elastic-tube-3d/solid-fenics preCICE mesh only includes line elements

While adding the elastic-tube-3d/solid-fenics case to the system tests, I noticed that the results comparison with fieldcompare was orders of magnitude slower than for the elastic-tube-3d/solid-calculix case, even though the exported mesh has fewer points. Paraview reports:

• solid-calculix: 3190 cells, 2588 points
• solid-fenics: 3281 cells, 1126 points

Loading the exported meshes in Paraview, I get the solid-calculix mesh rendering as a surface with clear inlet and outlet boundaries, and the solid-fenics mesh rendering as a wireframe:

I wonder if this means that

At the same time, fieldcompare takes significantly longer:

• solid-calculix: 6s for four time windows
• solid-fenics: 273s (45x) for two time windows

If I understand correctly, the points end up in a different order than in the reference (i.e., depends on the run):

elastic-tube-3d_fluid-openfoam_solid-calculix:

  field-compare-1  | Comparing '/runs/elastic-tube-3d_fluid-openfoam-solid-calculix_2026-06-29-172120/precice-exports/Solid-Mesh-Solid.init.vtu'
  field-compare-1  |       and '/runs/elastic-tube-3d_fluid-openfoam-solid-calculix_2026-06-29-172120/reference-results-unpacked/fluid-openfoam_solid-calculix/Solid-Mesh-Solid.init.vtu'
  field-compare-1  |   Reading '/runs/elastic-tube-3d_fluid-openfoam-solid-calculix_2026-06-29-172120/precice-exports/Solid-Mesh-Solid.init.vtu'
  field-compare-1  |   Reading '/runs/elastic-tube-3d_fluid-openfoam-solid-calculix_2026-06-29-172120/reference-results-unpacked/fluid-openfoam_solid-calculix/Solid-Mesh-Solid.init.vtu'
+ field-compare-1  |   Meshes did not compare equal (Differing connectivity detected for cell of type 'LINE'). Retrying with sorted points...
  field-compare-1  |   Meshes did not compare equal (Differing connectivity detected for cell of type 'LINE'). Retrying with sorted cells...
  field-compare-1  |   Wrote diff into '/runs/elastic-tube-3d_fluid-openfoam-solid-calculix_2026-06-29-172120/precice-exports/diff_Solid-Mesh-Solid.init.vtu.vtu'
  field-compare-1  |   File comparison PASSED with 3 PASSED / 0 FAILED / 0 SKIPPED

elastic-tube-3d_fluid-openfoam_solid-fenics:

  field-compare-1  | Comparing '/runs/elastic-tube-3d_fluid-openfoam-solid-fenics_2026-06-29-172244/precice-exports/Solid-Mesh-Solid.init.vtu'
  field-compare-1  |       and '/runs/elastic-tube-3d_fluid-openfoam-solid-fenics_2026-06-29-172244/reference-results-unpacked/fluid-openfoam_solid-fenics/Solid-Mesh-Solid.init.vtu'
  field-compare-1  |   Reading '/runs/elastic-tube-3d_fluid-openfoam-solid-fenics_2026-06-29-172244/precice-exports/Solid-Mesh-Solid.init.vtu'
  field-compare-1  |   Reading '/runs/elastic-tube-3d_fluid-openfoam-solid-fenics_2026-06-29-172244/reference-results-unpacked/fluid-openfoam_solid-fenics/Solid-Mesh-Solid.init.vtu'
+ field-compare-1  |   Meshes did not compare equal (Differing points - 'Deviation above tolerance detected -> [ 0.00160798 -0.00471205  0.037189  ] vs. [-0.0031449   0.00385612  0.00938227] ([ 295.58 -181.84   74.77] %)'). Retrying with sorted points...
  field-compare-1  |   Meshes did not compare equal (Differing connectivity detected for cell of type 'LINE'). Retrying with sorted cells...
  field-compare-1  |   Wrote diff into '/runs/elastic-tube-3d_fluid-openfoam-solid-fenics_2026-06-29-172244/precice-exports/diff_Solid-Mesh-Solid.init.vtu.vtu'
  field-compare-1  |   File comparison PASSED with 3 PASSED / 0 FAILED / 0 SKIPPED

Analyzing the mesh with meshio:

import meshio
m=meshio.read('Solid-Mesh-Solid.dt1.vtu')
print([(c.type, len(c.data), c.data.shape) for c in m.cells])

I get:

[('line', 3281, (3281, 2))]

while the equivalent file for CalculiX gives:

[('triangle', 1264, (1264, 3)), ('line', 1926, (1926, 2))]

Meshes: meshes.tar.gz

For all other FEniCS-based cases, fieldcompare processes the files similarly fast as all other cases. There seems to be something special about this case.

The case was added by @IshaanDesai (https://github.com/precice/tutorials/pull/222), who might have a lead on what is going on here.

Using the tutorials from https://github.com/precice/tutorials/tree/f7c0e34ade4545d1a67b9c831d1db509877fdb94

Start with the elastic-tube-3d/solid-fenics case and inspect the exported Solid-Mesh-Solid.dt1.vtu using meshio, then compare it with the solid-calculix output. Use the fieldcompare system-test run to reproduce the slowdown. Done means identifying why the FEniCS export contains only line elements and correcting or documenting the case so its mesh and comparisons behave as expected.