Roland
This is currently no implemented. There are currently two choices for calculating "upwind" derivatives (i.e. the shift-related spatial part of the Lie derivatives):
If[splitUpwindDerivsKranc, Upwind[dir_, var_, idx_] := dir PDua[var,idx] + Abs[dir] PDus[var,idx], Upwind[dir_, var_, idx_] := dir PDu[var,idx]];
By setting
Upwind[var_,dir_,idx_] = PD[var,idx];
you can change these "upwind" derivatives to be identical to regular derivatives. This happens at the level of Mathematica, before the equations are passed to Kranc, so that you will need one less ghost zone.
I would be happy to add a respective parameter to the Kranc script. For performance reasons this can't be a run-time parameter, though.
Note that this is always possible with the BSSN equations -- this is not a question of stability. We are using upwind derivatives since Peter Diener (?) found out that these lead to more accurate binary black hole evolutions, compared to centred differences.
-erik
On Sun, Apr 10, 2016 at 3:57 PM, Roland Haas rhaas@aei.mpg.de wrote:
Hello all,
just to be sure (I read McLachlan_BSSN.m but want to be sure): it is currently not possible to run a simulation using ML using shift advection beta^i\partial_i but without upwinding the advection derivatives, is it? Such an option is useful for simulations without black holes where one can potentially safe a ghost zone by using only centred stencils.
Yours, Roland
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