OpenCascade geometry Periodic Boundary Conditions

Hello,

I would like to impose periodic boundary conditions on the bottom and top of a space-time cylinder mesh.

To create a 2pi rotation in space-time of a subdomain in the domain, we use here the OpenCascade library of NGSolve, but it seems not to include he attribute PeriodicSurfaces() to impose the boundary condition:

 File "occ_periodic.py", line 86, in <module>
   geoOCC.PeriodicSurfaces(bottom,top)
AttributeError: 'netgen.libngpy._NgOCC.OCCGeometry' object has no attribute 'PeriodicSurfaces'

Is there a way to do this? We were looking at other solutions as well. For example the function GlobalInterfaceSpace(), but we are not sure of to use the mapping argument for our purposes (and if it is possible).

Thank you very much in advance for your help!

Alessio

Hi,
for opencascade the syntax is a little different, here a cube with 2 faces identified:

If your transformation between the faces is not a translation you have to create a netgen.occ.gp_Trsf and give the trafo as a fourth argument to identify (also if you want to identify multiple faces at once).

Look at the output if the identification was successful there should be something like

 Map face 1 -> 2

Also check identified points in the gui with view->Viewing Options->Mesh->Show identified Points

Best
Christopher

Hello Christopher,

thank you very much for your quick answer!

I have tried the command you suggested and it works in our case. We have a full 2*pi rotation and so a translation in Z-axis is fine.

Many thanks,
Alessio


 

Hi,

I am trying to impose periodic boundary conditions on the top and the bottom of a rotating object. Now, using a 360 degree rotation the periodic identification works pretty good. But when using only a 90 degree rotation, I have to create this trafo netgen.occ.gp_Trsf and use it as the fourth argument in the command as mentioned in the last post.

Here a short example, what I did so far:

from ngsolve import *
from netgen.occ import *
import math
import time
from ngsolve.internal import visoptions
from ngsolve.internal import viewoptions
from netgen.meshing import IdentificationType

origin = (0,0,0)
T = 1
desiredRad = 0.1
rotationAngle = pi/2

face1 = WorkPlane().Circle(0.25,0.25,desiredRad).Face()
spine = Segment(origin, (0,0,T))
cyl = Cylinder(origin, Z, r=0.5*desiredRad, h=T).faces[0]
heli = Edge(Segment((0,0), (rotationAngle, T)), cyl)
pipe1 = Pipe(spine, face1, auxspine=heli).mat("pipe1")
pipe1.faces.name="pipe_outer"
pipe1.faces.Min(Z).name ="bottom"

rot = Rotation(Axis((0,0,0), Z), 180)
pipe1.faces.Min(Z).Identify(pipe1.faces.Max(Z), "periodic", IdentificationType.PERIODIC, rot)

geoOCC = OCCGeometry(pipe1)
mesh = Mesh(geoOCC.GenerateMesh(maxh=0.1))
mesh.Curve(3)

Unfortunately, this does not work, after trying to adapt the rot object in several ways.

Thanks in advance for your help!

Best, Mario

Hi Mario,

you have to give the transformation from the first to the second face, which consists of rotation (by 90 degrees) and translation, then it works:

rot = Rotation(Axis((0,0,0), Z), 90)
trans = Translation(T*Z)
pipe1.faces.Min(Z).Identify(pipe1.faces.Max(Z), "periodic", IdentificationType.PERIODIC, rot*trans)

best, Joachim
 

Dear Professor Schöberl,

thank you very much for taking the time to check this for us and find the solution, or better said our mistake.

Now I am happy, it doesn't matter if the solution is or is not unique

All the best,
Alessio