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pythonmeshgmsh
Albacete
Albacete

Reputation: 23

Periodic Line in Gmsh Python api: affineTransform needed?

Using the Gmsh Python api with OpenCASCADE, I'm trying to create a periodic mesh, such that the left boundary matches the right one and the top matches the bottom. A .geo file works fine, but gmsh.model.mesh.setPeriodic() does not seem to work in the Gmsh Python api.

When specifying a .geo file, I just use Periodic Line {1} = {-3}; as follows:

SetFactory("OpenCASCADE");
// Define points (different meshSizes to check periodicity)
Point(1) = {0, 0, 0, 0.005};
Point(2) = {1, 0, 0, 0.05};
Point(3) = {1, 1, 0, 0.2};
Point(4) = {0, 1, 0, 0.2};

// Define lines
Line(1) = {1, 2};
Line(2) = {2, 3};
Line(3) = {3, 4};
Line(4) = {4, 1};

// Define line loop, turn into surface
Line Loop(1) = {1, 2, 3, 4};
Plane Surface(1) = {1};

// Define periodic boundaries
Periodic Line {1} = {-3};
Periodic Line {2} = {-4};

This works fine. However, in the Python api, I cannot get the function gmsh.model.mesh.setPeriodic() to work the same way. It gives an error that 'affineTransform' must be specified (which should not be necessary for a periodic line), but even if I try to specify the affineTransform as an identity matrix or actually as a translation from top to bottom or left to right, it still does not work. This is my script:

import gmsh

gmsh.initialize()
gmsh.model.add("example")

# Define points (different meshSizes to check periodicity)
points = [
    gmsh.model.occ.addPoint(0, 0, 0, 0.005),
    gmsh.model.occ.addPoint(1, 0, 0, 0.05),
    gmsh.model.occ.addPoint(1, 1, 0, 0.2),
    gmsh.model.occ.addPoint(0, 1, 0, 0.2),
]

# Define lines
lines = [
    gmsh.model.occ.addLine(points[0], points[1]),
    gmsh.model.occ.addLine(points[1], points[2]),
    gmsh.model.occ.addLine(points[2], points[3]),
    gmsh.model.occ.addLine(points[3], points[0])
]

# Create line loop, turn into a surface
line_loop = gmsh.model.occ.addCurveLoop(lines)
plane_surface = gmsh.model.occ.addPlaneSurface([line_loop])

# Set the periodic boundary conditions
gmsh.model.mesh.setPeriodic(1, lines[0:1], [-lines[2]])
gmsh.model.mesh.setPeriodic(1, lines[1:2], [-lines[3]])

# Generate mesh of dim 2
gmsh.model.mesh.generate(2)

# Write mesh to file
gmsh.write(r"example.msh")

# Launch the GUI to see the results:
gmsh.fltk.run()

# Finalize Gmsh
gmsh.finalize()

How can I create a periodic line using the gmsh Python api? Preferably without having to figure out an affineTransform from one line to another, although if necessary I can do that.

Upvotes: 0

Views: 41

Answers (1)

Michael Ermakov
Michael Ermakov

Reputation: 34

import gmsh

gmsh.initialize()
gmsh.model.add("example")

# Define points (different meshSizes to check periodicity)
points = [
    gmsh.model.occ.addPoint(0, 0, 0, 0.005),
    gmsh.model.occ.addPoint(1, 0, 0, 0.05),
    gmsh.model.occ.addPoint(1, 1, 0, 0.2),
    gmsh.model.occ.addPoint(0, 1, 0, 0.2),
]

# Define lines
lines = [
    gmsh.model.occ.addLine(points[0], points[1]),
    gmsh.model.occ.addLine(points[1], points[2]),
    gmsh.model.occ.addLine(points[2], points[3]),
    gmsh.model.occ.addLine(points[3], points[0])
]

# Create line loop, turn into a surface
line_loop = gmsh.model.occ.addCurveLoop(lines)
plane_surface = gmsh.model.occ.addPlaneSurface([line_loop])

gmsh.model.occ.synchronize()

# Set the periodic boundary conditions
dx=0
dy=-1
gmsh.model.mesh.setPeriodic(1, [lines[0]], [lines[2]],
                             [1, 0, 0, dx, 0, 1, 0, dy, 0, 0, 1, 0, 0, 0, 0, 1])

dx=1
dy=1
gmsh.model.mesh.setPeriodic(1, [lines[1]], [lines[3]],
                            [1, 0, 0, dx, 0, -1, 0, dy, 0, 0, 1, 0, 0, 0, 0, 1])

# Generate mesh of dim 2
gmsh.model.mesh.generate(2)

# Launch the GUI to see the results:
gmsh.fltk.run()

# Finalize Gmsh
gmsh.finalize()

A programming language requires Affine transform as the fourth parameter for SetPeriodic function to compile code. An example how to use affine transformation matrix in your case is presented. Concerning a use of something like Periodic Curve{1}={-2} in geo script, I suspect it works only for curves in rectangular quads or there is a way to build affine transformation based on the mentioned curves. It could be also seen in the Gmsh's docs and examples.

Upvotes: 0

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