Simplify Navier-Stokes BC imposition

[jlchan]

For Trixi.jl's implementation, the "divergence" boundary condition for Navier-Stokes is the same for both GradientVariablesPrimitive and GradientVariablesEntropy. This removes some duplicate implementations while also enabling slip wall BCs for GradientVariablesEntropy.

Todo: add 3D tests.

[github-actions]

Review checklist

This checklist is meant to assist creators of PRs (to let them know what reviewers will typically look for) and reviewers (to guide them in a structured review process). Items do not need to be checked explicitly for a PR to be eligible for merging.

Purpose and scope

• The PR has a single goal that is clear from the PR title and/or description.
• All code changes represent a single set of modifications that logically belong together.
• No more than 500 lines of code are changed or there is no obvious way to split the PR into multiple PRs.

Code quality

• The code can be understood easily.
• Newly introduced names for variables etc. are self-descriptive and consistent with existing naming conventions.
• There are no redundancies that can be removed by simple modularization/refactoring.
• There are no leftover debug statements or commented code sections.
• The code adheres to our conventions and style guide, and to the Julia guidelines.

Documentation

• New functions and types are documented with a docstring or top-level comment.
• Relevant publications are referenced in docstrings (see example for formatting).
• Inline comments are used to document longer or unusual code sections.
• Comments describe intent ("why?") and not just functionality ("what?").
• If the PR introduces a significant change or new feature, it is documented in NEWS.md with its PR number.

Testing

• The PR passes all tests.
• New or modified lines of code are covered by tests.
• New or modified tests run in less then 10 seconds.

Performance

• There are no type instabilities or memory allocations in performance-critical parts.
• If the PR intent is to improve performance, before/after time measurements are posted in the PR.

Verification

• The correctness of the code was verified using appropriate tests.
• If new equations/methods are added, a convergence test has been run and the results
  are posted in the PR.

Created with ❤️ by the Trixi.jl community.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.84%. Comparing base (12f0483) to head (8e68742).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3108   +/-   ##
=======================================
  Coverage   96.84%   96.84%           
=======================================
  Files         662      663    +1     
  Lines       51197    51203    +6     
=======================================
+ Hits        49579    49585    +6     
  Misses       1618     1618           

Flag	Coverage Δ
unittests	96.84% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[DanielDoehring]

Thanks for bringing this up!

I was thinking: Do we have any boundary condition, where the combination of momentum and energy/heat boundary condition actually has an influence? So I was wondering if we could reduce redundancy by only implementing e.g. no slip, adiabatic, isothermal etc. individually and then just call them after another.

[jlchan]

Thanks for bringing this up!

I was thinking: Do we have any boundary condition, where the combination of momentum and energy/heat boundary condition actually has an influence? So I was wondering if we could reduce redundancy by only implementing e.g. no slip, adiabatic, isothermal etc. individually and then just call them after another.

Good question - not that I know of. However, I was actually thinking it might be simpler and clearer to just combine them together with NoSlipAdiabaticWallBoundaryCondition, NoSlipIsothermalWallBoundaryCondition, and SlipAdiabaticWallBoundaryCondition.

At the moment, I can't think of too many other combinations that we use. Implementing these three individual cases is more redundant, but I think the API for calling one BC after another might introduce more complexity too.

[DanielDoehring]

Actually I was thinking about some additional BCs - namely a "do-nothing" equivalent for the momentum, analogous to the Adiabatic one. This would be used for cases where we would want free offstream. This would naturally be combined with Adiabatic heat BC.

Next, I would like to add a radiation-equilibrium BC for the temperature (which would be combined with a no-slip wall).

[jlchan]

Nice! If you're thinking of adding more options, then yes - calling separate momentum/energy BCs makes sense.

[DanielDoehring]

Thanks for refactoring this ! :)