[Users] issue with Outflow after BHB merger

[Federico Cattorini]

Roland,
 thank you for your precious suggestions.

I managed to solve the issue. After setting AHFinderDirect verbose level to
"algorithm debug" I found out that some points of the horizon surface fell
on an excised region:

AHFinderDirect::AHFinderDirect_find_horizons
>   in thorn AHFinderDirect, file
> /marconi/home/userexternal/fcattori/EinsteinToolkit/ET_2018_09/Cactus/arrangements/EinsteinAnalysis/AHFinderDirect/src/gr/expansion.cc:949:
>   -> interpolate_geometry():
>         one or more points on the trial horizon surface point
>         is/are in an excised region (or too close to the excision boundary)
>

Then I added to the .par file the string

AHFinderDirect::mask_is_noshrink                         = "false"
>

and now everything is working properly: the surface is valid (sf_valid[2] =
1 from t_merger onwards) and outflow correctly computes the flow across it.

Yours,
Federico

Il giorno lun 24 feb 2020 alle ore 16:20 Roland Haas <rhaas at illinois.edu>
ha scritto:

> Hello Federico,
>
> > I already track the SphericalSurface::sf_valid variable (line 570 of the
> > parfile). The sf_valid[2] ascii yields a -1 for each timestep, even after
> > t_merger = 1835.33. Yet, the other sf scalar outputs for the [2] surface
> > yield real numbers after t_merger (e.g., sf_max_radius[2] yields r =
> > 0.934217920021859 which is constant from the 13th iteration after the
> > merger onwards, and is *-nan* before t_merger).
> A value of "-1" means that (see the code snippet I had provided) that
> the horizon finder tried to look for a horizon and did not find it in
> that iteration.
>
> > As far as I can tell AHFinderDirect finds the 3rd horizon after the
> merger.
> > In my scalar output folder I have a BH_diagnostic.ah3.gp file which
> yields
> > real values starting from t_merger.
> If you see "sensible" values in the other fields but sf_valid is -1
> then this would mean that the horizon finder found the horizon at least
> once but is not currently finding it. Thus data on the surface if not
> guaranteed to actually be a good representation of the horizon since
> the "found it" could have been very long in the past.
>
> If you run eg
>
> git grep sf_max_radius
>
> in repos/einsteinanalysis/AHFinderDirect/src then you can see that
> sf_max_radius is only accessed in one place, namely in
> driver/BH_diagnostics.cc:
>
> --8<--
> 681 // only try to copy AH info if we've found AHs at this time level
> 682 if (! AH_data.search_flag) {
> 683   sf_valid[surface_number] = 0;
> 684   return;
> 685 }
> 686
> 687 // did we actually *find* this horizon?
> 688 if (! AH_data.found_flag) {
> 689   sf_valid[surface_number] = -1;
> 690   return;
> 691 }
> 692
> 693 sf_active       [surface_number] = 1;
> 694 sf_valid        [surface_number] = 1;
> 695 sf_origin_x     [surface_number] = this->origin_x;
> 696 sf_origin_y     [surface_number] = this->origin_y;
> 697 sf_origin_z     [surface_number] = this->origin_z;
> 698 sf_mean_radius  [surface_number] = mean_radius;
> 699 sf_min_radius   [surface_number] = min_radius;
> 700 sf_max_radius   [surface_number] = max_radius;
> --8<--
>
> and you can see that sf_max_radius is left at its old value if sf_valid
> is 0 (not trying to find) or -1 (not found).
>
> Thus if the horizon was ever found but is no longer then there will be
> sane values in sf_XXX but sf_valid will be -1.
>
> If you would like Outflow to continue using the (now invalid) horizon
> for a while until after it became invalid (for how long is up to you to
> decide) then you will have to add some grid scalar
> "horizon_last_valid_at" or so to Outflow (it must be checkpointed) and
> track when you last saw a "valid" horizon.
>
> Yours,
> Roland
>
> --
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> and signing email messages. Get my PGP key from http://pgp.mit.edu .
>
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