Add Unstructured quadrangular meshes example

examples/t11.py

@@ -0,0 +1,98 @@
+# ------------------------------------------------------------------------------  # noqa: D100
+#
+#  Gmsh Python tutorial 11
+#
+#  Unstructured quadrangular meshes
+#
+# ------------------------------------------------------------------------------
+from __future__ import annotations
+
+import sys
+
+import gmsh
+
+gmsh.initialize()
+
+gmsh.model.add("t11")
+
+# We have seen in tutorials `t3.py' and `t6.py' that extruded and transfinite
+# meshes can be "recombined" into quads, prisms or hexahedra. Unstructured
+# meshes can be recombined in the same way. Let's define a simple geometry with
+# an analytical mesh size field:
+
+p1 = gmsh.model.geo.addPoint(-1.25, -0.5, 0)
+p2 = gmsh.model.geo.addPoint(1.25, -0.5, 0)
+p3 = gmsh.model.geo.addPoint(1.25, 1.25, 0)
+p4 = gmsh.model.geo.addPoint(-1.25, 1.25, 0)
+
+l1 = gmsh.model.geo.addLine(p1, p2)
+l2 = gmsh.model.geo.addLine(p2, p3)
+l3 = gmsh.model.geo.addLine(p3, p4)
+l4 = gmsh.model.geo.addLine(p4, p1)
+
+cl = gmsh.model.geo.addCurveLoop([l1, l2, l3, l4])
+pl = gmsh.model.geo.addPlaneSurface([cl])
+
+gmsh.model.geo.synchronize()
+
+field = gmsh.model.mesh.field
+field.add("MathEval", 1)
+field.setString(1, "F", "0.01*(1.0+30.*(y-x*x)*(y-x*x) + (1-x)*(1-x))")
+field.setAsBackgroundMesh(1)
+
+# To generate quadrangles instead of triangles, we can simply add
+gmsh.model.mesh.setRecombine(2, pl)
+
+# If we'd had several surfaces, we could have used the global option
+# "Mesh.RecombineAll":
+#
+# gmsh.option.setNumber("Mesh.RecombineAll", 1)  # noqa: ERA001
+
+# The default recombination algorithm is called "Blossom": it uses a minimum
+# cost perfect matching algorithm to generate fully quadrilateral meshes from
+# triangulations. More details about the algorithm can be found in the
+# following paper: J.-F. Remacle, J. Lambrechts, B. Seny, E. Marchandise,
+# A. Johnen and C. Geuzaine, "Blossom-Quad: a non-uniform quadrilateral mesh
+# generator using a minimum cost perfect matching algorithm", International
+# Journal for Numerical Methods in Engineering 89, pp. 1102-1119, 2012.
+
+# For even better 2D (planar) quadrilateral meshes, you can try the
+# experimental "Frontal-Delaunay for quads" meshing algorithm, which is a
+# triangulation algorithm that enables to create right triangles almost
+# everywhere: J.-F. Remacle, F. Henrotte, T. Carrier-Baudouin, E. Bechet,
+# E. Marchandise, C. Geuzaine and T. Mouton. A frontal Delaunay quad mesh
+# generator using the L^inf norm. International Journal for Numerical Methods
+# in Engineering, 94, pp. 494-512, 2013. Uncomment the following line to try
+# the Frontal-Delaunay algorithms for quads:
+#
+# gmsh.option.setNumber("Mesh.Algorithm", 8)  # noqa: ERA001
+
+# The default recombination algorithm might leave some triangles in the mesh, if
+# recombining all the triangles leads to badly shaped quads. In such cases, to
+# generate full-quad meshes, you can either subdivide the resulting hybrid mesh
+# (with `Mesh.SubdivisionAlgorithm' set to 1), or use the full-quad
+# recombination algorithm, which will automatically perform a coarser mesh
+# followed by recombination, smoothing and subdivision. Uncomment the following
+# line to try the full-quad algorithm:
+#
+# gmsh.option.setNumber("Mesh.RecombinationAlgorithm", 2) # or 3  # noqa: ERA001
+
+# You can also set the subdivision step alone, with
+#
+# gmsh.option.setNumber("Mesh.SubdivisionAlgorithm", 1)  # noqa: ERA001
+
+gmsh.model.mesh.generate(2)
+
+# Note that you could also apply the recombination algorithm and/or the
+# subdivision step explicitly after meshing, as follows:
+#
+# gmsh.model.mesh.generate(2)  # noqa: ERA001
+# gmsh.model.mesh.recombine()  # noqa: ERA001
+# gmsh.option.setNumber("Mesh.SubdivisionAlgorithm", 1)  # noqa: ERA001
+# gmsh.model.mesh.refine()  # noqa: ERA001
+
+# Launch the GUI to see the results:
+if "-nopopup" not in sys.argv:
+    gmsh.fltk.run()
+
+gmsh.finalize()

src/skgmsh/__init__.py

@@ -278,7 +278,7 @@ def frontal_delaunay_2d(  # noqa: C901, PLR0912
         gmsh.model.mesh.embed(0, embedded_points, 2, 1)
 
     if recombine:
-        gmsh.model.mesh.set_recombine(2, 1)
+        gmsh.model.mesh.set_recombine(2, plane_surface_tag)
 
     gmsh.model.mesh.generate(2)
     mesh = pv.from_meshio(extract_to_meshio())