# [Users] Resolving two BH

Hamilton, Maria babiuc at marshall.edu
Tue Jun 7 10:02:36 CDT 2016

```Hello,

Does this paper come with access to a par file that will show how to set the dynamic shift?

I need some help as well in setting up a run with two unequal mass black holes, for an undergraduate visualization project.
My understanding is that the initial conditions are set with the parameters given in the TwoPunctures thorn.
For example, if I want the first black hole to be equal with the mass of the sun, and the second, 10 times the mass of the sun, I do something like this?

TwoPunctures::par_m_plus                = 1.5
TwoPunctures::par_m_minus            = 15

The last thing I need to set is the velocity for each hole, If I want each of them to move at 1/3 c tangential velocity?
I’m assuming I need to set:

TwoPunctures::par_P_plus [1] = +0.45
TwoPunctures::par_P_minus[1] = -4.5

Is this correct?
And one more important thing: if I want the distance between them to be for example 150, how do I set this? I see a parameter par_b which gives the x coordinate of the m+ puncture, but I’m not clear if this is what I need to set, and how.

All the other parameters I am assuming are to be kept the way they are. In Rahul’s parameter file I do not see the PunctureTracker thorn activated. I do want to know the position of the back holes on the grid, and the distance between them at each step of the evolution. Do I need to activate PunctureTracker? What output do I need for this?

Thank you,
Maria

On 6/7/16, 10:13 AM, "users-bounces at einsteintoolkit.org on behalf of Peter Diener" <users-bounces at einsteintoolkit.org on behalf of diener at cct.lsu.edu> wrote:

>Hi,
>
>This issue is discussed for example in
>
>http://arxiv.org/abs/1003.4681
>
>where a non-constant damping in the gamma driver shift is presented to
>handle unequal mass binaries. As far as I remember there may also be
>later papers with further improvements to the method.
>
>Cheers,
>
>   Peter
>
>On Tuesday 2016-06-07 01:00, Frank Loeffler wrote:
>
>>Date: Tue, 7 Jun 2016 01:00:59
>>From: Frank Loeffler <knarf at cct.lsu.edu>
>>To: rahul kashyap <rahulkashyap411 at gmail.com>
>>Cc: Einstein Toolkit Users <users at einsteintoolkit.org>
>>Subject: Re: [Users] Resolving two BH
>>
>> On Mon, Jun 06, 2016 at 06:50:04PM -0400, rahul kashyap wrote:
>>> Thanks for the reply. My apologies that I'm not completely familiar many of
>>> the physical and numerical issues while simulating bbh. I have experience
>>> in AMR astrophysical simulations.
>>
>> No need to apologize.
>>
>>> When I do the bbh simulation of ratio around 1, it evolves perfectly fine.
>>> With higher mass ratio, the smaller BH just becomes bigger and blows up.
>>> I'm assuming this as a problem of refinement.
>>
>> It is not. It looks like what I described earlier: your gauge
>> conditions, in particular the shift condition, isn't well suited for the
>> small black hole. Grid points are "falling in", making it appear growing
>> on the computational grid. You may try to play with the values such that
>> the small black hole evolves fine (you can try that with a simulation of
>> only a small black hole of that size), but then you might find that the
>> large black hole "vanishes from the grid" because grid points there are
>> pushed out too much. If this is the case, you might need
>> position-dependent values.
>>
>> Frank
>>
>>
>
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