Fix: Preserve periodic BC identifications in MergeMeshes

[ksugahar]

Summary

• Fix regression where periodic boundary conditions set via Identify() + Glue() are lost during volume meshing

Problem

The DeleteData() call on identification points was introduced in commit 27b8b5e ("Fix handling identified points in Compress and MeshVolume").

However, this causes a regression where periodic boundary conditions set via Identify() + Glue() are lost during volume meshing. The issue is that the original mesh contains the identification information, but when merging subdomain meshes, this information is deleted before subdomain identifications (which typically don't exist) are copied.

Solution

Remove the DeleteData() call, preserving the original mesh's identification points. The subsequent loop still copies any subdomain identifications if they exist.

Test case

from netgen.occ import *

box1 = Box(gp_Pnt(0,0,0), gp_Pnt(1,1,1))
box2 = Box(gp_Pnt(2,0,0), gp_Pnt(3,1,1))
box1.faces[0].Identify(box2.faces[0], 'periodic', IdentificationType.PERIODIC)
shape = Glue([box1, box2])
geo = OCCGeometry(shape)
mesh = geo.GenerateMesh(maxh=0.5)
print(len(mesh.GetIdentifications()))  # Should be > 0, was 0 before fix

Related

• Forum issue: https://ngsolve.org/forum/ngspy-forum/1918-periodic-b-c-regression
• Bug introduced in: 27b8b5e

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