[Users] McLachlan Shift Condition

[Peter Diener]

Hi Gwyneth,

On Wednesday 2016-12-28 15:00, Gwyneth Allwright wrote:

>Date: Wed, 28 Dec 2016 15:00:10
>From: Gwyneth Allwright <allgwy001 at myuct.ac.za>
>To: Peter Diener <diener at cct.lsu.edu>
>Cc: users at einsteintoolkit.org
>Subject: Re: [Users] McLachlan Shift Condition
>
>Hi Peter,
>Thanks so much! I asked because I'd like to try running simulations with the
>modified shift condition described in this paper (second
>page): https://arxiv.org/abs/gr-qc/0511103
>
>Some terms that feature in the usual gamma driver equations are dropped, and
>the lapse appears in the time derivative of beta. Would I have to make
>changes to the Kranc code in order to implement such a condition?

There is presently no parameter choice that would implement that 
particular version of the gamma driver shift in McLachlan. So, yes 
modifying the code would be the only option. It should be 
straightforward to modify the Kranc script and then regenerate the code
(if you have a Mathematica license).

The Kranc script can be found in:

Cactus/repos/mclachlan/m/McLachlan_BSSN.m

You'll have to modify the relevant part (i.e. the rules for dot[beta[ua]]
and dot[B[ua]]) of the calculation EverythingFromBSSNCalc.

It's probably easier for you to simply replace those complicated rules
by your version for testing instead of adding options to add it to the
existing shift.

If it turns out to be useful for you, we can then later think about how
to add it to McLachlan permanently.

>Best,
>
>Gwyneth

Cheers,

   Peter

>On Wed, Dec 28, 2016 at 10:57 AM, Peter Diener <diener at cct.lsu.edu> wrote:
>      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
>
>
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