[FLASH-USERS] Numerical Effects in Spherical Coordinates

[Rodrigo Fernandez]

Hi Michiel,

You could be having problems for a number of reasons. Some questions that
might help narrow things down:

1) Are you using gravity? This looks like a stellar envelope.
2) Which hydro solver are you using (split PPM, unsplit, or other)?
3) If using the split PPM solver, are you using the "hybrid_riemann" flag
to deal with the odd-even decoupling instability (due to shocks aligned
with the grid)?
4) Regarding the inner regions, how many cells per decade in radius are you
using (i.e., how well resolved is the density gradient)?
5) What is the largest Mach number of your flow? If it is too large, you
might want to use the "eint_switch" flag to evolve the internal energy
instead of total energy when the Mach number exceeds a threshold value.

Best,

Rodrigo




On Wed, Mar 22, 2017 at 7:10 AM, Michiel Bustraan <
michiel.bustraan at astro.su.se> wrote:

> Dear FLASH Users,
>
>
> I've been running 2D simulations of shocks in spherical simulations.
>
> The initial conditions involve a high density around the origin.
>
>
> I had to introduce random temperature fluctuations in the inner region to
> ensure refinement.
>
> I started with quite a high level of refinement, and the maximum level of
> refinement decreases over time (from 10 layers to 5).
>
>
> I've found that ripples appear in the inner portion of the domain, as well
> as larger numerical artifacts.
>
> I think that these might either be due to:
>
>
> - The initial temperature fluctuations I added, though I feel these would
> be too small (spatially) to cause all these issues.
>
> - Could it be that the higher pressure region in the center, combined with
> the lower radial velocity, cause the gas to bounce off the inner boundary?
>
> This might create ripples and internal shocks.
>
>
> Either way, I'm not certain how to improve my results from here.
>
> If anyone has had similar problems, I would greatly appreciate some advice.
>
>
> The figures attached highlight my problem.
>
>
> Kind Regards,
>
> Michiel Bustraan
>
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