[mesa-users] Radiative Turbulent Diffusion

[Ehsan Moravveji]

Dear Umberto and Bill,
Thanks for your clarifications and kind advices. I shall immediately read the paper …

Indeed, the list of proposed mixing mechanisms in radiative layers is non-exhaustive, and it is gradually growing over time. Even more serious than that is the interaction between different mixing mechanisms; e.g. what D_mix to adopt in a layer where semi-convection efficiency is roughly the same order of magnitude as radiative diffusion? This is indeed non-trivial, and the user can implement his subjective idea using extra hooks based on his physical insights.

Having said that, let’s take another look at lines 72 and 73 of the new turbulent_diffusion.f90
            if (D <= s% D_mix(k)) cycle
            if (D < s% D_mix_ov_limit) exit
The two “IF” clauses here are imposing two critical decisions. 

- Firstly, the associated radiative turbulent diffusion coefficient, D here, can be better added to s% D_mix(k), rather than just being compared and perhaps ignored (again my example above). If so, then D will serve as a background mixing coefficient and it can be easily overcome by more stronger mixing coefficients.

- Secondly, the s% D_mix_ov_limit has a default value of 100 cm^/sec which is too strong for some stars. Living in the era of high-precision frequencies from Kepler, we see evidences that D_mix_ov_limit should be fixed to much lower values to give room to smaller D_mix values to play. Thus, this default value sweeps out a whole deal of valuable information in the model. Additionally, I think the check is not needed at all. A user already has enough freedom to fix local changes of D_mix (also in the overshoot layer) through extra hooks.

Please consider my above words as few suggestions from a humble user; the Captain knows better how to steer the wheel ;-)

Best regards
Ehsan.


> On 04 Jun 2015, at 20:17, Bill Paxton <paxton at kitp.ucsb.edu> wrote:
> 
> 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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