Implement an alternative regrid request strategy.

[drwells]

We can measure the displacement of the structure directly instead of relying on
estimates from the fluid solver.

As requested by @boyceg.

This is only implemented fully for IBFEMethod - using it in other classes will result in the program being aborted. If people know how to implement this for other IBStrategy objects they should write patches for them.

IBAMR Pull Request Checklist

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[drwells]

Rewriting this took awhile but its now done.

[boyceg]

Let's not call the Cartesian grid velocity-based estimate a "fluid velocity"-based estimate. (The velocity in a solid location is the solid velocity, not the fluid velocity! 😀 ) Suggest something like "Eulerian" or "background grid" velocity.

[drwells]

The first two commits in this PR are taken from #1158 - we can do everything at once but that PR is logically separate (though this one is dependent on the changes made there).

[drwells]

The velocity in a solid location is the solid velocity, not the fluid velocity!

Yeah, but we are using the fluid velocity, it's just inside the structure :)

[drwells]

Either way - its now fixed.

[drwells]

@boyceg can you take a look soon? I don't want this to get stale.

[boyceg]

Suggested change

	* Eulerian patch hierarchy needs to be regridded.
	* Eulerian patch hierarchy needs to be regridded and/or if the assignment of Lagrangian data to Cartesian grid patches needs to be updated.

[drwells]

No, this isn't true - IBFEMethod deals with that problem (reassociating Lagrangian structures with the Eulerian grid) in an independent way. See IBFEMethod::preprocessIntegrateData().

[drwells]

In particular: the displacement since the last regrid does not necessarily equal the displacement since the last patch-element association.

[boyceg]

Could you help future me by adding to this comment by pointing to where this is handled and how this is distinct from the re-association trigger?

[boyceg]

I am stuck in the past, when regridding was the only way to update the associations between patches and elements.

[drwells]

That was only three months ago :)

[boyceg]

Looks good. A few minor suggested changes in wording in the comments. It might be more clear also to be able to allow users to explicitly specify a "fixed regrid interval" regridding strategy, and then the code use which ever triggers first?

[boyceg]

Looks good. A few minor suggested changes in wording in the comments. It might be more clear also to be able to allow users to explicitly specify a "fixed regrid interval" regridding strategy, and then the code use which ever triggers first?

Specifically, I can imagine being confused if I were to set a fixed regrid interval and then don't get it because a CFL-like criterion also is set.

Since we now can specify multiple strategies for triggering regridding, would it make sense to add that one too?

[drwells]

Specifically, I can imagine being confused if I were to set a fixed regrid interval and then don't get it because a CFL-like criterion also is set.

The current behavior is to ignore the fixed regrid interval when the CFL-like condition is also set. I'll update the documentation to reflect the status quo. Do you want to change the way this works? That might break what people expect to happen.

[boyceg]

Specifically, I can imagine being confused if I were to set a fixed regrid interval and then don't get it because a CFL-like criterion also is set.

The current behavior is to ignore the fixed regrid interval when the CFL-like condition is also set. I'll update the documentation to reflect the status quo. Do you want to change the way this works? That might break what people expect to happen.

I guess people may already be confused. 😀

[boyceg]

👍