[FLASH-USERS] Some questions about magnetic resistivity

[Yigeng Tian]

Dear FLASH users,

I'm working on a modified MHD laser-slab simulation. I tried to add
magnetic fields (~ 15T) and Biermann Battery effects (source version) for
the laser-slab simulation, it worked well. But when I turn on the magnetic
resistivity (SpitzerHighZ resistivity), the simulation is soon killed by a
"Negative 3T internal energy" error.

I noticed that FLASH manual (18.1.2) said "In general, implicit schemes are
numerically stable for any time step, so the runtime parameter
dt_diff_factor could be set to a large value to ignore the diffusion time.
However, there are cases in which HYPRE will have trouble converging on a
solution, and decreasing dt_diff_factor less than 1.0 may resolve the
issue."

So I changed the dt_diff_factor to 0.8, which makes the simulation run but
gives a pretty low diffusion time step (~ e-16 to e-15 s). Our past
simulation time step without magnetic resistivity can reach ~ e-13 to e-12
s, and the super time stepping method doesn't help a lot for overcoming
the small diffusion time step.

Besides, the FLASH manual (18.1) said "Flux-Based Diffusion Solvers are
used for magnetic resistivity", but 18.1.1 said "MHD implementations,
currently implement some diffusive effects in their own flux-based way that
does not use the DiffuseFluxBased unit."  I'm also confused which diffusion
solver should be used for magnetic resistivity, is the Flux-Based version
(explicit) or implicit version?

If you have any suggestions or any examples about how to set up the
magnetic resistivity correctly, I would appreciate it. Thanks.

Best regards,
Yigeng Tian
Department of Physics and Astronomy
University of Delaware
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