Different VTK stress dump file formats

40044600's picture
Submitted by 40044600 on Tue, 07/31/2018 - 17:47

Hello,

I am currently running simulations on 2 computers and have found a clash in the output of the same the dump command.

The dump command i'm using is "dump dumpstress all mesh/gran/VTK 1000 dump/dump*.vtk stress cad1"
However, the vtk dump files are formatted differently for each computer I use (and this is affecting my post processing work).

One computer will output the vtk dump command with the file header "# vtk Datafile version 3.0" and the other "# vtk DataFile Version 2.0".
Can anybody advise me on why two different vtk datafile versions are used and how I change it so that both are the same?

Many thanks in advance.
Jon

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Daniel Queteschiner | Wed, 08/01/2018 - 12:06

You are probably using two different LIGGGHTS versions. Prior to v3.8.0 LIGGGHTS used a custom implementation for writing VTK files (typically resulting in version 2.0). Since LIGGGHTS v3.8.0 methods from the VTK library are used for writing those files (typically resulting in vtk file version 3.0 or 4.0 depending on the version of the VTK library used). AFAIK, there shouldn't be any severe differences between those file versions. If you're post-processing makes specific assumptions that are not part of the file format specification, then the proper way to resolve this is to adapt your post-processing tools.

40044600's picture

40044600 | Wed, 08/01/2018 - 13:34

Thanks Daniel,

You were right. I am running two different versions of Liggghts however I am finding significant differences between the these two versions.

The key difference i'm finding lies in the order of the mesh elements in the dump vtk files. In vtk version 2.0, the order of the mesh elements under rotation remains the same in the dump files throughout the simulation (i.e mesh element number 1 will remain at the top of the dump file throughout). However, in vtk version 3.0, the order of the mesh elements in dump files is not consistent when the mesh undergoes a mesh rotate during the simulation (the order appears to be random for each dump file).

For my post-processing I need to be able to track each mesh element individually which I currently cannot do with vtk version 3.0. Do you think this issue is due to the vtk version or maybe something else? Everything else is run the same (same input script, different liggghts version). If it is an issue with version 3.0 I would need to downgrade my LIGGGHTS to use version 2.0

thanks

Daniel Queteschiner | Wed, 08/01/2018 - 14:15

I have not looked into the details, so it's quite possible that mesh elements are dumped in different order. This is not related to the file format though (as different VTK files are not interdependent) but probably owed to the way the data is internally stored. One option should be to additionally dump the mesh element ID to the VTK file (there's already an option for that). Although the order of elements may change, the ID should always refer to the same element. This of course requires a bit more work on the post-processing side as you would have to match IDs rather than rely on the order in the VTK file.