Hi Roland,
Additionally, I find it convenient to smooth all quantities near the
singularity, and to choose to advect both lapse and shift. I don't know whether the latter is necessary in theory, but I am always using it.
Concerning the smoothing it seems that some smoothing was necessary to not crash the code (via the Exact epsilon parameter to transform r -> r+epsilon). An alternative may have been to use NoExcision to fill the interior of the AH with smooth data.
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.
-Zach
* * * Zachariah Etienne Assistant Professor of Mathematics West Virginia University *https://math.wvu.edu/~zetienne/ https://math.wvu.edu/~zetienne/* https://blackholesathome.net
On Thu, Apr 25, 2019 at 12:30 PM Haas, Roland rhaas@illinois.edu wrote:
Hello Erik,
This is almost possible. For puncture data, one typically evolves lapse, shift, and a quantity B (the time derivative of the shift). For Kerr-Schild data, one also needs to evolve A, the time derivative of the lapse. Otherwise, Kerr-Schild data are not stationary. (One could instead add an offset alpha_0 to the evolution equations for K, but that is quite non-standard and not implemented in McLachlan.)
Ok so it is possible.
Additionally, I find it convenient to smooth all quantities near the singularity, and to choose to advect both lapse and shift. I don't know whether the latter is necessary in theory, but I am always using it.
Concerning the smoothing it seems that some smoothing was necessary to not crash the code (via the Exact epsilon parameter to transform r -> r+epsilon). An alternative may have been to use NoExcision to fill the interior of the AH with smooth data.
I can send a sample parameter file if that helps.
That would be great if you had something that you could send around easily.
Yours, Roland
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