Working with meshes

In this example (merge.py) we create two geometries (a cube and a sphere fitting inside), and mesh them. Then, we manually merge the surface meshes, and create a unified volume mesh, where the sphere and its complement are two different sub-domains.

from netgen.meshing import *
from netgen.csg import *

from ngsolve import ngsglobals
ngsglobals.msg_level = 2
# generate brick and mesh it
geo1 = CSGeometry()
geo1.Add (OrthoBrick( Pnt(0,0,0), Pnt(1,1,1) ))
m1 = geo1.GenerateMesh (maxh=0.1)
m1.Refine()

# generate sphere and mesh it
geo2 = CSGeometry()
geo2.Add (Sphere (Pnt(0.5,0.5,0.5), 0.1))
m2 = geo2.GenerateMesh (maxh=0.05)
m2.Refine()
m2.Refine()

print ("****************************")
print ("** merging surface meshes **")
print ("****************************")

# create an empty mesh
mesh = Mesh()

# a face-descriptor stores properties associated with a set of surface elements
# bc .. boundary condition marker,
# domin/domout .. domain-number in front/back of surface elements (0 = void),
# surfnr .. number of the surface described by the face-descriptor

fd_outside = mesh.Add (FaceDescriptor(bc=1,domin=1,surfnr=1))
fd_inside = mesh.Add (FaceDescriptor(bc=2,domin=2,domout=1,surfnr=2))
# copy all boundary points from first mesh to new mesh.
# pmap1 maps point-numbers from old to new mesh

pmap1 = { }
for e in m1.Elements2D():
    for v in e.vertices:
        if (v not in pmap1):
            pmap1[v] = mesh.Add (m1[v])


# copy surface elements from first mesh to new mesh
# we have to map point-numbers:

for e in m1.Elements2D():
    mesh.Add (Element2D (fd_outside, [pmap1[v] for v in e.vertices]))



# same for the second mesh:

pmap2 = { }
for e in m2.Elements2D():
    for v in e.vertices:
        if (v not in pmap2):
            pmap2[v] = mesh.Add (m2[v])

for e in m2.Elements2D():
    mesh.Add (Element2D (fd_inside, [pmap2[v] for v in e.vertices]))


print ("******************")
print ("** merging done **")
print ("******************")


mesh.GenerateVolumeMesh()
mesh.Save ("newmesh.vol")