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