[Users] defining a grid function constant in time

[Ian Hinder]

On 27 Oct 2017, at 10:28, Miguel Zilhão <miguel.zilhao.nogueira at tecnico.ulisboa.pt> wrote:

> hi Ian,
> 
>>> for the purposes of trying a particular gauge condition, we'd like to have a grid function (let's
>>> call it "shift_t0") that's set to be equal to the shift at t=0. we don't want this function to
>>> evolve at all; we just want this grid function to be accessible at all times (with exactly the same
>>> values).
>>> 
>>> is there something in particular that we need to watch out for, when doing this? we ask because
>>> we've noticed that when doing things in a "naive"* way, sometimes NaNs start developing at some
>>> point in the evolution (seemingly near refinement boundaries and when regriding). we noticed that
>>> exactly the same thing happens for the "puncture_u" grid function in TwoPunctures: this function is
>>> only used at t=0; however, if one activates storage for it and asks for its output, at some point
>>> NaNs start developing in the output, even though the function is never used again. so, what would be
>>> the standard way of defining a grid function that is constant in time?
>>> 
>>> thanks,
>>> Miguel & Helvi
>>> 
>>> * by "naive", we mean essentially the following
>>> 
>>> in schedule.ccl we have
>>> 
>>> STORAGE: shift_t0[1]
>>> 
>>> in interface.ccl we have
>>> 
>>> CCTK_REAL shift_t0 type=gf timelevels=1 tags='tensortypealias="U" tags='prolongation="none"'
>>> {
>>>   betax_t0 betay_t0 betaz_t0
>>> } "shift vector at t=0"
>>> 
>>> and we set its value (on the whole grid) at a function call at CCTK_INITIAL after ADMBase_PostInitial
>> Does this happen only after regridding?  By not prolongating the refinement boundaries, it's not clear how the "new" bits of the grid are supposed to be filled when the fine grids move.  Could this be the problem?  If you give the gridfunction 3 timelevels and remove "prolongation=none", do the NaNs go away?
> 
> it seems to happen only after regridding, yes. we did try precisely that (using 3 timelevels and prolongating) but the NaNs still appeared...

The past timelevels need to be initialised.  By default, Carpet will poison them with NaNs so that you can see when undefined values are used.  Theoretically, if there is no time evolution of this variable, you should be able to use a single timelevel and use a tag 'prolongation="copy"', which should perform only spatial prolongation.  Let me know what happens if you try this.  I expect you will get an assertion failure from Carpet, and then you might want to try cherry-picking this commit:

cd repos/Carpet
git cherry-pick d13a6e245a3d7d2b575094fd0def540d510a166c

which disables some code in Carpet that disables spatial prolongation in the case of "op_copy".  The problem here likely resulted in confusion due to the operator being badly named (https://martinfowler.com/bliki/TwoHardThings.html).  In arrangements/Carpet/CarpetLib/src/operators.hh, the implication is that the "copy" in op_copy refers to the time interpolation only, but the code in patched in the above commit assumes that "copy" means don't interpolate at all.  It's not clear how this can make sense when the data comes from another refinement level, which will always have a different resolution, but maybe there is some case where this would have made sense.

Erik, should this commit be merged to master?

-- 
Ian Hinder
http://members.aei.mpg.de/ianhin