[Users] Carpet moving grids

[Roland Haas]

Hello Erik,

> There are thorns that determine the location of the density maximum
> and/or the centre of mass for neutron stars, and those thorns can feed
> into Carpet's regridding method to have it track neutron stars. I have
> not evolved moving neutron stars myself, so I can't tell you any more
> details, but I hope others on this list will speak up.
> 
> Another method (that is probably not implemented yet) would be to
> track the centre of the neutron stars in the same manner as the
> puncture tracker, i.e. to derive an ODE that drives a pseudo-particle
> along the density gradient. (In the puncture tracker, this
> pseudo-particle moves along the shift vector instead.)
For mixed black hole neutron star systems we (Frank, Tanja, Bruno Mundim
and I) are using PunctureTracker and NSTracker (from
https://svn.aei.mpg.de/numrel/AEIThorns/PunctureTracker and
https://svn.cct.lsu.edu/repos/numrel/LSUThorns/NSTracker respectively).
Both cooperate with CarpetTracker to transfer the found location into
CarpetRegrid2. You will need something like this in your parfile:
{{{
CarpetTracker::surface[0] = 0
CarpetTracker::surface[1] = 1
NSTracker::NSTracker_SF_Index = 0
NSTracker::NSTracker_max_distance = 2
PunctureTracker::track                      [0] = "yes"
PunctureTracker::initial_x                  [0] = $x_BH
PunctureTracker::initial_y                  [0] = $y_BH
PunctureTracker::initial_z                  [0] = $z_BH
PunctureTracker::which_surface_to_store_info[0] = 1
}}}

NSTracker tracks the *global* maximum of the rest mass density. There
are options to mirror the found maximum which enable you to use it when
there are two equal mass neutron stars (ie. you have pi-symmetry). For
unequal mass neutron stars I am not aware of a public tracker.

You could either implement tracker particles that start at the centre of
a each star as Erik suggested or modify NSTracker to track multiple
maxima by only accepting a new position for a maximum if the new
location is with NSTracker_max_distance. The later is (more or less)
what I did in a private thorn of mine before I learned of NSTracker's
existence and it worked satisfactorily for my needs.

Yours,
Roland

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