
     // also not grown. This helps convergence of the layer addition process
@@ -313,30 +313,29 @@ addLayersControls
     // 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;
+    // Stop layer growth on highly warped cells
+    maxFaceThicknessRatio 0.5;
 
     // 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;
+    //- Use displacementMotionSolver to shrink mesh
+    meshShrinker displacementMotionSolver;
+
+    //- Use laplacian for shrinking
+    solver displacementLaplacian;
+
+    displacementLaplacianCoeffs
+    {
+        diffusivity quadratic inverseDistance ("two.*" igloo);
+    }
+
 
     // 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.