[Mesa-users] Setup for overshooting

Thu Apr 20 17:29:09 UTC 2023

Thank you for the correction and explanation Mathias!

-EbF

On Thu, Apr 20, 2023 at 5:53 AM Mathias Michielsen <
mathias.michielsen at kuleuven.be> wrote:

> Hello Wenyu and Ebraheem
>
>
> I'd like to add something to Ebraheem's answer to avoid a possible
> misconception, specifically the following sentence:
>
> "In (2), you are taking a 0.004*Hp step backward and then overshooting
> 0.004*Hp forward, which is technically not overshooting above the
> convective boundary and is instead enhancing the diffusion coefficient near
> the edge of the convection zone."
>
>
> This would be true if the overshoot scheme is 'step', but in this example
> the scheme was set to 'exponential'. Therefore you cannot make this same
> reasoning of subtracting f and f0 to get the extent of mixing beyond the
> convective boundary.
>
> In the exponential scheme, the value of f0 denotes the same step inwards
> over distance f0*Hp. However, f denotes an e-folding distance, meaning that
> the value of the mixing coefficient has decreased by a factor e when going
> a distance f*Hp outwards.
>
> So in this case there would still be overshooting across the convective
> boundary.
>
> Regardless of this difference, I would agree that it is in general a good
> idea to have f0 small so that f>f0, also when using the exponential scheme.
>
>
> Kind regards
>
> Mathias
>
>
> ------------------------------
> *Van:* Mesa-users <mesa-users-bounces at lists.mesastar.org> namens Ebraheem
> Farag via Mesa-users <mesa-users at lists.mesastar.org>
> *Verzonden:* donderdag 20 april 2023 12:11
> *Aan:* 辛文宇
> *CC:* mesa-users at lists.mesastar.org
> *Onderwerp:* Re: [Mesa-users] Setup for overshooting
>
> Hello Wenyu,
>
> I believe the order of precedence for overshooting is such
> that overshoot_scheme(1) supersedes overshoot_scheme(2). In the current
> form you have written, overshoot_scheme(2) will not take effect because
> overshoot_scheme(1) will overwrite it. By switching the ranking, so that
> your 'burn_z' overshoot scheme is (1), and your 'any' routine for all other
> regions is set to (2), your scheme should work as intended.
> ----
> Here is a pasted explanation from a previous email of how overshoot
> functions pasted from the overshooting section of
> $MESA_DIR/star/defaults/controls.defaults:
>
> Overshooting works by taking the diffusion mixing coefficient at the edge
> of the convection zone and extending it beyond the zone. But – and here’s
> the issue – at the exact edge of the zone the mixing coefficient goes to 0.
> So we don’t want that. Instead we want the value of the mixing coeff NEAR
> the edge, but not AT the edge. The “f0” parameter determines the exact
> meaning of “near” for this. It tells the code how far back into the zone to
> go in terms of scale height. The overshooting actually begins at the
> location determined by f0 back into the convection zone rather than at the
> edge where the diffusion coeff is ill-defined. The overshooting actually
> begins at the location determined by f0 back into the convection zone
> rather than at the edge where the diffusion coeff is ill-defined. So, for
> example, if you want overshooting of 0.2 scale heights beyond the normal
> edge, you might want to back up 0.05 scale heights to get the diffusion
> coeff from near the edge and then go out by 0.25 scale heights from there
> to reach 0.2 Hp beyond the old boundary. In the inlist this would mean
> setting the “f0” to 0.05 and the “f” to 0.25.
>
> ----
> A common practice, though not required, is to take a step back such that
> fo = 0.5*f, which helps establish a diffusion coefficient well inside the
> convection zone. In the current form, your diffusion coefficient for (1) is
> set to 0.006*Hp, as you are taking a 0.004*Hp step backward into the
> convection zone, and then overshooting 0.01*Hp from this position.
> In (2), you are taking a 0.004*Hp step backward and then overshooting
> 0.004*Hp forward, which is technically not overshooting above the
> convective boundary and is instead enhancing the diffusion coefficient near
> the edge of the convection zone.
> ----
> Here is a corrected overshoot scheme, which is what I believe you might be
> after? :
>
> &controls
> ! less overshooting above Z burning shell (e.g. C, O, Si), overshoot
> distance = 0.004*Hp
>       overshoot_scheme(1) = 'exponential'    ! one of exponential or step
>       overshoot_zone_type(1) = 'burn_z'         ! one of burn_H, burn_He,
> burn_Z, nonburn, any
>       overshoot_zone_loc(1) = 'shell'          ! one of core, shell, any
>       overshoot_bdy_loc(1) = 'any'           ! one of bottom, top, any
>       overshoot_f(1) = 0.008 ! was 0.004
>       overshoot_f0(1) =  0.004
>
> ! overshooting for all shells, superseded by overshoot_scheme(1) for
> burning Z shells, overshoot distance = 0.01*Hp
>       overshoot_scheme(2) = 'exponential'    ! one of exponential or step
>       overshoot_zone_type(2) = 'any'         ! one of burn_H, burn_He,
> burn_Z, nonburn, any
>       overshoot_zone_loc(2) = 'any'          ! one of core, shell, any
>       overshoot_bdy_loc(2) = 'any'           ! one of bottom, top, any
>       overshoot_f(2) = 0.015 ! was 0.01
>       overshoot_f0(2) = 0.005 ! was 0.004 and is fine, but changed by EbF
> for consistency with overshoot_scheme(1)
>
> ----
>
> >"Another question is that if I want to test with no overshooting for
> shell burning, can I set f = 0, or I need to give a very small value for
> f. Because f and f0 shoould >0."
>
> You can do this by setting: overshoot_scheme(1) = 'none' , instead of
> 'exponential' in the &controls I shared above.
>
> -EbF
>
> On Thu, Apr 20, 2023 at 2:35 AM 辛文宇 <xinwenyu16 at mails.ucas.ac.cn> wrote:
>
>> Hi Everyone,
>>
>>
>> I am going to change the overshooting of C-shell, O-shell and Si-shell
>> burning to a smaller value.
>>
>> Can I set the overshooting as follows:
>>
>>
>>       overshoot_scheme(1) = 'exponential'    ! one of exponential or step
>>       overshoot_zone_type(1) = 'any'         ! one of burn_H, burn_He,
>> burn_Z, nonburn, any
>>       overshoot_zone_loc(1) = 'any'          ! one of core, shell, any
>>       overshoot_bdy_loc(1) = 'any'           ! one of bottom, top, any
>>       overshoot_f(1) = 0.010
>>       overshoot_f0(1) = 0.004
>>
>>
>>       overshoot_scheme(2) = 'exponential'    ! one of exponential or step
>>       overshoot_zone_type(2) = 'burn_z'         ! one of burn_H, burn_He,
>> burn_Z, nonburn, any
>>       overshoot_zone_loc(2) = 'shell'          ! one of core, shell, any
>>       overshoot_bdy_loc(2) = 'any'           ! one of bottom, top, any
>>       overshoot_f(2) = 0.004
>>       overshoot_f0(2) = 0.004
>>
>>
>> In first part, the overshooting of all of the cores and shells are set to
>> 0.01.
>>
>> Then second part change the 'f' of only the shells to 0.004, right?
>>
>>
>> Another question is that if I want to test with no overshooting for shell
>> burning,
>>
>> can I set f = 0, or I need to give a very small value for f. Because f
>> and f0 shoould >0.
>>
>>
>> Thanks,
>>
>> Wenyu
>>
>> _______________________________________________
>> mesa-users at lists.mesastar.org
>>
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