<div dir="ltr">Kyle<div><br></div><div>To my knowledge -- and it seems I am mistaken here -- the previous behaviour corresponds to the "off" setting.</div><div><br></div><div>The parameter chooses between two variants for handling the advection terms in the lapse and shift evolution equations. Since these are gauge conditions, there is no "right" or "wrong" here, only historic precedence. Both lapse and shift can be evolved as PDEs that are first order in time, or second order in time, introducing new variables A and B^i that correspond to the time derivatives of lapse alpha and shift \beta^i. When this parameter is "on", then switching between first and second order equations does not change the gauge evolution equations, if the driver terms are neglected.</div><div><br></div><div>The driver terms in the gauge conditions choose how lapse and shift should look like in equilibrium. This term is different in the first order and second order formulation, and is the reason why people might want to use a second order formulation that otherwise only adds complexity. In a first order formulation, the lapse condition drives the gauge condition towards K=0 (maximal slicing), and the shift condition drives the gauge condition towards \tilde\Gamma^i=0 ("shear free"). Combined, this ensures that a Minkowski spacetime will end up in Minkowski coordinates, given appropriate boundary conditions. In a second order formulation, the gauge condition is looser and only drives towards \dot K=0 and \dot\tilde\Gamma^i=0. This is preferable if you have a coordinate system where e.g. K\ne0 (e.g. Kerr-Schild coordinates) or \tilde\Gamma^i\ne0 (don't know for sure -- maybe a highly spinning black hole?).</div><div><br></div><div>In short -- these days, I always set this parameter to "on". The whole reason the parameter is there is that old versions of the code effectively had set it to "off". Maybe this "old version" is so old that it is now irrelevant, and maybe some intermediate version of the code accidentally didn't have this parameter and implicitly set it to "on".</div><div><br></div><div>Regarding default behaviours: In Cactus, the default settings for parameters is not what is good for newcomers or what is least surprising, default values are for backward compatibility, as they allow re-running a parameter file two years later and getting the same result. This is eminently important in the long term.</div><div><br></div><div>-erik</div></div><div class="gmail_extra"><br><div class="gmail_quote">On Thu, Sep 29, 2016 at 12:47 PM, Slinker, Kyle Patrick <span dir="ltr"><<a href="mailto:kslink@live.unc.edu" target="_blank">kslink@live.unc.edu</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">Hello,<br>
<br>
I'm wondering if anybody can tell me about the<br>
ML_BSSN::fixAdvectionTerms parameter (which I think was new in<br>
Somerville). I have been trying to reproduce some old results and<br>
discovered this morning that having this parameter set to "off" is what<br>
was causing the discrepancy.<br>
<br>
What does it do? The description "Modify driver and advection terms to<br>
work better?" is vague enough I don't know what to make of it. From<br>
McLachlan_BSSN.m it looks like it's switching how the advection terms in<br>
the 1+log and Gamma-driver conditions are handled. What changes is it<br>
making to these equations? And what is the purpose of them? If setting<br>
the parameter to "on" reproduces old results (and if it lives up to its<br>
name and fixes things), should "on" be the default behavior?<br>
<br>
Thanks.<br>
<br>
Kyle Slinker<br>
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</blockquote></div><br><br clear="all"><div><br></div>-- <br><div class="gmail_signature" data-smartmail="gmail_signature">Erik Schnetter <<a href="mailto:schnetter@cct.lsu.edu" target="_blank">schnetter@cct.lsu.edu</a>><br><a href="http://www.perimeterinstitute.ca/personal/eschnetter/" target="_blank">http://www.perimeterinstitute.ca/personal/eschnetter/</a></div>
</div>