Save velocity at outlet of 3D mesh


I have 3D pipe mesh and I am solving Navier-Stokes equation with projection method.

I would like to save velocity at the outlet over time as h5 file but I am not sure how to do it.
I have access to the velocity all over the domain but how can I save the velocity only at the outlet ?
I created the mesh with gmsh and marked outlet with physical group.
Any idea would be much appreciated.


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Hello, you might be better off saving the whole velocity field and then extracting the outlet in Paraview or similar post-processing software. Is that unfeasible for any reason?

Thanks for the response ! I could save the entire velocity but I think it’s a waste of memory since I only need to save at the outlet.

Possibly a waste of memory, yes. In this case you might have a look at classes like SubMesh and BoundaryMesh. I’ve never used them so I don’t know if they do what you need.

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I finally solved this problem by using a function from fenicstools called Interpolation nonmatching mesh, and I did use SubMesh and BoundaryMesh. One sad thing is that SubMesh does not support parallel computing.
Thank you for the answer anyway !