Hi Gwyneth,

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

Date: Wed, 28 Dec 2016 15:00:10
From: Gwyneth Allwright <allgwy001@myuct.ac.za>
To: Peter Diener <diener@cct.lsu.edu>
Cc: users@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@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@myuct.ac.za>
           To: users@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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