[FLASH-USERS] Initialising the magnetic field, stagard mesh MHD

[Robert Fisher]

Hi Simon :

  You raise a number of interesting technical points, but I would advise
you to reconsider the setup itself. A key problem in numerical MHD is to
ensure the satisfaction of the div . B = 0 constraint, and by overwriting
the interior solution you will surely be breaking the constraint along the
surface of the star, leading to the development of numerical monopoles.
These monopoles will in turn create forces along the field lines, which
will corrupt the solution.

  Perhaps you could consider moving the interior boundary to the surface of
the star, and specifying the field as a boundary condition there.
Alternatively you could also reduce the magnetic wave speeds within the
star (e.g. by increasing the density) to such a great extent that the field
within the star will be effectively completely frozen over the duration of
your simulation. Either will achieve the same result as what you are
imposing by hand now, but while also maintaining div . B = 0.

  Best wishes,

  Bob

On Mon, Apr 24, 2017 at 6:25 AM, Simon Daley-Yates <
sdaley at star.sr.bham.ac.uk> wrote:

> Dear Flash users
>
> I'm in the process of putting together a simulation of a stellar wind
> based on parkers equation for the solar wind. I also need to put a dipole
> magnetic field on the star.
>
> I'm working in Cartesian coordinates and in the rotating frame of the
> star.
>
> My problem requires the following:
>
> 1/ The region inside the star (r < 1 R_star) needs to be held constant. To
> do this I am using the Simulation_adjustEvolution.F90 subroutine.
>
> 2/ A point source gravity acceleration vector and acceleration due to
> Coriolis and centrifugal forces. I am doing both of these with the
> Gravity_accelOneRow.F90 subroutine.
>
> 3/ I would like to use the unsplit stagard mesh MHD method for the
> magnetic field.
>
> There are a number of different examples of setting up a MHD simulation in
> the source/Simulation/SimulationMain/magnetoMHD folder but they all use
> the method of setting initial conditions using the solnData(DENS_VAR,i,j,k)
> array and not the call Grid_putPointData(blockId, CENTER, DENS_VAR,
> EXTERIOR, axis, rhoZone) function call as recommended in the user guide
> pdf. Which is the correct method?
>
> Also, there are no examples for initialising the face centred variables
> i.e. face centred magnetic fields. Is this also done with call
> Grid_putPointData(blockId, CENTER, DENS_VAR, EXTERIOR, axis, rhoZone)
> method and is there an example?
>
> When holding the region inside the star constant, I'm assuming I need to
> also hold the magnetic field (cell centred and face centred) constant. When
> I do this the output for the magnitude of the magnetic field sqrt(magx**2 +
> magy**2 + magz**2) is corrupted (please see attached figure).
>
> If anyone has had experience constructing this type of problem or they
> have any advice, it would be very appreciated.
>
> Many thanks in advance,
> Simon
>
>


-- 
Dr. Robert Fisher
Associate Professor / Graduate Program Director
University of Massachusetts/Dartmouth
Department of Physics
285 Old Westport Road
North Dartmouth, Massachusetts 02747
robert.fisher at umassd.edu
http://www.novastella.org
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