[FLASH-USERS] [EXT] Biermann Battery in 2D and 3D geometry

[Eddie Hansen]

Dear Xavier,

Thank you for your note and questions. This response should help answer
your questions:

1. It is correct that the signs are opposite. In 2D Cartesian, the positive
z-axis points *out of* the page. In 2D cylindrical (assuming for radius,
xmax > xmin), the positive phi axis points *into* the page. This preserves
right-handedness of the coordinate system. So if the domain is set up the
same way and is supposed to, for example, generate a B-field pointing out
of the page, then the two geometries will have opposite signs for MAGZ.

2. About flux-based Biermann vs. biermannSource: The biermannSource
implementation was originally intended for 2D cylindrical problems, since
the phi-generated Biermann field is cell-centered only and does not enter
in the staggered magnetic fields used in the USM solver to maintain
solenoidality. In most cases (especially in 3D cartesian), the flux-based
Biermann3T implementation is preferred because it preserves div(B) = 0 by
modifying the Godunov fluxes used in the E-field calculation. In the next
release we will include (at least) a warning message to discourage users
from using biermannSource for 3D problems.

On Thu, Jan 14, 2021 at 3:32 AM Xavier Ribeyre <ribeyre at u-bordeaux.fr>
wrote:

> Dear FLASH Code users,
>
> In the FLASH version 4.6.2 there is two Biermann battery test cases.
> One in 2D cartesian geometry and one in 2D axis-symmetric (cylindrical)
> geometry.
>
> *1-* Whatever in 2D Cartesian or 2D axis-symmetric cases: the magnetic z
> component is the same.
> And there is no magnetic field variation whatever I activated the
> "source BB terms" or the "3T BB term": i.e in flash.par:
> (use_Biermann3T = .false. hy_biermannSource = .true.
> or use_Biermann3T = .true.  hy_biermannSource = .false.)
> The z component values of the magnetic field magz are the same. *This
> behavior is correct.*
>
> *-However it seems that the SIGN of the magz component is not correct in
> 2D axis-symmetric case (it is the opposite).*
> *Are you agree with that?*
>
> *2- *I did the same test case in *3D geometry* with the the same
> temperature and density profile.
>
> When I deactivated the source term
> i.e. *use_Biermann3T = .true. hy_biermannSource = .false.* in the
> flash.par
> The values of *magz* seems correct (if we compare with the 2D previous
> cases)
> * BUT*
> When Biermann source term is activated:
> i.e. *use_Biermann3T = .false.* *hy_biermannSource = .true.* in flash.par
> *then now the magz values are very different and very low (20 times
> lower).*
>
> *-I don't know why there is a discrepancy between these two 3D simulations*
>
> I hope you can help to understand what happens.
>
> Best regards,
>
> Xavier Ribeyre
>
>
>
>

-- 
Eddie Hansen, PhD
Postdoctoral Associate
University of Rochester
607-341-6126 | Flash Center
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