[FLASH-USERS] E-MHD Aniso conductivity BUG

[Loffelmann, Jiri]

Hello FLASH users,

I have two questions for you.

#1
I noticed an error in the computation of anisotropic thermal conduction coefficients inside the new FLASH version 4.7 and I would like to check that my belief is correct.

In my E-MHD simulations, using all the newly added MHD terms (Seebeck, Nernst, CrossNernst, aniso. thermal cond.), the magnetic advection velocity corresponding to the Nernst term is in some situations similar or even greater than the advection of temperature due to grad(T) in the direction perpendicular to the magnetic field.
Using the formalism from this paper https://iopscience.iop.org/article/10.1088/1741-4326/ac25c1:
      gamma_perp >= kappa_perp
In addition, in the paper in Figure 2, there is clearly visible, that the ratio of these transport coefficients, kappa_perp / gamma_perp, should be always greater than 3.
Therefore, I suspect an implementation error in the computation of the kappa_pepr coefficient.

When reading through the two files that compute kappa_perp and gamma_perp coefficients:
/FLASH4.7/source/physics/materialProperties/Conductivity/ConductivityMain/JiHeld/Conductivity_anisoFullState.F90
/FLASH4.7/source/physics/materialProperties/Thermoelectric/ThermoelectricMain/DaviesWen/Thermoelectric_fullState.F90
I noticed that the computation is very similar and truly, for the same values of magnetization ('chi'), it gives the coefficients with the correct ratio.

Finally, I discovered that both codes compute different values of magnetization, with a difference of sqrt(4.0*PI).
I believe that in Conductivity_anisoFullState, the magnetic field is in Gaussian units and in Thermoelectric_fullState it is in different physical units. And since the computation of the magnetization is precisely the same, it differs in the end. I am not really sure why the units are different, as I am not as familiar with all of the code, but either way, I believe the magnetization should be the same...

I propose a quick fix:
add line 'xmag = xmag / sqrt(4.0*PI)' after line number 133, in file Conductivity_anisoFullState.F90.

I am happy to provide further details and the setup of the simulation where the incorrect fraction appears.


2#
Secondly, I would like to ask about the usage of flux limiters for the Nernst and CrossNernst terms in the E-MHD model inside FLASH.
I would like to use E-MHD for the laser-target problems and I am wondering what values to use for 'nernstflcoef' and 'crossfieldflcoef'?
Do you have any references or experience with this?
Should I use the same value as for thermal diffusion (diff_eleFlCoef)? When for laser-target problems, it is usually reasonable to set diff_eleFlCoef = 0.05-0.1...

Regards,
Jiří Löffelmann

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://flash.rochester.edu/pipermail/flash-users/attachments/20230515/cf35ee35/attachment.htm>