[mesa-users] Overshooting

Chris Mankovich cmankovich at ucsc.edu
Tue Jun 2 20:58:43 EDT 2015 

Hi Kenny and everyone else,

I think Rob identified the reason why M_cc looks so ratty as a function of
time.  With f=f0 you're effectively not adding an overshoot region.  You
can plot the mixing D's to see this, and in fact with the f=f0=0.1 case you
end up with a lot of isolated, borderline-stable zones that tend to merge
with each other and sporadically with the convective core.

If I run f0=0.1, f=0.2 so that there is an overshoot region extending 0.1
H_p beyond the standard boundary, things behave more like you'd expect.



Chris

On Tue, Jun 2, 2015 at 3:25 PM, Ehsan Moravveji <e.moravveji at gmail.com>
wrote:

> Along the line of Falk’s reasoning, I would like to point out that very
> recently, we studied the so-far richest main sequence B-type pulsator, KIC
> 10526294, which shows 19 consecutive dipole g-modes. We tested several
> input physics, among which, the choice of step-function versus exponential
> overshooting prescription, and examined which set of models can reproduce
> the observation better; indeed, our intuition dictates that the more smooth
> exponential presciption should be preferred over the steep step-function.
> However, we tried to be unbiased and let this star guide us through.
> What comes out is that the models with exponential prescription — as
> expected — outperform the other with a factor 2 to 3 in chi-square sense.
> We hope to keep testing this for our other B-type dwarfs too.
> More information can be found here:
> http://adsabs.harvard.edu/abs/2015arXiv150506902M
> Indeed, we all hope that a more physically grounded model, as Dave
> preaches, would soon liberate us from this shortcoming of MLT.
>
> Best regards
> Ehsan.
>
> On 02 Jun 2015, at 19:58, Falk Herwig <fherwig at uvic.ca> wrote:
>
> Dear Kenny,
>
> step overshooting is provided in MESA as an option to compare with
> historical computations that
> adopted this kind of overshooting treatment, usually at a time when it was
> numerically not practical
> to treat convective mixing as a time dependent process (say, in the
> 1970’s). Now, I would say that we
> have no evidence that overshooting is step-like. Any simulation of
> convective boundary mixing,
> including overshooting that has ever been made shows some gradual decline
> of the mixing efficiency
> in the transition from unstable to stable stratification (I admit that too
> much convective boundary mixing
> is the easy and wrong answer to obtain with too low grid resolution, a
> continuing challenge of such
> simulations). And any terrestrial example of overshooting or convective
> boundary mixing would support
> this notion. Whether or not the exponential decay of mixing model is the
> only one or even the best
> one can be debated. It does have some support from hydro simulation
> though. You may adopt
> another mixing model, but in any case it should allow for a smooth
> transition between stable
> and unstable layer. You can make this smooth transition as thin as you can
> computationally afford, but
> I am confident that it will note be the diffusion length-scale! One reason
> for a smooth and numerically resolved
> transition to be desirable comes from purely numerical considerations that
> it is impossible to converge
> in the presence of grid face discontinuities that are not somehow
> modelled. This may contribute to
> your results to not “seem very good” to you.
>
> Best, Falk.
>
> --
> Falk Herwig
> Dept of Physics & Astronomy, U of Victoria
> fherwig at uvic.ca, tel: +1 (250) 721-7743
>
>
>
> On Jun 2, 2015, at 8:34 AM, Kenny Van <kvan at ualberta.ca> wrote:
>
> Hi, I was trying to produce some results using overshooting and it seems
> as though regardless of timestep the plot produced doesn't seem very good.
> I have attached the three plots I created using my data. The overshooting
> and changes in timestep are said in the title. But just in case I will list
> all the properties of the star.
>
> &star_job
>
>    mesa_dir = '/home/kvan/mesa_svn'
>
>    create_pre_main_sequence_model = .true.
>
> / !end of star_job namelist
>
>
> &controls
>
> ! initial parameter
>
>  ! initial mass of star
>    initial_mass = 20.0
>
>  ! initial metallicity of star, MESA only goes from 0 to 0.04
>    initial_z = 0.014
>
>  ! initial helium mass fraction of star
>    initial_y = 0.266
>
>  ! changing the MLT to match that in Meynet
>    mixing_length_alpha = 1.6
>
> ! overshooting
>
>  ! all exponential overshooting is by default zero
>  ! setting step overshooting
>    step_overshoot_f_above_burn_h = 0.1
>    overshoot_f0_above_burn_h = 0.1
>    step_overshoot_f_below_burn_h = 0.1
>    overshoot_f0_below_burn_h = 0.1
>    step_overshoot_f_above_burn_he = 0.1
>    overshoot_f0_above_burn_he = 0.1
>    step_overshoot_f_below_burn_he = 0.1
>    overshoot_f0_below_burn_he = 0.1
>    step_overshoot_f_above_burn_z = 0.1
>    overshoot_f0_above_burn_z = 0.1
>    step_overshoot_f_below_burn_z = 0.1
>    overshoot_f0_below_burn_z = 0.1
>
> ! Mass loss
>
>  ! Changing mass loss so system acts like that of Ekstrom et al.
>  ! Use Vink mass loss rate
>    RGB_wind_scheme = 'Vink'
>    Vink_wind_eta = 1
>
> ! timestep limits
>  ! Limit magnitude of max change in log10 total pressure in any cell
>    delta_lgP_limit = 0.2
>
>  ! Limit magnitude of max change in log10 temperature in any cell
>    delta_lgT_limit = 0.2
>
> ! when to stop
>
>  ! max timestep size
>    max_years_for_timestep = 500
>
>  ! stop when we reach AGB to produce similar results to Maynet et al.
>    stop_at_TP = .true.
>
>  ! max age
>    max_age = 1.0d10
>
> / ! end of controls namelist
> <20M_Low_Lim_noOS_5d5.png><20M_Low_Lim_OS_5d5.png><20M_Low_Lim_OS_500.png>
>
> Thanks
>
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