[Users] defining a grid function constant in time

[Ian Hinder]

On 26 Oct 2017, at 23:23, Miguel Zilhão <miguel.zilhao.nogueira at tecnico.ulisboa.pt> wrote:

> hi all,
> 
> 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?

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

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