[FLASH-USERS] MHD simulations Eint=0 problem

[Slavin, Jonathan]

​Hi Seyit,

I have had to put that problem aside for a while, though I hope to get back
to it soon.  I have not solved the problem that leads to negative or zero
timesteps.  What I found was that what leads to the problem is that
suddenly in a single grid zone/pixel the thermal pressure jumps to a very
high value, which thus leads to a very small timestep.  I plan to probe the
problem further as soon as I can.

Regards,
Jon​

On Mon, Feb 6, 2017 at 1:00 PM, <flash-users-request at flash.uchicago.edu>
wrote:

> From: Seyit Hocuk <seyit at mpe.mpg.de>
> To: "flash-users at flash.uchicago.edu" <flash-users at flash.uchicago.edu>
> Cc:
> Date: Mon, 6 Feb 2017 10:56:22 +0100
> Subject: [FLASH-USERS] MHD simulations Eint=0 problem
> Hi community,
>
> Has anyone else encountered this problem before?
>
> Whenever I run MHD simulations with the USM solver (Flash4.4), I
> occasionally get Eint=0. This seems to occur only when I put a B-field > 0
> (I often simulate mass-to-flux ratio of 2). The problem is bigger with
> higher magnetic field strength. I have played around with the "eintswitch"
> parameter, thereby reducing reducing the issue it as much as I can
> (eintswitch =< 1e-4), but the problem still persists. A high eintswitch, on
> the other hand, thereby reverting to an independent internal energy
> calculation was giving me crazy values. It is as if the magnetic energy is
> not considered for the internal energy calculations when derived from the
> total energy (Eint = E_tot - Ekin - Emag), or something along those lines,
> but this is just guessing.
>
> Optimizing the parameters helped a little. See parameter choices below. My
> initial guess was that the solver has a hard time solving, because the
> problem is stiff on occasion. Playing with these options and relaxing the
> solver still did not completely remove this issue.
>
> I initially thought that this happens after new cells are created,
> however, the problem does still occur when lrefine_min=lrefine_max.
>
> The problem also seems to persist if:
> ForceHydroLimit = .True. (but the B-field is not set to zero; in the
> tested case it is 10.0e-6)
> So, forcing hydrolimit did not help! This while when the B = 0
> (+Forcehydrolimit=true), the Eint does not become 0, but very low instead
> (which is a different issue?). While with B=0, I also get a different
> refinement. Therefore, I am not entirely sure what forcehydrolimit does
> actually. Clearly, there is still impact by the B-field.
>
> Another funny thing is enableMaskedGCFill. I usually have
> enableMaskedGCFill = .True.
> but the module that made use of this is turned off. Setting this to true
> or false gives me somewhat different kind of errors. With .False. I also
> get negative values, i.e., WARNING after gc filling: min.
> unk(EINT_VAR)=-1487770975.081508
>
> The end results is sometimes (especially if I don't constrain by "small"
> variables, i.e., make the "small" values really small):
> DRIVER_ABORT: Computed new time step smaller than dr_dtMinContinue!
> similar to a problem mentioned recently "dt not positive in MHD
> simulations" by Jon Slavin. Was any solution fouond for that?
>
> What does help is:
> cfl = crazy low, but normally lowering cfl only delays the issue.
>
> What doesn't help is:
> Turning off gravity
> Turning off particles
> Turning off refinement
> Turning of all my modules (Heating/Cooling/Chemistry/Colmndensities...)
> Changing boundaries from periodic/isolated to outflow
> Viscosity
> And many other things
>
> With kind regards,
> Seyit
>
>
> Normal conditions (but I tested them all):
> mfieldstrength = 5.0e-6
> killdivb = .True.
> E_upwind = .True.
> energyFix = .True.
> RiemannSolver = "HYBRID"
> slopeLimiter = "HYBRID"
> order = 3
> use_steepening = .False.
> use_flattening = .True.
> use_avisc = .True.
> shockDetect = .True.
> cfl = 0.6
>
> Libraries/Compilers:
> hdf5-serial/1.8.15
> ifort: intel/15.0
> impi/5.0.3
> x86_64 x86_64 x86_64 GNU/Linux
>
>
> --
> Dr. Seyit Hocuk
> Post-doctoral researcher
> Center for Astrochemical Studies (CAS)
> Max-Planck-Institute for Extraterrestrial physics (MPE)
> Giessenbachstrasse 1, 85748 Garching bei Munchen
> Phone: +49-8930000-3012 | Fax: +49-8930000-3569
> Web  : www.mpe.mpg.de/~seyit
>
>



-- 
________________________________________________________
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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