Feature matrix

[ranocha]

As discussed today, it would be great to have a feature matrix of the meshes/solvers/whatever stuff in the docs.

[ranocha]

Here is a first second third suggestion how such a feature matrix might look like.

Feature	TreeMesh	StructuredMesh	UnstructuredMesh2D	P4estMesh
Spatial dimension	1D, 2D, 3D	1D, 2D, 3D	2D	2D, 3D
Coordinates	Cartesian	curvilinear	curvilinear	curvilinear
Connectivity	h-nonconforming	conforming	conforming	h-nonconforming
Adaptive mesh refinement (AMRCallback)	✔️	✖️	✖️	✔️
Domain	hypercube	mapped hypercube	arbitrary	arbitrary
Weak form (VolumeIntegralWeakForm)	✔️	✔️	✔️	✔️
Flux differencing (VolumeIntegralFluxDifferencing)	✔️	✔️	✔️	✔️
Shock capturing (VolumeIntegralShockCapturingHG)	✔️	✖️	✖️	✖️

[sloede]

This looks really great! I think we should put something like this in the docs and maybe also in the ICOSAHOM tutorial? Three and a half suggestions from my side:

• Use "Feature" as the first column title
• Add "AMR" row
• Use English terms for the DG features, eg, weak form, strong form, shock capturing. You can still put the type names in parents below
• left-align the first column

[ranocha]

☝️ Like this? (updated the draft above)

[sloede]

Yes, much better, thank you. I have a few more suggestions now 😉:

• use ✔️ instead of ✅ to make it stand out more
• use ✖️ instead of ❌ to make it stand out less
• for the "conforming" row: maybe use "connectivity" or "topology" as row header? Right now it looks somewhat empty
• drop the "Hennemann Gassner" from shock capturing - it makes the line wrap and is imho not a vital piece of information
• move the AMRCallback to the row header (like for the volume integral types)
• put the type links in (parens) to make it clear that this is a detail and not part of the general description

If you prefer, I can also create a new table below and modify your table above directly.

[ranocha]

Updated above

[ranocha]

I think we could also add

[ranocha]

I think we should put something like this in the docs and maybe also in the ICOSAHOM tutorial?

Could you add it to the ICOSAHOM tutorial?

[jlchan]

I really like this layout! Would it be OK for me to add simplicial meshes to the table after the PR is merged?

[ranocha]

Sure, that's the idea 👍

[andrewwinters5000]

I like this summary table a lot! It is a very nice snapshot of Trixi meshes and features. It also indicates directly where I need to work on the unstructured solver :)

[sloede]

I like this summary table a lot! It is a very nice snapshot of Trixi meshes and features. It also indicates directly where I need to work on the unstructured solver :)

We should really sit together (maybe after ICOSAHOM) and discuss what a good strategy would be to extend the unstructured solver. I mean, we can just leave it as it is, but in the back of my mind I have the feeling that the unstructured solver and the P4estMesh solver share too many similarities to be two completely separate solvers. Conceptually, I view the P4estMesh as the unstructured mesh plus h-adaptivity, thus from a very high-level point of view, I can't see a reason why there are so many DG-related methods implemented for the 2D P4estMesh (instead of reusing the unstructured stuff). But, as always, there's also a cost associated with unifying everything... Let's set together (maybe also with @efaulhaber) and figure out a good way forward.

[andrewwinters5000]

Conceptually, I view the P4estMesh as the unstructured mesh plus h-adaptivity,

I agree! Their structure is similar (apart from possible differences in local edge numbering) and the functionality goals are the same; real world, interesting problems in 2D/3D curvilinear coordinates.

Let's set together (maybe also with @efaulhaber) and figure out a good way forward.

Definitely, I am game to concoct a plan for these features moving forward.

EDIT: We should also include @Cczernik in this discussion. He is working on curvilinear shock capturing and already has some nice results for compressible Euler in 2D

[sloede]

EDIT: We should also include @Cczernik in this discussion. He is working on curvilinear shock capturing and already has some nice results for compressible Euler in 2D

As a side note to that, @efaulhaber also has done some work on 3D shock capturing for the P4estMesh, although I am not sure in what kind of state this is.

[efaulhaber]

So far, the 3D shock capturing works, but I haven't verified that thoroughly (yet). And my implementation definitely needs to be revised before it's ready to be merged anywhere, it's more of a proof-of-concept yet.

[sloede]

@ranocha I think you're right: While the table above looks great now directly on GH,

it is broken for Documenter.jl:

Thus my question is: will this table only be used in Documenter.jl or also in the README that's visible directly on GitHub? In the former case, I think we should just use proper unicode symbols, e.g., ✅ and ❌. That would make the table look like this (at least on my computer):

What do you think? I think @ranocha essentially suggested something like this in the first place, but I didn't realize what the issue really was.

[Cczernik]

The shock-capturing for the unstructured quad mesh in 2D seems to work fine (FSP, EC, Sedov).

But it‘s still not speed optimized. I will work on that and make a PR next week.

[ranocha]

Let's continue the discussion of the layout in #703

[ranocha]

I definitely agree that it would be great to unify these approaches and get shock capturing on curvilinear meshes. Some things that might be considered before we can really see P4estMesh{2} as superior to UnstructuredQuadMesh2D are

• performance problems such as Surface stuff is slow for the P4estMesh #642 and prolong2mortars! with P4estMesh allocates sometimes #628
• the ability to read and use mesh files generated by HOHQMesh.jl

By the way, I still think it is useful to have a pure Julia implementation of the unstructured mesh available since there can sometimes be subtle problems when relying on binary dependencies, cf. #665 and trixi-framework/P4est.jl#37

[sloede]

By the way, I still think it is useful to have a pure Julia implementation of the unstructured mesh available

Absolutely, I wholeheartedly agree. I was thinking/hoping it would be possible to implement it in such a way that all the numerics live in the unstructured solver and that P4estMesh essentially is a smart wrapper around an unstructured mesh. But this might turn out to be too high of a cost in terms of complexity, so we should tread carefully here.

What is imho definitely in the books, though, is to make the P4estMesh accept an UnstructuredMesh in the constructor as a base mesh. This would encapture very nicely that P4estMesh "just" adds adaptivity to the mix.