Add a Lax-Friedrichs-type entropy-stable dissipation operator and its specialization for multi-ion MHD

[amrueda]

This PR adds an entropy-stable dissipation operator DissipationLaxFriedrichsEntropyVariables and its specialization for multi-ion MHD.

[github-actions]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.36%. Comparing base (d01e4a7) to head (c98006f).
Report is 1 commits behind head on main.

Additional details and impacted files

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+ Coverage   96.35%   96.36%   +0.01%     
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+ Hits        37553    37654     +101     
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unittests	96.36% <100.00%> (+0.01%)	⬆️

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[ranocha]

I did not check the math

[andrewwinters5000]

I like adding the generic wrapper for an ES dissipation term. I just posed a few questions and comments regarding the multi-ion version.

[andrewwinters5000]

Thanks @amrueda ! The new comments are very helpful. I just left one small suggestion.

[DanielDoehring]

Cool stuff, I added some minor comments/suggestions regarding documentation.

[andrewwinters5000]

LGTM! I already left a comment with my two cents in the discussion about documentation for flux_ec.

[jlchan]

From the Trixi Zoom meeting discussion - what about DissipationEntropyVariables?

[DanielDoehring]

From the Trixi Zoom meeting discussion - what about DissipationEntropyVariables?

Maybe we also make use of abstract types and define an AbstractDissipationEntropyStable if that makes some functions more elegant.

[ranocha]

As discussed: Dissipation at the beginning, something like Scalar should be included to clarify what it is doing

[jlchan]

Maybe we also make use of abstract types and define an AbstractDissipationEntropyStable if that makes some functions more elegant.

Sure, that sounds nice as well, but I was just trying to suggest a precise name with minimal additional changes.

Since this dissipation is scalar (not a matrix dissipation in the sense of Roe, etc) and uses jumps of the entropy variables as opposed to conservative variables, maybe ScalarDissipationEntropyVariables?

[amrueda]

What about DissipationLaxFriedrichsEntropyVariables? While the name is a bit long, it clearly conveys the key aspects: it is a Lax-Friedrichs-type dissipation that uses the jump in entropy variables.

I think the use of the term "scalar" might be confusing for some users, as this flux can be used with non-scalar equations.

I am not suggesting DissipationLocalLaxFriedrichsEntropyVariables, even though it would be more consistent with the existing DissipationLocalLaxFriedrichs and DissipationGlobalLaxFriedrichs. That name feels unnecessarily long. Moreover, one can still apply a global Lax-Friedrichs strategy by prescribing a constant maximum wave speed with this flux.

[JoshuaLampert]

Thanks!