Sutherlands Law for temperature dependent viscosity

[DanielDoehring]

Closes #1195

[github-actions]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.07%. Comparing base (909abb4) to head (042b18e).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1808      +/-   ##
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- Coverage   96.30%   93.07%   -3.23%     
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  Files         440      441       +1     
  Lines       35793    35798       +5     
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- Hits        34470    33317    -1153     
- Misses       1323     2481    +1158     

Flag	Coverage Δ
unittests	93.07% <100.00%> (-3.23%)	⬇️

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[DanielDoehring]

Hey @sloede and @jlchan since you brought this up in #1195 can you take a brief look at this whether the fundamental choices (implementing this as a new set of equations) and form of temperature-dependent viscosity seem meaningful to you?

[jlchan]

This looks good to me - I am only familiar with Sutherlands law though and not anything else more complex.

[sloede]

Thanks for working on this! I haven't looked at your changes in detail yet, but I'm wondering if really a second set of equations is required or desirable. Would it also be possible to implement this as a equation-of-state-like property of the original NSE implementation? That is, make the viscosity type a type parameter of the NSE struct, or even just allow passing different viscosity_dynamic functions to the constructor of the NSE, allowing to either use a constant function, Sutherland's law, or something fancier?

I am worried about code duplication what I perceive (maybe wrongly) as an implementation detail in many other NSE codes I've seen. Maybe @andrewwinters5000 has a different take on this?

[DanielDoehring]

I haven't looked at your changes in detail yet, but I'm wondering if really a second set of equations is required or desirable. Would it also be possible to implement this as a equation-of-state-like property of the original NSE implementation? That is, make the viscosity type a type parameter of the NSE struct, or even just allow passing different viscosity_dynamic functions to the constructor of the NSE, allowing to either use a constant function, Sutherland's law, or something fancier?

That is a good idea, I will think about this. In principle, it should not be to difficult to pass in a function mu() that is then being called with teperature. This way, one could also use more complex models.
Question is whether this can be done without (significant) performance loss for the constant viscosity case.

[sloede]

In principle, it should not be to difficult to pass in a function mu() that is then being called with teperature.

I would just pass the entire state such that the user can freely decide what to do with it.

Question is whether this can be done without (significant) performance loss for the constant viscosity case.

If a function is defined that ignores it's input and just returns a constant value, it should in virtually all cases just be optimized away.

A question remains though as to whether we can efficiently provide a convenience layer that auto-generates the constant function for a given numerical value of mu, such that a user may just pass the constant value and does not have to always wrap it in a function.

[andrewwinters5000]

LGTM! This is a nice extension to the Navier-Stokes capabilities.

[sloede]

Minor formatting change, then LGTM!

[DanielDoehring]

It would be really nice if this gets merged sometime soon as I am currently working on the computation of drag & lift due to viscous terms (following up on #1812) for which the viscosity mu is needed.

[jlchan]

@andrewwinters5000 OK to merge?

@DanielDoehring I only checked a few of the CI failures - they look spurious. Did you see any actual failing tests?

[DanielDoehring]

@jlchan No, not except for codecov to fail.

[andrewwinters5000]

@jlchan Yes, everything looks good to me.

[jlchan]

Great, then unless someone objects, I'm ok with force merging this.

Do I need to disable auto-merge to do so?

[andrewwinters5000]

Yes, you need to disable auto-merge. Then there will be an override option

[DanielDoehring]

Thanks @jlchan !