Volume mesh of sphere has very acute tetrahedra

General
1

I noticed that meshing an OCC sphere with meshsize = radius / 2 produces very acute tetrahedra that are all connected to the same point in the volume mesh (see screenshot below).

Given that Netgen usually produces very homogeneous looking meshes, this surprised me. For my current use case, a valid workaround is to just set the mesh size a little smaller. So, this isn’t really a problem on my end, but I was wondering if this is the intended behavior?

image
image455×451 47.8 KB

Code to reproduce

from netgen import occ
from ngsolve import VTKOutput, Mesh
import pyvista as pv

# Create and save a sphere
sphere = occ.Sphere(occ.gp_Pnt(0, 0, 0), 1)
geo = occ.OCCGeometry(sphere)
ng_mesh = Mesh(geo.GenerateMesh(maxh=0.5))
vtk = VTKOutput(ng_mesh, coefs=[], names=[], filename="sphere")
vtk.Do()


# Load and visualize with PyVista
pv = pv.read("sphere.vtu")
pv.plot()

Best,
Michael