[ET Trac] [Einstein Toolkit] #898: use characteristic speed +/-c for Bx in HLLEM

[Einstein Toolkit]

#898: use characteristic speed +/-c for Bx in HLLEM
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 Reporter:  rhaas                  |       Owner:     
     Type:  enhancement            |      Status:  new
 Priority:  major                  |   Milestone:     
Component:  EinsteinToolkit thorn  |     Version:     
 Keywords:  GRHydro                |  
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 the attached patch (against trunk) adds a new parameter
 decouple_normal_Bfield which defaults to yes (and changes the default
 behaviour) to GRHydro's MHD code when divergence cleaning is used. Reasons
 for this are given eg in Mignonoe et al. arXiv:1001.2832v2 section 3.1.
 This patch allows us to run the rotor test without having to resort to the
 LLF flux formula (which also uses +/- characteristic speed but for all
 variables) and does not seem to cause any problems in the TOV tests that
 Philipp ran. Bruno says it helps with the cylindrical blast as well.

 Ok to include in the release (given that it is tunable at runtime) even
 though we are in the "no major change" phase?

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Ticket URL: <https://trac.einsteintoolkit.org/ticket/898>
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