Dear Jack,

Two-point fluxes are something we have not yet included in deal.II. I would be very interested if we could realize this feature. I must admit that I did not yet try to understand the complete formula in the link you provided, but in fact it should be possible to implement this with the internal evaluators used by FEEvaluation very efficiently. Would you be willing to share the FEValues variant, it is enough to provide it in 1D. I did work with the Trixi code before, but did not yet look into the algorithms, so I would be very curious to learn the details. My initial approach would be to encode the full D_split matrix to be fed into the tensor product evaluators in the internal namespace, and then add the respective boundary fluxes. Once we have a basic version, we can think about making the interface nicer.

Best,
Martin


Am 23.04.24 um 17:19 schrieb Jack Coughlin:
I am interested in using the deal.ii matrix-free infrastructure to implement an entropy-stable DG method. The entropy stable family of methods uses an interesting variation, where the volume flux is actually a consistent two-point flux function (similar to the numerical flux).

I've attached a screenshot of the formula in question, which is taken from this documentation page <https://trixi-framework.github.io/Trixi.jl/stable/tutorials/DGSEM_FluxDiff/#DGSEM-with-flux-differencing>. I've also attached a review of the method.

As far as I can tell, to do this you would need to be able to tell a class like FEEvaluation to integrate a given submission only for one test function. I think I can see how to do this integration manually with FEValues, but it would be nice to take advantage of the matrix-free infrastructure if possible.

Thank you,
-Jack C
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