Unable to run : nonpositive jacobian error

General Questions
1

Hello, i am new to PPM, when i try to solve model which is meshed using tetra mesh, i get below error. what am i doing wrong?

2

Are you using Tetrahedral Gmsh ? Try without it so that Netgen is used. Add only Meshing parameters and make sure that Midside nodes on geometry is set to No. Then there should be no negative jacobians.

3

ok thanks for your quick help

4

But if you want to take advantage of gmsh, stay with Tetrahedral gmsh and activate “netgen” for the 1st order optimization (i wish it would be activated by default). In most cases this should be enough to avoid negative Jacobians, and this should also work when selecting “Midside nodes on geometry - Yes”. It’s important to know that the 1st order optimization works also for 2nd order elements (only the positions of corner nodes will be taken into account). If 1st order optimization is not enough, activate additional the 2nd order optimization “high order elastic” but it needs much more time for meshing.

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5

Gmsh also has the SecondOrderLinear option. Could be useful in PrePoMax.

6

but wouldn’t it have the same effect as “Midside nodes on geometry - no”?

7

Nevermind, it’s already used:

if (!_gmshData.PartMeshingParameters.MidsideNodesOnGeometry) 
    Gmsh.Option.SetNumber("Mesh.SecondOrderLinear", 1);
8

Does this really help? Since, when using Netgen alone using mid-side nodes projected on geometry causes problems a lot of times.

9

OK it depends and can not be generalized. I tried to create a quick example but I couldn’t achieve any improvement with simple geometries…
That’s why I used the more complex geometry shown here again, where the 1st order optimization definitely helped to avoid creating “zero height tetrahedrons”. In other cases, the netgen optimization even leads to worse results than simple deactivating any optimization or using the gmsh optiimization.
You can try it out, just change the 1st order optimization for this example:

test.pmx (3.0 MB)

opt
opt1954Ă—2300 479 KB

Edit:

Here other examples from the models folder. While for the stl file “bracket” any 1st order optimization helped to prevent elements with negative jacobians, for the flange example only netgen optimization failed.

o
o2596Ă—2524 413 KB

In gmsh through GUI, netgen + gmsh can be activated together for optimization. Maybe this could be added to prepomax as fourth optimization option (if possible):

oo
oo911Ă—314 17 KB

10

Adding combinations of optimization techniques is not problem. I will only have to check if gmsh component I am using will take all of them into account. So the ones needed are:

  • Gmsh + Netgen
  • Gmsh + Netgen + Relocate 3D
1 Like
11

That would be great! Thanks