Poiseuille flow 3d setup

[svchb]

Extracted from #922

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 66.95%. Comparing base (3a5900a) to head (b0e2ede).

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HEAD has 1 upload less than BASE

Flag	BASE (3a5900a)	HEAD (b0e2ede)
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Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1075       +/-   ##
===========================================
- Coverage   88.70%   66.95%   -21.75%     
===========================================
  Files         128      128               
  Lines        9764     9744       -20     
===========================================
- Hits         8661     6524     -2137     
- Misses       1103     3220     +2117     

Flag	Coverage Δ
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unit	66.95% <ø> (ø)

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[efaulhaber]

I guess @LasNikas should take a look here, since you copied his code.

[LasNikas]

That works for me. However, it might be worth mentioning in the 2D and 3D examples that the pressure gradient is defined over the entire inflow-to-outflow extent and that the boundary zones free surfaces start and end at x=0 and x=L, respectively.
This means that the boundary zones are effectively part of the computational domain.
Otherwise, this could cause confusion during the validation process.

[svchb]

That works for me. However, it might be worth mentioning in the 2D and 3D examples that the pressure gradient is defined over the entire inflow-to-outflow extent and that the boundary zones free surfaces start and end at x=0 and x=L, respectively.
This means that the boundary zones are effectively part of the computational domain.
Otherwise, this could cause confusion during the validation process.

I have added a comment to the setups