[Users] evolving a BH hole with Kerr-Schild ID in ET

[Zach Etienne]

Hi Roland,

> I generally suggest shifting the grid so that the closest gridpoint
> > to the origin is (dx/2, dy/2, dz/2), where dx,dy,dz are the
> > resolutions on the finest grid. I've used this trick many times to
> > stabilize evolutions. Also you may want to try the ShiftedKerrSchild
> > thorn (in the ETK), which enables a radial offset that shrinks the BH
> > coordinate size but at the same time kills off enormous constraint
> > violations near r=0. Unlike the existing KerrSchild thorn(s), this
> > one uses the standard Kerr-Schild metric written in spherical
> > coordinates and does the basis xform.
> Right, I had completely forgotten about that one. Thank you. So
> ShifterKerrSchild is actually different from Exact with its
> Kerr_KerrSchild__epsilon parameter (ie it would do proper coordinate
> transformation r -> r+r0 rather than just add r0 in some of the 1/r
> terms)?
>

KerrSchild_radial_shift sets r0.

For more documentation, my Master's student George Vopal developed as part
of his Master's research project a couple of interactive NRPy+ tutorial
modules on the shifted Kerr-Schild solution (even confirming constraint
convergence to zero in a standalone code):
Basic equations (takes a minute to load):
https://mybinder.org/v2/gh/zachetienne/nrpytutorial/master?filepath=Tutorial-ADM_Initial_Data-ShiftedKerrSchild.ipynb
Numerical validation (takes a minute to load):
https://mybinder.org/v2/gh/zachetienne/nrpytutorial/master?filepath=Tutorial-Start_to_Finish-BSSNCurvilinear-Setting_up_Shifted_KerrSchild_initial_data.ipynb


>
> Note that the goal is use it only as ID and not to reset the metric at
> each time so I will have to modify ShifterKerrSchild a bit.
>

Should be a pretty simple change.


>
> Yours,
> Roland
>
> --
> My email is as private as my paper mail. I therefore support encrypting
> and signing email messages. Get my PGP key from http://pgp.mit.edu .
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://lists.einsteintoolkit.org/pipermail/users/attachments/20190425/57be6ffe/attachment.html