Hi Kyle,
I am not sure the origin of this parameter, but it is indeed necessary. With fixAdvectionTerms=0, the evolution of the time derivative of the lapse A is given by
partial_t A = (partial_t - Lie_beta) K - A
where K is the trace of the extrinsic curvature, beta is the shift, and Lie is the Lie derivative. It turns out that this prescription is incompatible with 1+log slicing. Worse, if you try to impose 1+log like gauge conditions the gauge for a static black hole will in fact be dynamic. In other words, the coordinates will evolve.
To get 1+log slicing, you need to set fixAdvectionTerms=1, which gives the prescription
( partial_t - Lie_beta ) A = (partial_t - Lie_beta) K
which is compatible with 1+log slicing and with a static coordinate system for a black hole.
As you can see, it's called "fixAdvectionTerms" because it changes the partial time derivative of A into an advective total derivative.
I'm not sure the origin of the parameter so I don't know the reason for the current default settings. Somebody else should have some insight there.
Best, Jonah
On 16-09-29 12:47 PM, Slinker, Kyle Patrick 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 _______________________________________________ Users mailing list Users@einsteintoolkit.org http://lists.einsteintoolkit.org/mailman/listinfo/users