[ET Trac] #2706: Update default TwoPunctures parameters, or at least default parameters in BBH gallery example

[Zach Etienne]

#2706: Update default TwoPunctures parameters, or at least default parameters in BBH gallery example

 Reporter: Roland Haas
   Status: new
Milestone: 
  Version: 
     Type: enhancement
 Priority: minor
Component: EinsteinToolkit website

Changes (by Zach Etienne):

title: Update default TwoPunctures parameters, or at least default parameters in BBH gallery example (was increase TwoPunctures resolution in BBH gallery example)

Comment (by Zach Etienne):

There are two major improvements I would recommend. The first is for TwoPunctures, and the second for the BBH gallery example:

1. For most binary black hole simulations, the early evolution constraint violations are dominated by the initial data, when using the default TwoPunctures grid resolution of 30x30x16. I strongly recommend increasing the default resolution to at least 48x48x20.
2. The BBH gallery example has a rather unusual choice of initial lapse, setting it to the average value of 1 - m\_i / \(2r\_i\) for both punctures i. More typically the initial lapse is set to the “pre-collapsed” value of psi^\{-2\} which goes like 1 - M/r at large radii for total binary mass M. This is the same asymptotic behavior that the will lapse relax to, and so this choice results in less gauge dynamics than the “averaged lapse” value. TwoPunctures supports `initial_lapse = "psi^n"` and chooses a perfectly fine default value of `n=initial_lapse_psi_exponent=-2` . 

Note that the BBH gallery example is negatively impacted by \(1\) as well.

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Ticket URL: https://bitbucket.org/einsteintoolkit/tickets/issues/2706/update-default-twopunctures-parameters-or
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