[mesa-users] diffusion causing model to get stuck

[Jean McKeever]

Thanks for the tips!

Setting diffusion_min_dq_at_surface to a larger value seems to have fixed
the issue.

-Jean

2017-06-05 14:34 GMT-06:00 Evan Bauer <ebauer at physics.ucsb.edu>:

> Hi Jean,
>
> Another control you might find useful is
>
>          !### diffusion_v_max
>
>          ! Max velocity (cm/sec).
>          ! We can get extremely large velocities in the extreme outer
> envelope
>          ! that cause problems numerically without really effecting the
> results,
>          ! so we allow a max for the velocities that should help the
> numerics
>          ! without changing the results.
>          ! Note: change `diffusion_v_max` to at least 1d-2 when using
> radiative levitation.
>
>       diffusion_v_max = 1d-5
>
> Setting this to a smaller number can help models that are having trouble
> with fast moving material in the outer atmosphere. In your model, I suspect
> the stuff that’s causing problems at the surface will eventually sediment
> away from where it’s moving so fast, and there will be nothing left that
> requires enforcing an artificial velocity cap, so you shouldn’t hurt the
> overall physics of diffusion much.
>
> Another option is just to force the model to take smaller time steps for a
> while until things sediment away from the very surface layers. When all
> else fails, I find it is often useful to set max_years_for_timestep to
> something rather small to keep diffusion well behaved for a while. Then I
> turn that control off and restart after the fast sedimentation at the
> surface has taken place.
>
> Cheers,
> Evan
>
>
> On Jun 5, 2017, at 12:52 PM, Aaron Dotter <aaron.dotter at gmail.com> wrote:
>
> Hi Jean,
>
> I recommend trying the following control;
>
>          ! treat at least this much at surface as a single cell for
> purposes of
>
>       diffusion_min_dq_at_surface = 1d-9
>
> 1d-9 is the default value.  Try setting it to a larger number, say 10^-4
> or 10^-3.  This tells the diffusion module to use treat the outer fraction
> of the star by mass as one zone in the diffuison calculation.  That
> calculation can get messy near the stellar surface, especially in stars in
> around ~1.5 Msun (as yours is) because they have a very thin -- or no --
> surface convection.
>
> Good luck!
>
> Aaron
>
>
>
>
>
>
> On Mon, Jun 5, 2017 at 3:42 PM, Jean McKeever <jeanm12 at nmsu.edu> wrote:
>
>> Hello,
>>
>> I was trying to create a grid of models using the astero module and the
>> grid got stuck on a particular point. After some testing of turning things
>> on and off I have discovered that do_element_diffusion appears to be the
>> culprit. For a 1.4 solar mass star with a metallicity of -.4, the model
>> seems to get caught in a series of retries where dt keeps decreasing to be
>> prohibitively small. This happens very early on in the model. If I turn off
>> diffusion, the model proceeds normally.
>>
>> Can anyone provide insight as to why diffusion is causing the model to
>> get stuck? I have attached some modified inlists and starting model to help
>> reproduce this. I checked the newest version of mesa and it is still a
>> problem in 9793.
>>
>> Thank you,
>> Jean
>>
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