[Mesa-users] Question about controls in intermediate-mass stars evolution

[Josiah Schwab]

Hi Francisco,

In this test suite there are two commands included in the test suite for
> which i didnt find information in the web page:
>
> min_cell_energy_fraction_for_dedt_form
> use_correction_for_KE_plus_PE_with_eps_grav_in_energy_eqn
>
> The chemical structure at the end of the C-burning seems to be affected
> whether these commands are considered or not, and Im not sure what they do.
> Are these just simple numerical improvements in the equations ?
> Are they worth considering in v12778?
>

I disliked both of these options once I learned about
their existence/implementation and so have removed them in
subsequent versions. If I were forced to set them I would probably choose
min_cell_energy_fraction_for_dedt_form = 0 and
use_correction_for_KE_plus_PE_with_eps_grav_in_energy_eqn = .false. .

The option min_cell_energy_fraction_for_dedt_form controlled whether there
was per-cell switching between the different forms of energy equation
("dedt" or "eps_grav") .  I never really understood the motivation for this
option and because it was framed in terms of a per-cell fraction of the
total internal energy, it introduces a dependence of the equations on cell
size.

The option use_correction_for_KE_plus_PE_with_eps_grav_in_energy_eqn added
in a correction term when using the eps_grav form that effectively
amounts to accounting for heating from numerical viscosity.  In a converged
model, this should approach zero, so I believe that it should not have any
significant effect.

More specifically, looking at what's in the 7M_prems_to_AGB test case in
r12778

      use_dedt_form_of_energy_eqn = .true.
      min_cell_energy_fraction_for_dedt_form = 0
      use_correction_for_KE_plus_PE_with_eps_grav_in_energy_eqn = .true.

and since 'min_cell...' is 0, MESA should use the 'dedt' form in all cells,
unless some other switch condition is active, meaning the setting of the
'use_correction_...' option shouldn't matter because no cell is using the
'eps_grav' form.

If you're seeing significant differences in your models, I would suggest
confirming that your results for a given run are insensitive to variations
of the space and time resolution.  Note that the test suite cases are often
not converged, either due to neglect or in an attempt to reduce their
runtime.

Hope that helps,
Josiah
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