[FLASH-USERS] Problems with strong shocks

[Dongwook Lee]

Dear Jeremy,

Do you see this instability issue only with FLASH4, but NOT with FLASH3?
More specifically, I wonder what happens if you run the exact same problem
with FLASH3. Have you tried this type of FLASH3 and FLASH4 comparison tests?

Cheers,
Dongwook

On Sun, Feb 18, 2018 at 10:41 AM, Jeremy S Ritter <jritter at mail.utexas.edu>
wrote:

> 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
>
>


-- 

=========================================
Dongwook Lee, Ph.D., Assistant Professor
Applied Mathematics and Statistics
University of California, Santa Cruz
Baskin Engineering, Room 353C
1156 High Street, Santa Cruz, CA 95064
https://users.soe.ucsc.edu/~dongwook/
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