TOPIC: Still Mesh Import; but more generally, Elmer and Mesh Simulation Accuracies and Compatibilities.

Hello Peter; thanks for the request for the 'minimalistic example';

Following upon the above conversation thread, the Elmer tutorial for elasticity has an already-meshed beam to work with. Replacing its material with Elmer's Al metal, which has the same character as the US grade 6061 Al alloy, yields a displacement of about 9mm under load, which also agrees with the SolidWorks and theory results.

Attached is the "BeamElmerTuteAlMetal.STP" file I used to attempt meshing with Elmer, Gmsh, and Salome.

So firstly, here are the crazy results from Elmer and Gmsh:

With Elmer standard mesh settings, I got 1.4m deflection. Putting H=10 in nglib yielded 7.9m deflection. Remeshing with ElmerGrid gave 6-8 trapezoids deep meshing through the beam height, but 7.9m deflection.

Using GMSH simple meshing, order 2, with 1 or 2 trapezoid depth, 8.9m defl. Putting max size = 5, order 2 yielded no convergence for Elmer. With max size = 10, Frontal 2 + 3D yielded 7.3m defl. With max size 20, order 2; 9m defl.

In general Gmsh quads and frontal choices yield a mesh but Elmer solver did not run with such meshes.

Final Gmsh test was with Frontal all, no recombine triangles, min/max 0/10, element order 2, with about 6 mesh depth on short side, plus tetrahedral optimisation, yielded a deflection of 4m after 26 Elmer iterations.

And now here are the more realistic results using Salome's meshes ...

Hexahedral regular mesh, exported as "unv", about 15 elements per short side face of the beam, and running Elmers' TFQMR solver, ILU0, 21 iterations with mostly 500 internal iterations, started to converge at the 15th run, yielding 5.4 mm deflection, which is only a factor of about 2 from reality.

Slight change to GCR, ILU1 after 7 iterations gave 5.4mm deflection.

With auto-hexahedralisation in Salome, using 6 members per side with widths scaled from edge to centre to other edge as 1, 0.7, 0.4, 0.4, 0.7, 1 at the positions 0, 0.2, 0.4, 0.6, 0.8, 1 positions yielded 1.9mm deflection. With 10 members per side, 3.7mm deflection; and 15 members per side, 5.4mm deflection. Changing the density of members to attain a lower density in the centre yielded 5.1mm deflection for 15 members; but going to 30 elements per side yielded finally 7.7mm, within about 10% of reality but with very slow Elmer convergence.

With tetrahedral meshes in Salome, 0.1 'length' parameter, got 1.7mm defl. But with 0.01 'length', got 10-20 elements per side and 7.9mm deflection - at last! And with 0.005 'length', yielding 170,000 volumes and a very regular mesh with about 40 elements on the 100mm face, was very slow to mesh but Elmer analysed very fast, for the pretty exact 8.8mm result.

For similar testing that I performed on a simple Al disk 200mm dia 10mm thick, with atmospheric pressure on one side and with a fixed rim, where the expected deflection is 25 microns, again I found Elmer and Gmsh did not work, but Salome with 'length' of 0.002 and 170,000 elements (seems like a lot, doesn't it!) gave 21 microns whereas 0.005 'length' was only 17 micron deflection, too far off to be useful. Salome kept working well yielding up to 22 micron deflection, until I reached 0.001 'length', where it failed. With such dense meshes, Elmer's result, and time to converge, was independent of the solution method.

So my conclusion from the above is that you can put a lot of work into Elmer's and Gmsh's meshes, but Salome yields reasonable results right away. And Salome works best with Tet meshes, is almost accurate enough with 10-20 elements per side but 40 elements per side (or almost 200,000 elements on a simple shape) will bring it up to expectation levels; and that Salome's quadratic mesh output always crashes Elmer, or is a null field as you found.