[FLASH-USERS] noisiness in run with high ekin/eint

[yueh wang]

Jon,
I remember I have emailed you.
I did some HD, MHD, and RHD work on supernova remnants, though not using FLASH.
You have to check the ghost zones, implementing the velocity gradiant.  As pressure is trivial, the problem should arise from the velocity gradiant of the ghostzone not setting correctly, if not from PPM. (Density gradient is important, too).
You need to set up the condition like a free-expanding ejecta. Don't just use the boundary condition options from the code; modify the BC.

Chih-Yueh Wang
________________________________
From: flash-users-bounces at flash.uchicago.edu <flash-users-bounces at flash.uchicago.edu> on behalf of Slavin, Jonathan <jslavin at cfa.harvard.edu>
Sent: Thursday, March 29, 2018 9:29 PM
To: flash-users
Subject: [FLASH-USERS] noisiness in run with high ekin/eint

Hi,

I've been doing runs in 1D spherical symmetry of supernova remnants starting with fast cool ejecta.  This is one of those situations in which the kinetic energy far exceeds the thermal energy (at least in the early stages).

I know about eintSwitch which is supposed to help with numerical issues that arise in such situations, but I've been finding that setting it lower actually results in less noisiness, though the noisiness that remains is, if nothing else, unsightly.  What happens is that the pressure develops fairly large fluctuations that seem to be purely numerical since there doesn't seem to be any other source for them.  The pressure is small in those regions, relative to the kinetic energy, but still the noise seems to affect the density.

So I'm wondering if others have found parameter settings or approaches that work well in this sort of situation.  If so, I'd like to hear about them.  I should mention that I've been using split PPM.  I did try to use unsplit hydro but it failed with the pressure dropping to the smallp value very quickly.  I may try to investigate that some more.

Regards,
Jon

--
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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
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