[Users] Excision with McLachlan

[Jonah Miller]

Hi Miguel,

1. For the geometry, I believe you can choose between extrapolated and 
radiative boundary conditions. I think the excision boundary has the 
same conditions as the outer boundary, which are set in the parameter 
file as:

ML_BSSN::initial_boundary_condition = "extrapolate-gammas"
ML_BSSN::rhs_boundary_condition     = "NewRad"
Boundary::radpower                  = 2

Others can correct me if this is not the case. For matter, outflow 
boundary conditions are probably appropriate. Usually it doesn't matter 
what boundary conditions you set at the excision boundary, since nothing 
will escape the horizon anyway.

2. I don't think it's possible to excise dynamically... at least not 
with Llama. The way this infrastructure works is that you create a grid 
made out of 6 cubed spheres. So the excision is actually built into the 
coordinate system--it's the inner boundary of each cubed sphere.

You could get around this by, for example, stopping the simulation when 
the apparent horizon appears and then interpolating onto the sixpatches 
grid and restarting the simulation.

Or you could follow Frank's suggestion and, rather than excising, simply 
under-resolve/dissipate away whatever is going on inside the black hole.

Best,

Jonah

On 17-03-28 07:05 PM, Miguel Zilhão wrote:
> hi all,
>
> many thanks for the many replies, in particular to Jonah for providing 
> the parameter file!
> i have just a couple of follow up questions:
>
>  - what sort of boundary conditions are imposed on the evolved 
> variables inside the excised region, and how is this controlled?
>
>  - would it then be possible to do this dynamically, ie, start 
> excising only when a certain condition (like formation of an apparent 
> horizon) is fulfilled?
>
> and for my purposes having a single excised region is fine, the setup 
> i have in mind would be a collapse to a single black hole.
>
> thanks,
> Miguel
>
>
> On 27/03/17 16:59, Jonah Miller wrote:
>> Hi Miguel,
>>
>> I'm not sure about excising dynamically during evolution--maybe 
>> someone else has insight there.
>> However, excision is definitely possible. I have been using the 
>> sixpatches Llama thorn to perform
>> excision for a single black hole. I attach a parameter file to excise 
>> a Kerr-Schild black hole that
>> I based my simulations on.
>>
>> Best,
>>
>> Jonah
>>
>>
>> On 2017-03-27 05:01 AM, Miguel Zilhão wrote:
>>> hi all,
>>>
>>> i was wondering whether one could straightforwardly use the ET to 
>>> perform black hole
>>> evolutions using excision instead of punctures.
>>>
>>> specifically, i had in mind a collapsing configuration where i'd 
>>> like to excise the inner
>>> domain once a black hole horizon is formed... is this possible?
>>> i had a look around and noticed the LegoExcision thorn, but i'm not 
>>> sure whether it's
>>> actively maintained, nor whether it can interface with McLachlan...
>>>
>>> thanks,
>>> Miguel
>>> _______________________________________________
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>>
>>
>>
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