[mesa-users] Problem with the overshoot parameters

Aaron Dotter aaron.dotter at gmail.com
Mon Dec 17 17:51:17 EST 2012

Hi Umang,

As Matteo and Dennis have already pointed out, the way overshoot is handled
in MESA is different than the "classic" way, which is the way most existing
codes do it.

I know from experience that at least YREC treats an overshoot zone as fully
mixed. So whatever fraction of a pressure scale height your overshoot
parameter is set to will be fully mixed with the neighboring convection

MESA treats mixing by solving a diffusion equation and overshoot mixing is
(by default) treated as an exponential decay, see paper


You can approximate the fully mixed zone treatment with the setting

            ! optional step function for overshooting to be used in place
of exponential
            overshoot_step_fraction = 0 ! use this if > 0

               ! as above, f0*Hp determines r0 where switch from convection
to overshooting.
               ! D0 = diffusion coefficient D at point r0
               ! Hp0 = the scale height at r0.
               ! overshooting extends a distance f*Hp0 from r0
               ! with constant diffusion coef D = overshoot_step_fraction*D0

               ! NOTE: for step function overshoot, the overshooting extends
               ! beyond the location where grad_ad == grad_rad by a distance
               ! (f - f0)*Hp.

You'll still need to set the overshoot_f* and overshoot_f0* to be the
equivalent of what the other code is using, and then set the
overshoot_step_fraction > 0.


On Tue, Dec 18, 2012 at 9:01 AM, Umang Gupta <umanggupta92 at gmail.com> wrote:

> Hi,
> I tried evolving some models with overshoot parameters (0.05), however the
> resulting models are not convincing as they do not match with the other
> tested older codes like YREC, and CESAM. I compared the results with this
> codes and there is a huge difference. The overshoot effect is much higher
> in MESA as compared to the other codes. I think there is some problem with
> the overshoot implementation
> Kindly, let me know if there is a fix
> Umang Gupta
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