Fem: Add partial support for hexahedral elements - fixes #12526

src/Mod/Fem/femmesh/gmshtools.py

@@ -151,6 +151,17 @@ class GmshTools():
         else:
             self.HighOrderOptimize = "0"
 
+        # SubdivisionAlgorithm
+        algoSubdiv = self.mesh_obj.SubdivisionAlgorithm
+        if algoSubdiv == "All Quadrangles":
+            self.SubdivisionAlgorithm = "1"
+        elif algoSubdiv == "All Hexahedra":
+            self.SubdivisionAlgorithm = "2"
+        elif algoSubdiv == "Barycentric":
+            self.SubdivisionAlgorithm = "3"
+        else:
+            self.SubdivisionAlgorithm = "0"
+
         # mesh groups
         if self.mesh_obj.GroupsOfNodes is True:
             self.group_nodes_export = True
@@ -858,6 +869,20 @@ class GmshTools():
         geo.write("Mesh.Algorithm3D = " + self.algorithm3D + ";\n")
         geo.write("\n")
 
+        geo.write("// subdivision algorithm\n")
+        geo.write("Mesh.SubdivisionAlgorithm = " + self.SubdivisionAlgorithm + ";\n")
+        geo.write("\n")
+
+        geo.write("// incomplete second order elements\n")
+        if (self.SubdivisionAlgorithm == "1"
+            or  self.SubdivisionAlgorithm == "2"
+            or self.mesh_obj.RecombineAll):
+            sec_order_inc = "1"
+        else:
+            sec_order_inc = "0"
+        geo.write("Mesh.SecondOrderIncomplete = " + sec_order_inc + ";\n")
+        geo.write("\n")
+
         geo.write("// meshing\n")
         # remove duplicate vertices
         # see https://forum.freecad.org/viewtopic.php?f=18&t=21571&start=20#p179443

src/Mod/Fem/femobjects/mesh_gmsh.py

@@ -73,6 +73,12 @@ class MeshGmsh(base_fempythonobject.BaseFemPythonObject):
         "Elastic",
         "Fast curving"
     ]
+    known_mesh_SubdivisionAlgorithms = [
+        "None",
+        "All Quadrangles",
+        "All Hexahedra",
+        "Barycentric"
+    ]
 
     def __init__(self, obj):
         super(MeshGmsh, self).__init__(obj)
@@ -305,3 +311,13 @@ class MeshGmsh(base_fempythonobject.BaseFemPythonObject):
                 "For each group create not only the elements but the nodes too."
             )
             obj.GroupsOfNodes = False
+
+        if not hasattr(obj, "SubdivisionAlgorithm"):
+            obj.addProperty(
+                "App::PropertyEnumeration",
+                "SubdivisionAlgorithm",
+                "FEM Gmsh Mesh Params",
+                "Mesh subdivision algorithm"
+            )
+            obj.SubdivisionAlgorithm = MeshGmsh.known_mesh_SubdivisionAlgorithms
+            obj.SubdivisionAlgorithm = "None"

src/Mod/Fem/femtest/data/elmer/group_mesh.geo

@@ -35,6 +35,12 @@ Mesh.Algorithm = 2;
 // 3D mesh algorithm (1=Delaunay, 2=New Delaunay, 4=Frontal, 7=MMG3D, 9=R-tree, 10=HTX)
 Mesh.Algorithm3D = 1;
 
+// subdivision algorithm
+Mesh.SubdivisionAlgorithm = 0;
+
+// incomplete second order elements
+Mesh.SecondOrderIncomplete = 0;
+
 // meshing
 Geometry.Tolerance = 1e-06; // set geometrical tolerance (also used for merging nodes)
 Mesh  3;