[Users] Problem with accretion of magnetised gas and mesh refinement

[Petra Suková]

Hello everyone,

I am a postdoc at CFT PAN Warsaw and a relatively new user of ET. I am 
trying to use ET for simulating accretion of low angular momentum magnetised 
gas onto Kerr black hole.

I use GRHydro and GRHydro_Init thorns on stationary Kerr spacetime. Using 
mesh refinement with nonzero magnetic field always leads to crash of the 
computation, after few hundreds or thousands of iterations. It looks like 
the cause of the crash is violation of divB=0 constrain in the buffer zones 
on the edge of refined level, which grows and later starts to propagate into 
the grid.

More details:
I have adapted GRHydro_BondiM.c and run some tests with low angular momentum 
accretion, B=0, spin=0 and several refinement levels, which after some 
modifications worked quite well. Now I am trying to include magnetic field. 
I have set initially vertical magnetic field. After first iteration, divB in 
the buffer zones increases to values around 10e-7 and it is growing during 
the evolution. After some time (depending on the Bfield strength, grid size 
etc, typically tens or hundreds of M), the values of divB in the buffer 
zones exceed 0.1 and start to spread inside the grid zones and spoil the 
simulation.
I thought, that maybe some of my changes caused this, so I tried to run an 
example par file MBondi_B5.70_Mdot12.57_Npts100_Sch.par with clean ET 
release 12, but that crashed immediately in the first iteration. So I tried 
to lower bondi_bmag from 5.7 to 0.1 and this worked and was stationary, but 
it runs only on unigrid without refinement. When I added the mesh refinement 
and fixed GRHydro_BondiM_Boundary to not act on the refinement boundary, the 
same problem with buffer zones appears. I am attaching the par file of this 
computation together with few snapshots of divB, rho and Bvec.
My guess is that during the prolongation and restriction of magnetic field 
vector the DivB=0 condition is violated and this error is continuously 
increasing when the Bfield and matter is accreted through the refinement 
boundary.
Has anyone faced similar problem? Or does anyone have some working parfile 
with similar computation with magnetised gas and at least two refinement 
levels, which he can send me? I could test that and hopefully discover my 
problem.

Thanks a lot,
Petra 


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