posti_visu generates invalid VTU (XML parsing error / appended data corrupted)

[abadashahbary-dotcom]

Hello,

I am running the Basic_Re100 tutorial and everything works fine until the visualization step.

FLEXI simulation runs successfully and produces valid state files (*.h5).
However, when converting the result using posti_visu, the generated VTU file cannot be opened in ParaView.

ParaView error:
Error parsing XML: not well-formed (invalid token)
vtkXMLParser: error parsing XML in stream

Additionally, xmllint reports:

• Char 0x0 out of allowed range
• Premature end of data in tag AppendedData

This suggests that the VTU file is not well-formed and the appended binary section might be corrupted or incomplete.

[flexi-framework]

Hello and thanks for checking out FLEXI!
Which OS and ParaView version do you use?

We had this issue on Ubuntu24 which could be attributed to the version of libexpat (XML parser library) used by the ParaView installation from the apt package manager - see this thread
https://discourse.paraview.org/t/i-cannot-read-a-vtp-file-i-could-open-yesterday-can-someone-try-to-open-it/13938

Can you try to open the vtu-files with another ParaView version, i.e. download one of the newer pre-built binaries from the ParaView-Homepage?
Alternatively, you can also use our Docker image from the package registry (tag v2025.10.0), which has a newer version of ParaView as well as all other dependencies already installed.