[mesa-users] Radiative Turbulent Diffusion

[Ehsan Moravveji]

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.

- 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?

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

Best regards
Ehsan.