[mesa-users] turbulent diffusion

[Aaron Dotter]

Hi folks,

I've been experimenting with turbulent diffusion in a solar model this week
and found that it doesn't work as expected.  I mean this one

            !### turbulent_diffusion_D0

            ! Turbulent diffusion below outer convection zone.
            ! Similar effect to overshooting.
            ! Proffitt, C.R., and Michaud, G.,
            ! GRAVITATIONAL SETTLING IN SOLAR MODELS,
            ! ApJ, 380:238-290, 1991.
            ! e.g., 8000 cm^2 s^-1

         turbulent_diffusion_D0 = 0

This was developed in the context of the solar model so that's the only
place I've tested it so far.  I'm using turbulent_diffusion_D0 = 7500 as
suggested by the authors but it does not produce any extra mixing.  I get
the same result with 7500 as I do with 0.

The problem is in star/private/turbulent_diffusion.f90:

            if (s% mixing_type(loc-1) == anonymous_mixing) then
               call do_turbulent_diff(ierr)
               if (ierr /= 0) return
            end if

The if statement does not function as we'd like it to.  I'm not sure of the
original intent of this code.  What we're looking for is if the zone
*below* the
bottom of the surface convection zone is currently unmixed, then the
turbulent diffusion coefficient is applied.

I've tried replacing that if statement with

            if (s% mixing_type(loc+1) == no_mixing) then

and this works as expected in my solar case.  First, it is checking the
zone below the boundary (loc+1) not the one above the boundary (loc-1).
Second, it is checking to see that the zone is not mixed -- sorry I'm not
familiar with the use or meaning of anonymous_mixing.

However, this might not catch all the possible cases for which the extra
mixing should be applied.


- Is there a more MESAonic* way to write this?


- Can we get that change added to MESA trunk?


Thanks!

Aaron



* I'm thinking of a word like "pythonic" in the python realm but mesonic
and Masonic are already taken.  ;-)
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