Add Velocity volume output for compressible flow solvers

[MastermindScope]

Disclaimer

This is my first ever contribution to any GitHub project. Please give feedback on what I can Improve if I haven't done something that is expected of me.

Proposed Changes

Added Velocity output, since I am interested in that and adding it only in the config just gets ignored and no output happens

Related Work

PR Checklist

• I am submitting my contribution to the develop branch.
• My contribution generates no new compiler warnings (try with --warnlevel=3 when using meson).
• My contribution is commented and consistent with SU2 style (https://su2code.github.io/docs_v7/Style-Guide/).
• I have added a test case that demonstrates my contribution, if necessary.
• I have updated appropriate documentation (Tutorials, Docs Page, config_template.cpp), if necessary.

[pcarruscag]

Welcome to SU2. You need to add these variables to the "PRIMITIVE" group, there is a comment // Primitive variables.
Look also for SetVolumeOutputValue("PRESSURE" because for each variable you need to add it and then set its value.
Please make the same change in CNEMOCompOutput.cpp
And add yourself to AUTHORS.md

[MastermindScope]

Hi, thank you for the warm welcome. I added the changes and hope they are sufficient.

[pcarruscag]

Thanks, you are still missing the part of the code that sets the output value, AddVolumeOutput only adds the name.
Look for SetVolumeOutputValue in the files you changed, should be very close.

[MastermindScope]

I've added SetVolumeOutputValue now. Thank you for being patient and letting me make mistakes.

[EvertBunschoten]

Hi! Just out of curiosity, if you're interested in just the velocity and not the momentum, why not just divide the momentum vector by the density during postprocessing?

[EvertBunschoten]

Make sure you retrieve the velocity from the Primitive vector stored in the CVariable classes, not the Solution vector.

[MastermindScope]

why not just divide the momentum vector by the density during postprocessing?

There are several reasons why:

1. I thought of this, but it looks patchy somehow
2. It makes life easier for people who want to use this software but have little experience (like me)
3. I want to start contributing to this project and this was one of the things I noticed and thought I could implement myself