Hi, I post an error of netgen here: bc and mat will be erroneous merged in netgen.csg · Issue #49 · NGSolve/netgen · GitHub

I want to add 2 small cylinders (electrodes) to 1 big cylinder, the boundary conditions are set on the outer surface of the small cylinder. However, If I place them at the same z axis with 0.5 spacing, the bc will be automatically merged. If I offset 1 small cylinder with theta=theta+0.01, then its bc will be preserved. Any hint on this issue?

```
import numpy as np
from netgen import csg
from ngsolve.comp import Mesh
def cylinder(p0, p1, unit_vec, r, bc="bc"):
""" a pointed cylinder (numpy interface) """
# generate CSG primitives
csg_p0 = csg.Pnt(p0[0], p0[1], p0[2])
csg_p1 = csg.Pnt(p1[0], p1[1], p1[2])
csg_vec = csg.Vec(unit_vec[0], unit_vec[1], unit_vec[2])
# generate cylinder shape
bot = csg.Plane(csg_p0, -csg_vec)
top = csg.Plane(csg_p1, csg_vec)
cy_surface = csg.Cylinder(csg_p0, csg_p1, r)
# specify bc on top
top.bc(bc)
# geo
cy = top * cy_surface * bot
return cy
def unit_cylinder(h=1.0, r=0.5, bc="cyl"):
""" scaled unit z-axis cylinder """
p0 = [0, 0, 0]
p1 = [0, 0, h]
unit_vec = [0, 0, 1]
return cylinder(p0, p1, unit_vec, r, bc)
def electrode_cylinder(p0=[0, 0, 0], unit_vec=[1, 0, 0], shape=[0.1, 0.1],
bc="e0"):
"""
cylinder electrode (numpy interface)
shape: NDArray
[radius, thickness]
"""
r, thick = shape
# calculate the center of the other face
p1 = np.asarray(p0) + thick * np.asarray(unit_vec)
# generate cylinder electrode with annotation
ele = cylinder(p0, p1, unit_vec, r, bc)
ele.mat('agcl')
return ele
def cylinder_with_electrodes():
""" [test only, will be deleted] """
h = 1.0
r = 0.5
g0 = unit_cylinder(h=h, r=r)
g0.mat('g0')
# generate cylinder electrode
ele_shape = [0.1, 0.1] # radius, thick
thick = ele_shape[1]
ele_pos = [30, h/2.0] # angle (degree), z
# infer locations
rad = ele_pos[0] * 2 * np.pi / 360.0
unit_vec = np.array([np.cos(rad), np.sin(rad), 0])
# warning: a thin contact of electrode with domain will cause
# netgen failed to generate proper mesh
p0 = (r - thick/2.0)*unit_vec
p0[2] = ele_pos[1]
# place 1st electrode
ele = electrode_cylinder(p0, unit_vec, shape=ele_shape, bc="e0")
# geo = g0 + ele
# place 2nd electrode
p0[2] += 0.25
# p0[0] += 0.01 # <-- https://github.com/NGSolve/netgen/issues/49
ele2 = electrode_cylinder(p0, unit_vec, shape=ele_shape, bc="e1")
# geo = geo + ele2
ele = ele - g0
ele2 = ele2 - g0
return g0, ele, ele2
if __name__ == "__main__":
g, ele, ele2 = cylinder_with_electrodes()
geo = csg.CSGeometry()
geo.Add(g)
geo.Add(ele)
geo.Add(ele2)
ngmesh = geo.GenerateMesh(maxh=0.1)
mesh = Mesh(ngmesh)
# mesh.Curve(3)
print(mesh.GetBoundaries())
print(mesh.GetMaterials())
```