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

[Petra Suková]

Hello,

thanks for you reply. I was not aware about the Llama code. I will have a 
look at this and also at the overlap zones, if it will help.

Best regards,
Petra

-----Původní zpráva----- 
From: Philipp Moesta
Sent: Wednesday, March 9, 2016 6:46 PM
To: users at einsteintoolkit.org
Cc: psukova at cft.edu.pl
Subject: Re: [Users] Problem with accretion of magnetised gas and mesh 
refinement

Hi Petra,

to add to this. The problem you are describing sounds very familiar to
me. I had faced similar issues in simulating magnetized core-collapse
with AMR.

One thing that helped me a lot was utilizing additional buffer zones
(called 'overlap' zones). Using these seem to effectively decouple the
divB violations from the bulk of the grid when using constrained transport.

The relevant parameter is

Carpet::use_overlap_zones = "yes"

All that said, you may want to follow Erik's suggestion and use
multi-block for simulating gas accretion onto a black hole.

Best,
Philipp

On 3/9/16 7:23 AM, Erik Schnetter wrote:
> Petra
>
> 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
> <http://llamacode.bitbucket.org>?
>
> -erik
>
>
> 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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>
>
>

-- 
Philipp 


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