[Users] Excision with McLachlan

[Frank Loeffler]

On Mon, Mar 27, 2017 at 11:12:26AM -0500, Peter Diener wrote:
>To perform simulations a lot of infrastructure (a la what is available 
>in SpEC) to control the shape and size of the horizons during a 
>dynamical evolution.

Also maybe adding to this: In matter simulations one also often ends up 
with at least one black hole, even a moving black hole. In those cases 
it seems "good enough" to apply a little more dissipation well within 
the horizon to not crash - and that is really all you need. Nothing bad 
seems to "come out" of the black hole. You simply evolve everywhere and 
"ignore" what is going on well "inside the BH".

I didn't test this for a binary black hole system, but I wouldn't see 
why this shouldn't work as well. I don't think excision is really 
needed; but that of course also depends on how you evolve the space 
time. Punctures used to be evolved differently, when their nature was 
exploited to get rid of some of the singular behavior, but that is isn't 
used anymore, at least not in the Einstein Toolkit afaik, not even for 
binary black holes.

So I guess as short answer: you cannot use excision and Mclachlan in the 
Einstein Toolkit, but you should not need to for binary black holes.

Frank

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