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Segfault when creating 1D mesh

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4 years 6 months ago #1934 by JanWesterdiep
Segfault when creating 1D mesh was created by JanWesterdiep
Hey team,

I'm trying to build a 1D mesh with two subdomains, somewhat like ngsolve.org/docu/nightly/i-tutorials/uni...mains-and-boundaries

I was unable to find helper methods for 1D meshes, so I tried something more manual, like
Code:
from netgen.meshing import Element0D, Element1D, Element2D, MeshPoint, Pnt from netgen.meshing import Mesh as NetGenMesh import ngsolve N = 10 ngmesh = NetGenMesh(dim=1) pids = [] # Add points to the mesh. for i in range(N + 1): pids.append(ngmesh.Add(MeshPoint(Pnt(i / N, 0, 0)))) # Left half of the domain is material 1, right half material 2. for i in range(N): ngmesh.Add(Element1D([pids[i], pids[i + 1]], index=1 if i <= 5 else 2)) ngmesh.SetMaterial(1, "domain_left") ngmesh.SetMaterial(2, "domain_right") # Add BC to the mesh. ngmesh.Add(Element0D(pids[0], index=1)) # Added index is 1 ngmesh.Add(Element0D(pids[N], index=2)) ngmesh.SetBCName(0, "l") # Index is retrieved as 0 ngmesh.SetBCName(1, "r") # Create NGSolve mesh and refine it once uniformly. mesh = ngsolve.Mesh(ngmesh) for el in mesh.Elements(): mesh.SetRefinementFlag(el, True) mesh.Refine()

Now, this code segfaults:
Code:
$ netgen mwe.py NETGEN-6.2-dev Developed by Joachim Schoeberl at 2010-xxxx Vienna University of Technology 2006-2010 RWTH Aachen University 1996-2006 Johannes Kepler University Linz optfile ./ng.opt does not exist - using default values togl-version : 2 loading ngsolve library NGSolve-6.2.1907 Using Lapack Including sparse direct solver UMFPACK Running parallel using 8 thread(s) importing NGSolve-6.2.1907 (should) load python file 'mwe.py' Mesh bisection Segmentation fault: 11

I was unable to run it through valgrind (no support yet on newer MacOS systems, :-() so I have little idea what is going wrong. In any case, I have two questions:
- how would I make this code work, or at least achieve the same goal?
- In setting the boundary conditions, there is a discrepancy between the indices (see comment) -- what am I doing wrong?

Thank you!!
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4 years 6 months ago #1935 by joachim
Replied by joachim on topic Segfault when creating 1D mesh
Hi Jan,

thank you for the report ...

using Netgen/NGSolve in 1D is not the main focus, and some things are a bit crooked.
Yes, we have this 0/1-based discrepancy, and the way out is to use labels wherever possible.

I just added an exception telling that mesh refinement in 1D is not supported, instead of segfaulting.

Joachim
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4 years 6 months ago #1944 by joachim
Replied by joachim on topic Segfault when creating 1D mesh
Hi,

we have now an updated method to get rid of the 0/1-based indexing for the element-index.

We first create a 0/1/2/3-dimensional region, and get the region-index back.
This index is used for the elements. We should not worry whether it is 0 or 1-based:
Code:
idx_doml = ngmesh.AddRegion("domain_left", dim=1) idx_domr = ngmesh.AddRegion("domain_right", dim=1) for i in range(N): ngmesh.Add(Element1D([pids[i], pids[i + 1]], index=idx_doml if i < 5 else idx_domr))

the whole example is attached.

the tutorials are also updated.
you need the latest nightly build.

Joachim
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