<div dir="ltr"><div class="gmail_extra"><div class="gmail_default" style="font-family:arial,helvetica,sans-serif;font-size:small">Hi Seyit,</div><div class="gmail_default" style="font-family:arial,helvetica,sans-serif;font-size:small"><br></div><div class="gmail_default" style="font-family:arial,helvetica,sans-serif;font-size:small">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.</div><div class="gmail_default" style="font-family:arial,helvetica,sans-serif;font-size:small"><br></div><div class="gmail_default" style="font-family:arial,helvetica,sans-serif;font-size:small">Regards,</div><div class="gmail_default" style="font-family:arial,helvetica,sans-serif;font-size:small">Jon</div><br><div class="gmail_quote">On Mon, Feb 6, 2017 at 1:00 PM, <span dir="ltr"><<a href="mailto:flash-users-request@flash.uchicago.edu" target="_blank">flash-users-request@flash.uchicago.edu</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div id=":1ew" class="a3s aXjCH m15a14952389b85ad">From: Seyit Hocuk <<a href="mailto:seyit@mpe.mpg.de">seyit@mpe.mpg.de</a>><br>To: "<a href="mailto:flash-users@flash.uchicago.edu">flash-users@flash.uchicago.edu</a>" <<a href="mailto:flash-users@flash.uchicago.edu">flash-users@flash.uchicago.edu</a>><br>Cc: <br>Date: Mon, 6 Feb 2017 10:56:22 +0100<br>Subject: [FLASH-USERS] MHD simulations Eint=0 problem<br>Hi community,<br>
<br>
Has anyone else encountered this problem before?<br>
<br>
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.<br>
<br>
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.<br>
<br>
I initially thought that this happens after new cells are created, however, the problem does still occur when lrefine_min=lrefine_max.<br>
<br>
The problem also seems to persist if:<br>
ForceHydroLimit = .True. (but the B-field is not set to zero; in the tested case it is 10.0e-6)<br>
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.<br>
<br>
Another funny thing is enableMaskedGCFill. I usually have<br>
enableMaskedGCFill = .True.<br>
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)=-1487770975.0815<wbr>08<br>
<br>
The end results is sometimes (especially if I don't constrain by "small" variables, i.e., make the "small" values really small):<br>
DRIVER_ABORT: Computed new time step smaller than dr_dtMinContinue!<br>
similar to a problem mentioned recently "dt not positive in MHD simulations" by Jon Slavin. Was any solution fouond for that?<br>
<br>
What does help is:<br>
cfl = crazy low, but normally lowering cfl only delays the issue.<br>
<br>
What doesn't help is:<br>
Turning off gravity<br>
Turning off particles<br>
Turning off refinement<br>
Turning of all my modules (Heating/Cooling/Chemistry/Col<wbr>mndensities...)<br>
Changing boundaries from periodic/isolated to outflow<br>
Viscosity<br>
And many other things<br>
<br>
With kind regards,<br>
Seyit<br>
<br>
<br>
Normal conditions (but I tested them all):<br>
mfieldstrength = 5.0e-6<br>
killdivb = .True.<br>
E_upwind = .True.<br>
energyFix = .True.<br>
RiemannSolver = "HYBRID"<br>
slopeLimiter = "HYBRID"<br>
order = 3<br>
use_steepening = .False.<br>
use_flattening = .True.<br>
use_avisc = .True.<br>
shockDetect = .True.<br>
cfl = 0.6<br>
<br>
Libraries/Compilers:<br>
hdf5-serial/1.8.15<br>
ifort: intel/15.0<br>
impi/5.0.3<br>
x86_64 x86_64 x86_64 GNU/Linux<br>
<br>
<br>
-- <br>
Dr. Seyit Hocuk<br>
Post-doctoral researcher<br>
Center for Astrochemical Studies (CAS)<br>
Max-Planck-Institute for Extraterrestrial physics (MPE)<br>
Giessenbachstrasse 1, 85748 Garching bei Munchen<br>
Phone: <a href="tel:%2B49-8930000-3012" value="+4989300003012" target="_blank">+49-8930000-3012</a> | Fax: <a href="tel:%2B49-8930000-3569" value="+4989300003569" target="_blank">+49-8930000-3569</a><br>
Web : <a href="http://www.mpe.mpg.de/~seyit" rel="noreferrer" target="_blank">www.mpe.mpg.de/~seyit</a><br>
<br></div></blockquote></div><br><br><br clear="all"><div><br></div>-- <br><div class="gmail_signature" data-smartmail="gmail_signature"><div dir="ltr"><div><div dir="ltr">________________________________________________________<br>Jonathan D. Slavin Harvard-Smithsonian CfA<br><a href="mailto:jslavin@cfa.harvard.edu" target="_blank">jslavin@cfa.harvard.edu</a> 60 Garden Street, MS 83<br>phone: (617) 496-7981 Cambridge, MA 02138-1516<br>cell: (781) 363-0035 USA<br>________________________________________________________<br><br></div></div></div></div>
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