MHD implementation details and discussions

Random bits and pieces

• Reuse parts of Harm3D
• Used Harm3d as a reference, but the only parts of Harm3d directly included were for the Con2PrimM routines.

Conventions / Variable names etc.

• Principally follow the Valencia formulation as out-lined in Anton et al. (2006)
• HydroBase's Bvec[1-3] GF will be identified with Valencia's B^i vector
• Bvec[] will be used both as an evolved quantity and a primitive variable
• During the Carpet workshop, we concluded it is easier to define B^i to be the undensitized magnetic field; this requires some care, since psi^6 B^i is the quantity that appears in the evolution equation, but simplifies the rest of the code
• Only use locally calculated "comoving magnetic tensor" b^a when needed
• In general, but especially for MHD, it seems worthwhile to promote sqrt(gamma)=psi^6 to an independent gridfunction
• See Josh Faber's notes here [1], or check out the code (still a work in progress!!!) here [2].
• Josh Faber will commit the routines mentioned in these notes soon.
• Scott Noble is working Con2PrimM, will hopefully commit by the end of this week (or next week at least).

Subprojects

Task	Main people	Notes
Riemann Problem	Scott Noble
Con2Prim	Christian Ott	from Harm3D
Constraint transport	Scott Noble	from Harm3D
Reconstruction	Frank Löffler
Failure Recovery Procedure	Scott Noble
Carpet Refluxing	Erik Schnetter	Finished?
EMF Restriction/Prolongation
Constraint Damping?

During the Cactus workshop, some progress was made on implementing MHD within GRHydro by reimplementing various subroutines with alternate versions for MHD and calling them out with the MHD flag in the scheduler. Basically, instead of GRHydro_filename.F90, we will have GRHydro_filenameM.F90.