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/
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