[Users] Fw: Questions about the nsnstohmns.par

[Hamilton, Maria]

From: Roland Haas <rhaas at illinois.edu>

Sent: Tuesday, March 3, 2020 6:29 PM
To: Hamilton, Maria
Cc: Peter Diener
Subject: Re: Questions about the nsnstohmns.par

Hello Maria,

>   1.  The parameter has refinement levels, and regrid radius, but
> does not have time_refinement_factors. Is this on purpose? Should it
> be there?
Without setting time_refinement_factors it uses the default which is a
factor of 2 between each level. Many of the BBH parfiles use a factor
of 1 between the coarsest level, however for NS this is often not
required (the differing factor is only there to cure an instability in
the shift condition).

>   2.  CarpetRegrid2 is given 3 num_centers, if first and second are
> the NSs, what is the role of the third, why does it have only 1
> num_level, and why is not centered at 0?
The third one is created without any boxes. I will contain the
merged star / black hole once it forms. The Triger thorn  (see your
question 4) is used to enable refinement levels for this center once a
horizon is about to form (lapse drops below a given value). Since it is
only a placeholder initially it is set to 1 level and since it will
capture the black hole that forms at the origin it is centered on the
origin.

>   3.  CarpetRegrid2 is given position_x_1 and position_x_2, if those
> are the centers for the NSs, why is 15.1875 for a 45 radius? If I
> want the radius to be 50, how do I change this?
45 is in km. So 45km is ~30Msun distance which gives a radius of
~15Msun (Msun being the code unit as well).

>   4.  Why do I need Trigger?
Trigger is needed to enable grid changes once a horizon is about to
form.

>   5.  If I  want to search for a BH as final result, how do I add
> AHFinderDirect?
Isn't it already there? If not then you can add it as usual but set eg
the parameter "find_individual_after[n]" of the horizon to some huge
value (1e6 or so) and have the Trigger thorn steer that value to "0."
once it enables the central grid patch so that the horizonfinder starts
looking for horizons (there may also be a more direct "enable"
parameter listed in AHFinderDirect's param.ccl that you could use
instead of hacking find_invidual_after, or whatever its name actually
is.

>   6.  SphericalSurface is given 5 surface, but surface [2] is
> skipped. Why?
That one was likely intended / used for a black hole horizon. There is
no reason for the set of used surfaces being contiguous really.

> Thank you for your time, and I hope you won't mind my questions.
No worries. As usual I would like to encourage these question to be
asked on the users mailing list so that others can contribute (and
benefit from the answers eventually).

Yours,
Roland

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