[Users] logic of scheduling SelectBoundConds in McLachlan?

[Erik Schnetter]

On Mon, Feb 18, 2013 at 3:11 PM, Kelly, Bernard J. (GSFC-660.0)[UNIVERSITY
OF MARYLAND BALTIMORE COUNTY] <bernard.j.kelly at nasa.gov> wrote:

> [re-sent, with smaller attachment]
>
> Hi Roland, and thanks for your reply. I'm still a bit confused, I confess
> (see below) ...
>
>
> On 2/16/13 12:38 AM, "Roland Haas" <roland.haas at physics.gatech.edu> wrote:
>
> >Hello Bernard,
> >
> >> Hi. Can someone explain to me why ML_BSSN calls SelectBoundConds within
> >> MoL_PostStep? It seems like the kind of once-off routine that would
> >>happen
> >> near the start of a simulation, rather than something that has to be
> >> performed every single timestep.
> >In the "new" boundary/symmetry interface (ie using thorn Boundary and
> >Symbase) one has to Select the variables for boundaries each time before
> >ApplyBCs is scheduled. There is a routine in Boundaries that clears the
> >selection.
>
> OK; but can you tell me *why* this is? Why should we ever have to
> re-specify what kind of boundary conditions we use during a simulation,
> any more than we re-specify the evolution equations? Perhaps I don't
> really understand what "select the variables" means here.
>

Boundary conditions can be applied at different points during a simulation,
and one may want to apply different boundary conditions at different times.

The canonical example is the conformal factor: While calculating initial
conditions (e.g. with an elliptic solver), one wants to apply Robin
boundary conditions; during evolution, one wants a radiative boundary
condition. Similar arguments would hold if one were to enforce constraints
during evolution.

-erik

-- 
Erik Schnetter <schnetter at cct.lsu.edu>
http://www.perimeterinstitute.ca/personal/eschnetter/
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