[Mesa-users] Question about using other_cgrav with the PPISN test_suite

[Adam Jermyn]

Hi Jeremy,

I’m not familiar with the PPISN test case specifically, but changing cgrav and then loading a pre-existing model can be tricky. The pre-existing model likely had the standard_cgrav value. When you change that value even by a small amount you’re kicking the model way out of hydrostatic equilibrium, which will require significant adjustment to restore.

For instance, if you take a solar model and change cgrav down by 1% with hydro on I’d expect the surface to expand at ~1%*(escape velocity), which can be large…

Someone more familiar with PPISN might be able to say more though...

-Adam
On Jul 2, 2020, 3:48 PM -0400, Jeremy Sakstein <sakstein at hawaii.edu>, wrote:
> Hi everyone,
>
> I have a question about the PPISN test suite.
>
> I am trying to use other_cgrav with G rescaled by a constant (1 + a small number) and I have run into some issues. When I change G by a few % or more I am finding that the code terminates due to reaching the time step limit (I set this to 1d-99).
>
> My initial explorations of changing various controls led me to believe that the star was going away from hydrostatic equilibrium, my evidence for this being that I kept running into various stropping conditions telling me that the surface velocity was too high. These errors show up before hydro is turned on for the first time.
>
> My two leading hypotheses are:
>
> 1. Hydro is turning on too late i.e. after the star begins to expand or
> 2. other_cgrav is not implemented in every equation being solved in this test_suite leading to cumulative errors.
>
>
> I tested (1) by turning hydro on from the start of the run. In this case I didn't get any termination codes but the runs take a very long time. I was able to get past the temperatures and densities where the code previously terminated however.
>
> I tested (2) by turning off other_cgrav and instead setting:
>
> standard_cgrav = 1.05*standard_cgrav
>
> in extras_controls in run_star_extras. I have used this procedure before for e.g. the RSP module.
>
> Is anyone able to give me any insight into what is happening here, and whether I am on the right track with (1) or (2).
>
> My own tests suggest that my procedure (2) is working as expected i.e. the value of G is indeed altered for every equation being solved by setting it in extras_controls but it would be great if someone were able to confirm this for me.
>
> I'd be very grateful for any help you can give.
>
> Cheers,
>
> Jeremy
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