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Hi Bob<br>
<br>
Yes, you are absolutely correct that this setup does lead to
numerical difficulties of the type you mention. I have previously
run this simulation using the code PLUTO. This was using the
Divergence cleaning method and not CT. The results are stable with
minimal pollution of the solution outside the star (I'm only
interested in the dynamics of the wind).<br>
<br>
Your suggestion of moving the interior boundary to the surface of
the star I'm guessing would involve using
Simulation_defineDomain.F90 to remove the blocks inside the stellar
radius? This sounds like a plan. However, reading the source code
and the guide, it is a bit opaque how boundary conditions are then
applied to the removed blocks. The surface of my star is revealed by
successively refining blocks. If I'm removing hole blocks from the
simulation, and one block is comparable in size to my star, this my
not work.<br>
<br>
I may try using your suggestion of raising the density in the star
to freeze in the field.
<br>
<br>
In all honesty, after using PLUTO for a couple of years, I was
curious to see what other established codes could do, so if I can't
get this working in FLASH, never mined.<br>
<br>
Thank you for your help.<br>
<br>
Simon<br>
<br>
<div class="moz-cite-prefix"><br>
On 24/04/2017 11:56, Robert Fisher wrote:<br>
</div>
<blockquote
cite="mid:CAD40uowEXqYkBnjV_LAhu5YjiqXR4ypi5ALuRgJcEeabG9f8Sg@mail.gmail.com"
type="cite">
<div dir="ltr">Hi Simon :
<div><br>
</div>
<div> 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.</div>
<div><br>
</div>
<div> 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.</div>
<div><br>
</div>
<div> Best wishes,</div>
<div> </div>
<div> Bob</div>
</div>
<div class="gmail_extra"><br>
<div class="gmail_quote">On Mon, Apr 24, 2017 at 6:25 AM, Simon
Daley-Yates <span dir="ltr"><<a moz-do-not-send="true"
href="mailto:sdaley@star.sr.bham.ac.uk" target="_blank">sdaley@star.sr.bham.ac.uk</a>></span>
wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0
.8ex;border-left:1px #ccc solid;padding-left:1ex">Dear Flash
users<br>
<br>
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.<br>
<br>
I'm working in Cartesian coordinates and in the rotating
frame of the star.
<br>
<br>
My problem requires the following:<br>
<br>
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. <br>
<br>
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.<br>
<br>
3/ I would like to use the unsplit stagard mesh MHD method
for the magnetic field.<br>
<br>
There are a number of different examples of setting up a MHD
simulation in the source/Simulation/SimulationMa<wbr>in/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? <br>
<br>
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? <br>
<br>
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).<br>
<br>
If anyone has had experience constructing this type of
problem or they have any advice, it would be very
appreciated.<br>
<br>
Many thanks in advance,<br>
Simon<br>
<br>
</blockquote>
</div>
<br>
<br clear="all">
<div><br>
</div>
-- <br>
<div class="gmail_signature" data-smartmail="gmail_signature">
<div dir="ltr">Dr. Robert Fisher<br>
Associate Professor / Graduate Program Director<br>
University of Massachusetts/Dartmouth<br>
Department of Physics<br>
285 Old Westport Road<br>
North Dartmouth, Massachusetts 02747<br>
<a moz-do-not-send="true"
href="mailto:robert.fisher@umassd.edu" target="_blank">robert.fisher@umassd.edu</a><br>
<a moz-do-not-send="true" href="http://www.novastella.org"
target="_blank">http://www.novastella.org</a></div>
</div>
</div>
</blockquote>
<br>
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