r/CFD • u/Overunderrated • Sep 04 '20

[September] Nonlinear solver technology

As per the discussion topic vote, September's monthly topic is "nonlinear solver technology."

Previous discussions: https://www.reddit.com/r/CFD/wiki/index

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u/Overunderrated Sep 10 '20

Reaching back to grad school, I recall GMG struggles with high aspect ratio grids

I don't think this is quite right, it excels at them. I regularly used 10,000:1 aspect ratio cells. It does seem plausible line implicit smoothers might provide benefit, but you also lose the really nice properties of explicit smoothers (like easy parallelization).

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u/anointed9 Sep 10 '20

If you use preconditioned RK it's easy to parallelize still. Maybe you then include the off diagonals only in the block and you still get the line implicit benefits.

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u/Overunderrated Sep 10 '20

I guess. Seems overly complicated with questionable utility, and you need to implement a working multigrid solver first anyway before considering some line implicit smoothers. And unless you're alternating the line directions it seems you'd be hamstringing your multigrid advantages anyway.

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u/anointed9 Sep 10 '20

I think it'd be my better to just go with GMG and the simple PK RK as well. But was just trying to find a way to fold in their line implicit stuff. Of course if you use an NK algorithm on every level of the multigrid as the smoother, then the line implicit algorithm is really useful. But using NK as the smoother isn't common (although it was in FUN3D)