Hayley No, there should not be any loss of precision. The difference in the ADMBase variables should be at round-off level.
-erik On Wed, Aug 23, 2023 at 12:37 PM Hayley Macpherson <[email protected]> wrote: > > Hi Erik, > Thanks for the quick response! > > I’m not using mesh refinement, and yes I ignore the ghost zones in my > calculations. I use periodicity within the original (no ghosts) grid for > derivatives. > > And yes I’m using the ADMBase metric and extrinsic curvature. I get the same > result whether I do the calculation in the postpostinitial stage or using the > 3D dumps in post-processing. Would there be a loss of precision in ADMBase > gij and kij when translating back and forth to BSSN metric/kij, which could > then be propagated to the output files? > > Best, > Hayley > > On 23 Aug 2023, at 11:31, Erik Schnetter <[email protected]> wrote: > > Hayley > > Are you using mesh refinement in your calculations? If so, the ET > would fill the ghost and the buffer zones with interpolated data. > These are generally less accurate than properly calculated data. If > so, are you ignoring ghost and buffer zones? These should not be used > for visualization and post-processing. > > Which metric variables are you accessing? I assume you are looking at > ADMBase for the metric and extrinsic curvature? These should change, > unless you look at them too early: During initialization, we set up > the ADMBase variables, then define the BSSN variables from these, and > then recalculate the ADMBase variables. This would change them > slightly. Analysis tools should only look at the ADMBase variables > after they have been reset. > > -erik > > > On Wed, Aug 23, 2023 at 11:56 AM Hayley Macpherson > <[email protected]> wrote: > > > Hi all, > > I’m the author of a cosmological initial data thorn in the ET; FLRWSolver. > I’m currently working on improving the initial data to solve the constraint > equations exactly (instead of previously using an approximation), for a given > metric and Kij. The way this works is to calculate the metric terms on the > LHS of both the constraints and solve for the relevant rest mass density and > peculiar 3-velocities from the matter terms. > > I have my own routines from a post-processing analysis code to calculate the > metric terms, and I’ve incorporated these into the ET for both generating the > initial data but also then double checking the constraint violation after the > initial data is set up. > > I noticed something strange: my checks immediately after FLRWSolver is called > in the ET show the constraints are satisfied essentially to roundoff error > (i.e. the momentum constraint violation has magnitude ~ 1.e-15), but when I > take the 3D dump of the initial time slice and run this through my analysis > code (which uses the same routines as I use to set up initial data), I see > the momentum constraint is violated at the ~ 1e-7 level. > > I thought this might be my post processing code, so I added a second call to > check the constraints using my routines after the full initial process is > finished. The first call which immediately follows my FLRWSolver routine is > placed in group HydroBase_Initial, and I added another call in > CCTK_POSTINITIAL which gives the same result, however, if I instead schedule > this call in POSTPOSTINITIAL I see the momentum constraint violation is > identical to what I see when post processing the initial dump, at ~ 1.e-7. I > can see the specific terms which are causing this difference are the terms > which use finite-difference derivatives (the curvature terms in the momentum > constraint), while all others are identical. > > So my question is the following: is there any way that the metric and > curvature variables could suffer a loss of precision between the POSTINITIAL > and POSTPOSTINITIAL phases of the run? Especially, a loss in precision which > is then translated to the 3D dumps. > > The momentum constraint violation I have is satisfactory, but I am trying to > pinpoint why this jump happens to make sure it’s not a bug in my separate > code somewhere (also to explain why the constraints aren’t satisfied to > roundoff level when they should be, by construction of my initial data :) ). > > Any help is much appreciated! > Best wishes, > Hayley > > ---- > > Hayley Macpherson | NASA Einstein Fellow > > Email: [email protected] > Pronouns: she/her/hers > > Office: ERC 479 > Kavli Institute for Cosmological Physics > Eckhardt Research Center > 5640 South Ellis Avenue > Chicago, IL, 60637 > > _______________________________________________ > Users mailing list > [email protected] > http://lists.einsteintoolkit.org/mailman/listinfo/users > > > > > -- > Erik Schnetter <[email protected]> > http://www.perimeterinstitute.ca/personal/eschnetter/ > > -- Erik Schnetter <[email protected]> http://www.perimeterinstitute.ca/personal/eschnetter/ _______________________________________________ Users mailing list [email protected] http://lists.einsteintoolkit.org/mailman/listinfo/users
