[mesa-users] Radiative Turbulent Diffusion

[Umberto Battino]

Dear Ehsan, Bill and mesa-users,

I would like to bring to yopur attention the work by Alecian and Michaud (2005) ( http://www.aanda.org/articles/aa/pdf/2005/07/aa1259.pdf ) where, as you can read from the abstract, "It is shown that the process called radiative diffusivity by Morel & Thévenin (2002, 390, 611) does not exist as they describe it. Their description is based on a confusion between atomic diffusion and turbulent transport".

The implementation made by Morel (2002) turns out to be at least controversial... 

Have a look at this paper and let me know what you think!
Cheers,

 Umberto






________________________________________
From: Bill Paxton [paxton at kitp.ucsb.edu]
Sent: Thursday, June 04, 2015 8:17 PM
To: Ehsan Moravveji
Cc: mesa-users
Subject: Re: [mesa-users] Radiative Turbulent Diffusion

Hi Ehsan,

On Jun 3, 2015, at 9:39 AM, Ehsan Moravveji wrote:

> Dear mesa-users,
> Today, I’ve been fiddling around the radiative turbulent diffusion (Morel & Thevenin, 2002, A&A), and its influence on the net mixing coefficient. I came up with few questions that I would like ask.
>
> One only needs to set radiation_turbulence_coeff to something greater than 0 to activate this. The relevant routine sits in star/private/turbulent_diffusion.f90. Reading through lines 64 to 84, you see (briefed in here):
>
>         do k = 2, nz
>            if (s% mixing_type(k) /= no_mixing) cycle
>            …
>
>            D = coeff*4*crad*T_face*T_face*T_face*T_face/(15*clight*opacity_face*rho_face*rho_face)
>            if (D < s% D_mix_ov_limit) exit
>            …
>
>            s% cdc(k) = cdc
>            s% D_mix(k) = D
>
>            s% conv_vel(k) = vc
>            s% mixing_type(k) = anonymous_mixing
>         end do
>
> - Based on the original paper, the radiative turbulent mixing coefficient (D_rad) should be used across the whole star, although its effect will be swamped by the convective mixing in convective layers; thus, I would like to know why it is ignored if a zone is not flagged as no_mixing? I also cannot convince myself why D_rad cannot be “added” to other mixing coefficients, say semi-convective, thermohaline, etc.? Indeed, we do not know well how different mixing mechanisms interplay/interact. Some words from black belts would be very appreciated here.

good point.  the code is assuming that any other form of mixing will dominate, so it can skip calculating radiative turbulence at that cell boundary.
i'll change it to calculate the radiative turbulence mixing in all cases and use it if it is stronger than the previous mixing.
i.e., delete the initial test of mixing_type, and after calculation of D, add "if (D <= s% D_mix(k)) cycle".

>
> - Furthermore, why the associated convective diffusion coefficient and velocity, cdc and vc here, are stored in s% cdc and s% conv_vel, whereas these variable names are reserved for convection-related quantities? Are there specific design reasons behind this?

those names go back to the dark ages of mesa when all mixing was convective.  i haven't renamed them.

>
> - And finally, is the add_radiation_turbulence() subroutine identical to the original CESAM version?

I'll leave that one for you to check.

The above changes are in version 7626 if you'd like to test them.

Bill




>
> Best regards
> Ehsan.
>
>
>
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