Update domain.py

[keiyamamo]

Two main reasons for this PR

1. There was a problem restarting the problem. The problem should be fixed with this PR. (I tested restarting and it works now)
2. domain.py was quite complicated to understand without enough documentation. Therefore, I added args/returns as well as underlying assumption for some of the variables that was not explicitly stated.

Please review my comments and feel free to add comments on them!

Kei

[dbruneau-mie]

Hey Kei, these comments look clear to me

[dbruneau-mie]

Hey Kei, have you checked that these edits work with the following:

1. Single Solid Region, like the Turek Flag
2. Multiple Solid regions
3. Single Fluid region
4. Multiple Fluid regions
5. St. Venant Kirchoff material
6. Mooney Rivlin material
7. With Robin BC?
   I would like to make sure this doesn't break any of the existing functionality before incorporating this PR.

[keiyamamo]

Hi Daivd,

Good point!
I did test 1, 3, 5, 7, but not 2, 4, 6. Do you have any problems with multiple solid/fluid regions that I can test?

Best,
Kei

[dbruneau-mie]

Hey Kei, here is a simulation with mutliple fluid regions in the Aneurysm Workflow FSI:
https://github.com/dbruneau-mie/Aneurysm_Workflow_FSI/blob/master/simulations/stenosis_FC/pulsatile_vessel.py

[dbruneau-mie]

And in the solver itself there is a cylinder with 2 solid regions
https://github.com/KVSlab/turtleFSI/blob/master/turtleFSI/problems/Test_Cylinder/problem_aneu_2solid.py

[keiyamamo]

Hi @dbruneau-mie

Thanks for pointing out! I just reverted the change and will re-open the pull-request after I test the changes.
We should consider adding multiple fluid/solid regions as a part of test...

Best,

[dbruneau-mie]

Yes, I think we need more tests for the solver. A restart test would also be very useful

[keiyamamo]

I did work on that, but found that the problem is multi-folded, meaning we need to fix time loop, drag/lift evaluation and not-designed-to restart. But I will work on this now that it turned out that it is important we have well designed test to avoid manually performing many kinds of test.