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 _______________________________________________ Users mailing list Users@einsteintoolkit.org http://lists.einsteintoolkit.org/mailman/listinfo/users