[mesa-users] Overshooting

[Falk Herwig]

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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