[Users] using WeylScal4 and Multipole for WF output -- scheduling issues

[Erik Schnetter]

On Friday, October 22, 2010, Ian Hinder <ian.hinder at aei.mpg.de> wrote:

> The difference is related to prolongation of mesh refinement
> boundaries.  Psi4 is computed using spatial derivatives, and hence
> needs the boundaries of the grid to be filled in via prolongation from
> the next coarser grid.  Functions scheduled in ANALYSIS cannot have
> time interpolation, due to the time at which ANALYSIS is scheduled
> relative to the Berger-Oliger time stepping.  In order that all points
> in the grid have sensible values, analysis quantities which take
> spatial derivatives should be scheduled in EVOL so that time
> interpolation onto their refinement boundaries is possible.  It is
> also necessary to recompute these quantities after regridding and in
> several other places.  To make this convenient,  a new schedule group,
> MoL_PseudoEvolution, was introduced in EVOL in which such quantities
> should be computed.  Variables set in this group should have three
> timelevels, just like evolved variables, since time interpolation
> requires this.
>
> Erik: is this description accurate?

Yes, this is accurate.

> When Multipole calls the interpolator to find the values of Psi4 on
> the extraction spheres, Carpet uses the finest grid that the point
> exists on.  This could mean the last points in a buffer zone, and if
> you haven't filled these correctly with prolongation, these will be
> poisoned (or undefined, if you're not using poisoning).  So if your
> extraction spheres intersect your refinement boundaries (very
> difficult to avoid in 3D), you might pick up poison from them.
>
> In my opinion, the interpolator should not give points from buffer
> zones, and instead use the coarser grid.  I think this might be the
> case in the new (Mercurial) version of Carpet.  Erik: is that the case?

This is partly correct. The centre of an interpolation stencil will
not be in a buffer zone in the Mercurial version. However, the stencil
itself may extend into buffer / ghost / symmetry / outer boundary
zones. That means that one still needs to prolongate, synchronise and
apply symmetry and boundary conditions before interpolating.

Thanks for the explanations, Ian!

-erik