[Users] init_3_timelevels and nans on past timelevels

[Erik Schnetter]

Yosef

As you know, init_3_timelevels uses a special algorithm developed by
Scott Hawley (see <https://arxiv.org/abs/gr-qc/0310042>) to initialise
3 time levels with initial conditions when you only have initial
conditions for a single time. It uses a carefully orchestrated dance
to step forward and backward in time, without having to do time
interpolation (which isn't possible yet since there are no other time
levels to interpolate from). The end result are (supposed to be) 3
time levels full of good initial data that are second order accurate.
To my knowledge, this algorithm is rarely used, and over time, it
might be that a bug has crept in that would leave certain grid points
uninitialized.

As you do, I would have recommended using VisIt to analyze the
resulting initial conditions and look for nans. I don't know how
multiple time levels are mapped to VisIt variables; what you do looks
correct. Another option would be to run the NaNChecker with its option
to output all nan locations that it finds.

-erik


On Tue, Aug 3, 2021 at 9:41 PM Yosef Zlochower <yosef at astro.rit.edu> wrote:
>
> Hi,
>
>    I have been running into an issue where I when I interpolate an
> evolved function I get nans even though there are no nans on the
> gridfunction itself. The nans appear to arise when the past timelevels
> (at iteration 0) are initialized by init_3_timelevels and occur near the
> buffer regions. As the evolution proceeds, these bad past timelevels are
> overwritten with seemingly good data. Is this the expected behavior of
> init_3_timelevels?
>
> Also, I have been trying to use visit to see what is going on. I used
> CarpetIOHDF5::output_all_timelevels = true, and I think this means that
> the past timelevels show up as different components of the same
> gridfunction although I haven't been able to determine how the component
> number is related to the actual components of the grid and the different
> timelevels.
>
> Thanks
>
>
> --
> Dr. Yosef Zlochower
> Center for Computational Relativity and Gravitation
> Associate Professor
> School of Mathematical Sciences
> Rochester Institute of Technology
> 85 Lomb Memorial Drive
> Rochester, NY 14623
>
> Office:74-2067
> Phone: +1 585-475-6103
>
> yosef at astro.rit.edu
>
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-- 
Erik Schnetter <schnetter at cct.lsu.edu>
http://www.perimeterinstitute.ca/personal/eschnetter/