[Users] McLachlan Shift Condition

Gwyneth Allwright allgwy001 at myuct.ac.za
Wed Dec 28 15:00:10 CST 2016

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):

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?



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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