[Users] Hyperbolic encounters using example parameter files

[Roland Haas]

Hello Øyvind Christiansen,

I am not an expert on BH simulations (though there are some on this
list, so it is best to keep the conversation there), but I will try to
at least give some pointers.

When changing the location of the black holes you need to also change
the parameters to TwoPunctures.

Namely:

TwoPunctures::par_b which is half the distance between the black holes

and (this is not present in bbh.par)

TwoPunctures::center_offset[0] which is the "x" coordinate of the point
on the grid from which "par_b" is measured. Ie the black holes are at 

TwoPunctures::center_offset[0] + TwoPunctures::par_b

and

TwoPunctures::center_offset[0] - TwoPunctures::par_b

You should ensure that the center of mass between the two black holes
is at the origin, otherwise eg extraction of gravitational waves on
spheres centered around the origin will give strange results (will mix
modes).

Note that TwoPunctures (and in particular its internal coordinate
system) was not designed with par_b very large (more than 20 or so
might be difficult, more than 100 would find "interesting" though the
RIT group in particular has used TwoPunctures to produce initial data
for BH that were separated by that much). You may have to increase
TwoPunctures and npoints_A, npoints_B parameters (possibly npoints_phi
as well though that seems less likely).

You physical accuracy will depend largely on the resolution used I
would say. It is almost impossible to predict it without having done at
least a couple of trial simulations at lower resolution.

When changing the mass (mass ratio really as you should try and ensure
that the total mass is ~1 since many parts of the code and formulation
implicitly assume this) you need to keep in mind that you may have to
add extra refinement levels around the small black hole to resolve it.
You will also have to adjust the 

ML_BSSN::BetaDriver

parameter which should be ~ 1/2*mi around a black hole with mass mi.
There is a fair amount of literature on this on the arXiv, by the RIT
group, and (if my memory serves) a couple years back by Doreen Mueller,
and even further back in the past by Erik Schnetter and many others.

With changing mass the the black hole radius changes since the radius
is proportional to the mass so you will have to adjust the initial
guess.

You usually find at least some information about thorn parameters in
the thorn's param.ccl file (which sometimes is the only documentation
available unfortunately).

Yours,
Roland

> Hi,
> 
> 
> I'm trying to change at first the bbh.par file from the
> arxiv-1111.3344 folder in Cactus and later the GW150914 example at
> the gallery at the einsteintoolkit website, to simulate hyperbolic
> encounters at large impact parameters where there's overlap of
> validity with the post-Newtonian approximation.
> 
> 
> I'm naively changing the TwoPunctures parameters to aim momentum a
> bit off of each other and start out at a larger distance. Changing
> the distance I also change the origin_x and
> initial_guess__coord_sphere__x_cent, and initial_x parameters in
> AHFinderDirect and PunctureTracker. I also adjust dtfac because of
> using larger momenta.
> 
> 
> My questions:
> 
> Are there any other parameters I should be changing as well to
> accommodate this scenario?
> 
> How accurate and physical results will bbh.par yield? Is GW150914
> gallery necessary? If so, should I expect reasonable results using
> it, when working with it black-box like this?
> 
> Are there any subtleties to changing the masses? For example
> regarding the initial_guess__coord_sphere__radius in AHFinder?
> 
> 
> I couldn't find too much documentation on TwoPunctures.
> 
> I'm assuming the units are G=c=Ms=1.
> 
> Thank you for any help and/or direction!
> 
> Øyvind Christiansen
> 
> Master student, Oslo



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