[FLASH-USERS] dt not positive in MHD simulations

[Slavin, Jonathan]

Hi all,

I've been doing MHD simulations involving shocks in 2D (cylindrical
symmetry) and have been occasionally running into cases where the runs
aborts with:
 dtCheck=  0.000000000000000E+000
 DRIVER_ABORT: [Hydro]: Computed dt is not positive! Aborting!

I believe that this is occurring for cells where the plasma beta, i.e.
Pthermal/Pmagnetic, is very small.  These cells are not near any
boundaries, so that doesn't seem to be the problem.  I do see that the
dt_hydro is dropping quickly just before, and drops below the dtmin that I
set.  So it's likely that the proximate cause is the violation of the
courant condition in the sense that the timestep is too long given the
signal speed.  What I really need to know then is how to prevent the very
low density with high magnetic field condition from occurring which leads
to the very high alfven speed that leads to this problem.

Note also that the simulations that I'm doing include cooling, which
decreases the thermal pressure.  Some parameters that I've set, partially
based on information in the Flash User's Guide are:
killdivb = .true.
flux_correct = .true.
E_modification = .true.
energyFix = .true.
shockLowerCFL = .true.
RiemannSolver = "HYBRID"
prolMethod = "BALSARA_PROL"

I'm wondering if anyone else has run into problems like this with MHD runs
and if anyone has suggestions for getting around this.  There are many
possible parameter settings and it could be that I just need to find the
best ones for my problem.

Regards,
Jon

-- 
________________________________________________________
Jonathan D. Slavin                 Harvard-Smithsonian CfA
jslavin at cfa.harvard.edu       60 Garden Street, MS 83
phone: (617) 496-7981       Cambridge, MA 02138-1516
cell: (781) 363-0035             USA
________________________________________________________
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