[Users] defining a grid function constant in time

[Miguel Zilhão]

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...

thanks,
Miguel