# Setting up boundary condition numbers of an STL object

Dear Joachim:
I cannot thank you enough for providing such a wonderful software, netgen, which I use most of my meshing work.

I have a technical question of netgen meshing using an STL file of a plate heat exchanger.
The length and width are 1 m and the thickness is 7 mm. The top and bottom surfaces are curvy and four sides are perpendicular. So, it literally looks like a book or magazine dried after someone spilled water on it.

I was able to make a mesh directly from the STL file, using a command line with -batchmode option to netgen mesh (.vol) and neutral format. Since I have many pieces, I would like to do this without using GUI too much. As I have technically six surfaces, i.e., top, bottom, left, right, front and back. I would like to have distinct six boundary numbers. In some cases, netgen did what I expected, and I guessed that netgen understands an angle between two surfaces and assign a single boundary number per distinct surface. Unfortunately, it was not general for all the cases.

So, I would like to know if there is any method that I can assign specific boundary numbers to surfaces of unique conditions. For example, the left and right surfaces have points that have the minimum and maximum in x-axis, and similar to the front and back surfaces in y-axis.

Could you please let me know or suggest any methods?
Thank you so much.

Albert

Hi Albert,
if the mesh is ok and you just want to change the boundary conditions of some elements you can use the Python interface like this:

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

mesh = unit_cube.GenerateMesh(maxh=0.2)

isinXplane = lambda el: all((abs(mesh[v][0]) < 1e-12 for v in el.vertices))
mesh.SetBCName(fd_id-1, "new_bc")

for el in mesh.Elements2D():
if isinXplane(el):
el.index = fd_id``````

This adds boundary conditions (FaceDescriptors) to the mesh and changes the index of the desired elements to match the new bc index. The SetBCName function is 0 based and the facedescriptor index 1 based (legacy reasonsâ€¦) thats why I need the -1

Best
Christopher

Dear Christopher:
Thank you so much!
I may need some study of python, but I think I can figure them out.