Second order meshing of 2D domains (with multiple subdomains)

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4 years 5 months ago #2887 by bhaveshshrimali
Second order meshing of 2D domains (with multiple subdomains) was created by bhaveshshrimali
Hi everyone,
I was trying to generate a second order mesh for a domain comprising of a circle inside square. But was facing an issue. Here is the MWE with a very coarse mesh to delineate the problem:
Code:
from ngsolve import * from netgen.geom2d import SplineGeometry periodic = SplineGeometry() pnts = [ (0,0), (1,0), (1,1), (0,1) ] pnums = [periodic.AppendPoint(*p) for p in pnts] top = periodic.Append( ["line", pnums[2], pnums[3]], bc="outer") right = periodic.Append ( ["line", pnums[1], pnums[2]], bc="periodic") btom = periodic.Append( ["line", pnums[1], pnums[0]], leftdomain=0, rightdomain=1, copy=top, bc="outer") left = periodic.Append( ["line", pnums[0], pnums[3]], leftdomain=0, rightdomain=1, copy=right, bc="periodic") # adding the circle periodic.AddCircle((0.5, 0.5), 0.3, leftdomain=0, rightdomain=1, bc = "inner") msh = periodic.GenerateMesh(maxh=5) msh.SecondOrder()

This, however, generates a mesh where the mid-points of the edges along the circle aren't properly placed. I am guessing it has to do with how I am defining the geometry



Any suggestions on what could be going wrong.

Thank you
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4 years 5 months ago - 4 years 5 months ago #2888 by bhaveshshrimali
Replied by bhaveshshrimali on topic Second order meshing of 2D domains (with multiple subdomains)
[attachment=undefined]paraview_Ordered.png[/attachment]I think I confused `triplot` of the corresponding linear mesh with the actual mesh. However I noticed something. The ordering of vertices is slightly different than what one would expect in Paraview (vertices followed by edge midpoints in the same order as the vertices)

For instance:
Code:
points = [] cell_arr = [] for pt in msh.Points(): points.append([*pt]) points = np.array(points) for elem in msh.Elements2D(): cell_arr.append([int(str(i)) for i in elem.points]) points = points[:, :2] cell_arr = np.array(cell_arr, int)-1
when fed to paraview gives:

Attachment not found



whereas adjusting the last 2 points as
Code:
cell_arr = cell_arr[:, [0,1,2,5,3,4]] #the last three points are ordered different than what one would expect

gives the desired ordering

[attachment=undefined]paraview_Ordered.png[/attachment]
Last edit: 4 years 5 months ago by bhaveshshrimali.
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