Hi Gwyneth, On Wednesday 2016-12-28 15:00, Gwyneth Allwright wrote:
Date: Wed, 28 Dec 2016 15:00:10 From: Gwyneth Allwright <allgwy...@myuct.ac.za> To: Peter Diener <die...@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 <die...@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 <allgwy...@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 Disclaimer - University of Cape Town This e-mail is subject to UCT policies and e-mail disclaimer published on our website at http://www.uct.ac.za/about/policies/emaildisclaimer/ or obtainable from +27 21 650 9111. If this e-mail is not related to the business of UCT, it is sent by the sender in an individual capacity. Please report security incidents or abuse via cs...@uct.ac.za
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