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

[Haas, Roland]

Hello Zach,

> 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)?

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.

Yours,
Roland

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