[FLASH-USERS] MHD simulations Eint=0 problem

[Seyit Hocuk]

Hi Jon,

Thanks for the quick response. I too have a sudden increase/decrease in 
temperature and internal energy in a single cell (I assume that E_int 
--> T --> P). It is possible that there is a single origin to this 
problem. Let us notify one another about any progress on this matter, 
and anyone else from the community.

Best wishes,
Seyit



On 06/02/17 19:24, Slavin, Jonathan wrote:
> ​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 
> <mailto:flash-users-request at flash.uchicago.edu>> wrote:
>
>     From: Seyit Hocuk <seyit at mpe.mpg.de <mailto:seyit at mpe.mpg.de>>
>     To: "flash-users at flash.uchicago.edu
>     <mailto:flash-users at flash.uchicago.edu>"
>     <flash-users at flash.uchicago.edu
>     <mailto: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 <tel:%2B49-8930000-3012> | Fax:
>     +49-8930000-3569 <tel:%2B49-8930000-3569>
>     Web  : www.mpe.mpg.de/~seyit <http://www.mpe.mpg.de/%7Eseyit>
>
>
>
>
>
> -- 
> ________________________________________________________
> Jonathan D. Slavin                 Harvard-Smithsonian CfA
> jslavin at cfa.harvard.edu <mailto:jslavin at cfa.harvard.edu>       60 
> Garden Street, MS 83
> phone: (617) 496-7981       Cambridge, MA 02138-1516
> cell: (781) 363-0035             USA
> ________________________________________________________
>

-- 
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

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