Meshing Fails for Scaled Sheet

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1

Hi everybody!

I have tried to mesh a sheet.
First I mesh it in meters, than in decimeters, than in centimeters and finally in millimeters.
It is quite simple in my opinion, however it does not work properly.
Meter, dm, cm work fine but mm fails.

What do I do wrong?

Cheers, Valentin

from netgen.csg import *

def foo(scale = 1):
    thickness = 0.18

    origin = Pnt(0, 0, 0)
    corner = Pnt(9*scale, 4*scale, thickness*scale)
    left  = Plane (origin, Vec(-1,0,0) )
    right = Plane (corner, Vec( 1,0,0) )
    front = Plane (origin, Vec(0,-1,0) )
    back  = Plane (corner, Vec(0, 1,0) )
    bot   = Plane (origin, Vec(0,0,-1) )
    top   = Plane (corner, Vec(0,0, 1) )

    cube = left * right * front * back * bot * top
    geo = CSGeometry()
    geo.Add (cube)

    mesh = geo.GenerateMesh(maxh=1)
    mesh.Save("cube.vol")


if __name__ == "__main__":
    from ngsolve import *
    ngsglobals.msg_level = 0
    print("it works with scale 1:")
    foo(1)
    print("it works with scale 0.1:")
    foo(0.1)
    print("it works with scale 0.01:")
    foo(0.01)
    print("it does  not work with scale 0.001:")
    foo(0.001)
    

    # from ngsolve import *
    # mesh = Mesh("cube.vol")

    # import netgen.gui
    # Draw(mesh)
    print("done")
    # input()
2

Thanks to the information in the (personal) replies, I was able to fix the issue.

One suggested work-around would be to use the non-scaled model and fit the other parameters.
The proper solution redefines the bounding box of the mesh generator by

geo.SetBoundingBox(Pnt(-10*scale, -10*scale, -10*scale), Pnt(10*scale, 10*scale, 10*scale))

Under consideration of this adaption the original code works fine:


from netgen.csg import *

def foo(scale = 1):
    thickness = 0.18

    origin = Pnt(0, 0, 0)
    corner = Pnt(9*scale, 4*scale, thickness*scale)
    left  = Plane (origin, Vec(-1,0,0) )
    right = Plane (corner, Vec( 1,0,0) )
    front = Plane (origin, Vec(0,-1,0) )
    back  = Plane (corner, Vec(0, 1,0) )
    bot   = Plane (origin, Vec(0,0,-1) )
    top   = Plane (corner, Vec(0,0, 1) )

    cube = left * right * front * back * bot * top
    geo = CSGeometry()
    geo.Add (cube)
    geo.SetBoundingBox(Pnt(-10*scale, -10*scale, -10*scale), Pnt(10*scale, 10*scale, 10*scale))

    mesh = geo.GenerateMesh(maxh=1)
    mesh.Save("cube.vol")


if __name__ == "__main__":
    from ngsolve import *
    ngsglobals.msg_level = 0
    print("it works with scale 1:")
    foo(1)
    print("it works with scale 0.1:")
    foo(0.1)
    print("it works with scale 0.01:")
    foo(0.01)
    print("it does  not work with scale 0.001:")
    foo(0.001)
    

    # from ngsolve import *
    # mesh = Mesh("cube.vol")

    # import netgen.gui
    # Draw(mesh)
    print("done")
    # input()