[Users] McLachlan Shift Condition

[Peter Diener]

Hi Gwyneth,

The parameters to control the gamma driver shift condition is something 
that we would like to make more transparent. I have to go and look at 
the Kranc code, in order to figure out how things work.

When ShiftBCoeff = 1 and ShiftAdvectionCoeff = 1 (both are the default 
values) we are evolving the gamma driver shift that is listed on the 
wiki you point to.

ShiftGammaCoeff should be set to 0.75 (it's the 3/4 in the equation for 
the shift). It's unfortunate that 0.75 is not the default value for 
ShiftGammaCoeff, as this is the only value that is ever used (as far as 
I know). BetaDriver is the eta value in the eqaution for B. 
ShiftAlphaPower is a leftover parameter from before the moving puncture 
approach came into use and currently has no effect in the code.

Let me know if you have any other questions.

Cheers,

   Peter

On Tuesday 2016-12-27 10:31, Gwyneth Allwright wrote:

>Date: Tue, 27 Dec 2016 10:31:55
>From: Gwyneth Allwright <allgwy001 at myuct.ac.za>
>To: users at einsteintoolkit.org
>Subject: [Users] McLachlan Shift Condition
>
>Hi All,
>
>I'm new to the Einstein Toolkit and would like to double check what the
>ML_BSSN parameters ShiftGammaCoeff, BetaDriver and ShiftAlphaPower refer to.
>I tried looking in the param.ccl file, but it still wasn't 100% clear to me.
>
>As far as I understand, the shift condition used by McLachlan is the gamma
>driver, given here: http://grwiki.physics.ncsu.edu/wiki/Shift_Conditions.
>What value of eta does ML_BSSN use?
>
>Thanks,
>
>Gwyneth
>
>