Thank you Jonah and Erik for your quick replies! I think I understand better
what's going on now.
Kyle
On 09/29/2016 01:53 PM, Erik Schnetter wrote:
Kyle
To my knowledge -- and it seems I am mistaken here -- the previous behaviour
corresponds to the "off" setting.
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.
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?).
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".
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.
-erik
On Thu, Sep 29, 2016 at 12:47 PM, Slinker, Kyle Patrick
<ksl...@live.unc.edu<mailto:ksl...@live.unc.edu>> wrote:
Hello,
I'm wondering if anybody can tell me about the
ML_BSSN::fixAdvectionTerms parameter (which I think was new in
Somerville). I have been trying to reproduce some old results and
discovered this morning that having this parameter set to "off" is what
was causing the discrepancy.
What does it do? The description "Modify driver and advection terms to
work better?" is vague enough I don't know what to make of it. From
McLachlan_BSSN.m it looks like it's switching how the advection terms in
the 1+log and Gamma-driver conditions are handled. What changes is it
making to these equations? And what is the purpose of them? If setting
the parameter to "on" reproduces old results (and if it lives up to its
name and fixes things), should "on" be the default behavior?
Thanks.
Kyle Slinker
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Erik Schnetter <schnet...@cct.lsu.edu<mailto:schnet...@cct.lsu.edu>>
http://www.perimeterinstitute.ca/personal/eschnetter/
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