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

Erik Schnetter schnetter at cct.lsu.edu
Wed Mar 9 09:23:22 CST 2016


Without going into the details of the issues you are encountering:

Accretion onto a black hole has a near-spherical or axisymmetric
symmetry. In my experience, this is better modelled using a
multi-block structure than mesh refinement. Are you aware of


On Wed, Mar 9, 2016 at 10:10 AM, Petra Suková <psukova at cft.edu.pl> wrote:
> 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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Erik Schnetter <schnetter at cct.lsu.edu>

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