<html>#2288: Lean's boundary condition setting for BSSN constraints only fills in a single ghostzone
<table style='border-spacing: 1ex 0pt; '>
<tr><td style='text-align:right'> Reporter:</td><td>Zach Etienne</td></tr>
<tr><td style='text-align:right'> Status:</td><td>new</td></tr>
<tr><td style='text-align:right'>Milestone:</td><td></td></tr>
<tr><td style='text-align:right'> Version:</td><td></td></tr>
<tr><td style='text-align:right'> Type:</td><td>enhancement</td></tr>
<tr><td style='text-align:right'> Priority:</td><td>minor</td></tr>
<tr><td style='text-align:right'>Component:</td><td>EinsteinToolkit thorn</td></tr>
</table>
<p>In developing BaikalETK (a NRPy+-based BSSN thorn), I set up a schedule.ccl file that follows almost exactly the same pattern as Lean.</p>
<p>I noticed that when BaikalETK evaluated the constraints without initializing the variables to zero (as is done in Lean), I would get uninitialized on the boundaries **after ApplyBCs was called**. This confused me since BaikalETK, like Lean, calls for "flat" boundary conditions on the constraints. Sure enough when I performed the same run with Lean, most boundary points were filled with zeros (unlike BaikalETK, the constraints in Lean are initialized to zero). I spoke to Roland (Haas) about this, and he was also confused until he noticed that when BaikalETK/Lean calls Boundary_SelectVarForBC/Boundary_SelectGroupForBC <em>a la</em>:</p>
<p>Boundary_SelectVarForBC(cctkGH, CCTK_ALL_FACES, +1, -1, "BaikalETK::somegridfunction", "flat") ,</p>
<p>the "+1" indicates that only one ghost zone point should be filled in. You might then wonder what would happen if we just replaced that "+1" with "cctk_nghostzones[0]". Well, since this function in Lean & BaikalETK is called in "LEVEL" mode, "cctk_nghostzones[0]" does not exist (actually it compiles and gives a garbage value). </p>
<p>Roland's brilliant solution: call the function in "Local" mode instead. This applies "flat" boundary conditions on the constraints to *all* ghostzones. It's currently implemented in BaikalETK (here's the schedule.ccl: <a data-is-external-link="true" href="https://nbviewer.jupyter.org/github/zachetienne/nrpytutorial/blob/master/Tutorial-BaikalETK.ipynb#scheduleccl" rel="nofollow">https://nbviewer.jupyter.org/github/zachetienne/nrpytutorial/blob/master/Tutorial-BaikalETK.ipynb#scheduleccl</a>) and seems to work great.</p>
<p>--<br/>
Ticket URL: <a href='https://bitbucket.org/einsteintoolkit/tickets/issues/2288/leans-boundary-condition-setting-for-bssn'>https://bitbucket.org/einsteintoolkit/tickets/issues/2288/leans-boundary-condition-setting-for-bssn</a></p>
</html>