Imposing boundary flags in 3D

[Sander Rhebergen]

Hello,
I am new to using ngsolve/ngs-py and struggling a little to impose boundary conditions in 3D. I took the 2D ng-spy navierstokes.py demo and want to extend this to 3D. I created a geo file describing a brick and give number to the different planes:

algebraic3d

solid wall_t = plane (0, 0, 0; 0, 0, -1) -bc=3000;
solid wall_b = plane (0, 0, 0; 0, -1, 0) -bc=3000;
solid inflow = plane (0, 0, 0; -1, 0, 0) -bc=1000;

solid wall_f = plane (3, 0.5, 0.5; 0, 0, 1) -bc=3000;
solid wall_ba = plane (3, 0.5, 0.5; 0, 1, 0) -bc=3000;
solid outflow = plane (3, 0.5, 0.5; 1, 0, 0) -bc=2000;

solid rec = inflow and outflow and wall_t and wall_b and wall_f and wall_ba;

tlo rec -maxh=0.08;

I then create the grid using netgen which then gives me a brick.vol.gz. When I open this file I indeed see the bc imposed on the different planes. I now load this into navierstokes.py and create my velocity space:

mesh = Mesh("brick.vol.gz")
V = H1(mesh,order=2,dirichlet=[1000])

I then want to impose u[0]=2 on boundary with label 1000, which I do as follows:

uin = CoefficientFunction(2.0)
u.components[0].Set(uin, definedon=mesh.Boundaries("1000"))

However, when I run my code, I just get zero everywhere. Does anyone know what I am doing wrong here?
(Eventually, I would like to impose different bcs on the different planes).

Thanks,
Sander

[Joachim Schoeberl]

Hi Sander,

welcome to NGS-Py !

You can use either 1-based numbers, or labels (= strings) for bound markers. The recommendation are strings, what are C++11 regex. You use them with:

solid wall_t = plane (0, 0, 0; 0, 0, -1) -bc="bottom"

with

print ("boundary labels: ", mesh.Boundaries())

you see whether everything went ok.

Best, Joachim

[Sander Rhebergen]

Hi Joachim,

Thanks for the response. So I did as you suggested. In my brick.geo file I wrote

solid wall_t = plane (0, 0, 0; 0, 0, -1) -bc="wall"

This gave me an error though:

Load CSG geometry file brick.geo
caught error Parsing error in line 3:
token ';' expected

Problem with input file:
Parsing error in line 3:
token ';' expected

So I changed this to:

solid wall_t = plane (0, 0, 0; 0, 0, -1) -bc=wall;

(I had to remove the double quotation mark)

I added the line

print ("boundary labels: ", mesh.Boundaries())

to the navierstokes.py file after I read the mesh, but then get the error

Import expr.py
(should) load python file 'navierstokes.py'
Traceback (most recent call last):
File "<string>", line 12, in <module>
TypeError: Boundaries(): incompatible function arguments. The following argument types are supported:
1. (self: ngsolve.comp.Mesh, pattern: str) -> ngsolve.comp.Region

Invoked with: <ngsolve.comp.Mesh object="" at="" 0x7fe84d16d518="">
Finished executing navierstokes.py

Also, if I just open the brick.vol.gz file, I don't see the labels anywhere. Did I misunderstand something?

thanks,
Sander

[Joachim Schoeberl]

Hi Sanders,

I was to quick:
in the .geo file you have to write -bcname="label"
You can also model the csg-geometry within python, see the cmagnet.py example.

and it is mesh.GetBoundaries() to get the list of labels.

Joachim

[Sander Rhebergen]

Hi Joachim,
I think that seems to work now, thanks!
Sander