[mesa-users] overshooting

[Bill Paxton]

Hi Umberto,

Thanks for your question -- I think this is a case where we should
use grep to track down the truth in the code in star/private.
But first let me say that these parameters allow you to set "standard"
values for the overshoot f parameters (such as appropriate for a 2 Msun star)
and have them automatically reduced for lower mass stars.  The automatic
reduction takes effect over a mass range that you specify using the
controls mass_for_overshoot_full_on and mass_for_overshoot_full_off.
You might check with Falk Herwig for the physical motivation for this.
For the code details, grep will tell us where to look.

/Users/bpaxton/mesa/star/private: grep mass_for_overshoot_full_on *
ctrls_io.f:         mass_for_overshoot_full_on, &
ctrls_io.f:         s% mass_for_overshoot_full_on = mass_for_overshoot_full_on
overshoot.f:            if (s% star_mass <= s% mass_for_overshoot_full_on) then
overshoot.f:                      (s% mass_for_overshoot_full_on - s% mass_for_overshoot_full_off)

The references in ctrls_io are defining the names for the &controls inlist, so that's not what we want.
We'll want to check the code in overshoot.f -- here's the relevant section from the add_overshooting routine:

         do i=1, n_conv_bdy ! from center to surface

            if (s% conv_bdy_q(i) < s% min_overshoot_q) then
               if (dbg) write(*,*) 'skip since s% conv_bdy_q(i) < min_overshoot_q', i
               cycle
            end if
            
            call set_f(i)
            
            have_h1_czb = .false.
            
            if (f_above == 0 .and. f_below == 0) cycle
            if (s% star_mass <= s% mass_for_overshoot_full_off) cycle
            if (s% star_mass <= s% mass_for_overshoot_full_on) then
               frac = (s% star_mass - s% mass_for_overshoot_full_off) / &
                      (s% mass_for_overshoot_full_on - s% mass_for_overshoot_full_off)
               frac = 0.5d0*(1 - cos(pi*frac))
               if (dbg) write(*,1) 'overshoot frac', frac
               f_above = f_above*frac
               f_below = f_below*frac
            end if

We are looping over the convective boundaries deciding whether or not to add overshooting.
The test of min_overshoot_q prevents adding overshooting very near the center.
The call on set_f sets the f_above and f_below values depending on what kind of convective zone it it.
If star_mass is <= mass_for_overshoot_full_on, then we cycle to the next boundary without adding overshooting.
If star_mass is > mass_for_overshoot_full_off but less than mass_for_overshoot_full_off, then the values
of f_above and f_below are reduced by a value that depends on where the star_mass lies in the range.

Good luck,
Bill







On Dec 1, 2011, at 2:34 AM, umberto.battino at unibas.ch wrote:

> Hi all,
> I'm performing some tests varying overshooting.. I saw in  
> star_defaults.dek that  overshoot_f0_above/below parameter has to be  
> set.. moreover I saw that a range of star masses over which  
> overshooting is gradually enabled has to be set as well.. more  
> precisely the two parameters mass_for_overshoot_full_on and  
> mass_for_overshoot_full_off.. but what do they mean? Do they define a  
> mass range such as overshooting starts from   
> mass_for_overshoot_full_off and reaches its plenty efficiency at   
> mass_for_overshoot_full_on and major masses? Can anybody clarify this  
> point? :)
> All the best,
> 
>  Umberto
> 
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