[Users] McLachlan Shift Condition
Gwyneth Allwright
allgwy001 at myuct.ac.za
Sat Dec 31 04:30:44 CST 2016
Hi Peter,
Thanks! I had a look at the part of the code you pointed me to, and it does
seem like a straightforward modification.
The rules for dotbeta[ua], dot[beta[ua]] and dot[B[ua]] make sense to me,
besides for the sign of the first IfThen term in dot[B[ua]]:
dot[B[ua]] -> IfB[+ dotXt[ua]
+ IfThen[fixAdvectionTerms != 0,
Upwind[beta[ub], Xt[ua], lb],
0]
- (betaDriverValue
(+ B[ua]
+
IfThen[fixAdvectionTerms != 0 && advectShift != 0,
Upwind[beta[ub], beta[ua], lb] /
(shiftGammaCoeffValue alpha^
shiftAlphaPower),
0]))
+ IfThen[fixAdvectionTerms == 0 && advectShift !=
0,
Upwind[beta[ub], B[ua], lb],
0]
+ Dissipation[B[ua]],
0]
Comparing with the gamma driver condition in the wiki (
http://grwiki.physics.ncsu.edu/wiki/Shift_Conditions), shouldn't this term
be negative, or am I missing something?
You mentioned that shiftAlphaPower doesn't affect the code any more, but it
seems to feature in the rule for dotbeta[ua]. Wouldn't I be able to make
the time derivative of beta proportional to alpha by setting the parameter
shiftAlphaPower = 1?
Thanks again for your help, and Happy (soon-to-be) New Year!
Best,
Gwyneth
On Fri, Dec 30, 2016 at 12:04 PM, Peter Diener <diener at cct.lsu.edu> wrote:
> 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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>>
>>
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