Inconsistent definition of B in evolution thorns

[Samuel Cupp]

GRHydro and IllinoisGRMHD use different definitions for several quantities. This is mostly fine, as IllinoisGRMHD converts vel to get its desired velocity variable. However, the magnetic variable B also differs between these two thorns by a factor of sqrt(4pi). This means that GRHydro couldn't use, for example, initial data from the Seed_Magnetic_Fields thorns without rescaling them. Without an expectation for what B is, diagnostic thorns that need this variable will produce incorrect results if used with anything except for the evolution thorn they were designed for. Presumably, all the WVUThorns are using IlliniosGRMHD’s B, and GRHydro_InitData uses GRHydro’s B. Anything that isn't explicitly tied to an evolution thorn would be ambiguous. Is there a consensus on what B in HydroBase is expected to be?

Especially with the transition to CarpetX, having a single definition for B would likely help keep thorns mutually compatible.

[Roland Haas]

Documented in:

‌

https://bitbucket.org/einsteintoolkit/einsteinbase/src/db5777d1f921254d3e8d489c5b2e2b1c336113ef/HydroBase/interface.ccl#lines-15

and

https://bitbucket.org/einsteintoolkit/einsteinbase/src/master/HydroBase/doc/documentation.tex#lines-152

which hopefully is determining all factors for sqrt(4pi) uniquely. This has been a bit of a headache since the very beginning of GRHydro.

[Erik Schnetter]

I am surprised that there is no consensus about HydroBase; this should be documented in that thorn. Given historic precedence, I would go with the definition used in GRHydro.

HydroBaseX should then use the same definition.