[FLASH-USERS] Problems with strong shocks

[Jeremy S Ritter]

Hello Flash Users,

I have been using FLASH since version 3 to run simulations of supernova
blastwaves in cosmological simulations. Since updating to version 4 I have
had a lot of stability problems with the unsplit solver and strong shocks.
The setup is similar to a Sedov example, with a spherical blastwave
inserted into a realistic cosmological environment. We zoom in to high
resolution to insert the blastwaves with radii of ~1 pc and 16-32 grid
blocks across the diameter. The background density/temperature/pressure in
the vicinity of the blastwaves is effectively constant when zoomed in to
this scale. We refine a large volume around the blastwave so that the
entirety of it is kept at the same refinement level while expanding.

The blastwaves will evolve as they should for many years, doubling or more
in radius, and then suddenly within 1 step the solution will break down.
This happens in the free expansion phase, before the reverse shock. The
timestep dt will drop from ~1 year to 10^-7 seconds, tracer particles in
the blastwave will be flung outside of the simulation box, gas density in
some cells will drop by 20 orders of magnitude, etc.

I have tried both the HLLC and the hybrid HLLC solver with shock detection
and shock lower CFL. By lowering the CFL even further, to 0.1, I was able
to push the simulation further but eventually it breaks down too. I never
had any similar issues in FLASH3. I was wondering if anybody has dealt with
anything similar in FLASH4?

Cheers,
-Jeremy
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://flash.rochester.edu/pipermail/flash-users/attachments/20180218/0d6a97e3/attachment.htm>