[FLASH-USERS] Unexpected instable shell interface in cylindrical implosion simulation

[Ryan Farber]

Hi Mathieu,

To my untrained eye, I don't notice any obvious grid artefacts, so it's not
clear to me the instability you observe is not unphysical. If you're trying
to do a simulation without magnetic fields, you might also try setting
ForceHydroLimit = .true. in your flash.par. Visualizing the pressure might
be helpful as well.

Best wishes,
--------
Ryan


On Fri, Dec 20, 2024 at 11:13 AM Mathieu Bailly-Grandvaux <
mbaillygrandvaux at ucsd.edu> wrote:

> Dear FLASH Users,
>
> I am using FLASH to simulate a cylindrical implosion for the National
> Ignition Facility (NIF). The simulations are performed in 2D r-z geometry.
>
> Initially, I utilized the 3D-to-2D ray tracing algorithm, incorporating
> the 128 beams of the experimental configuration. I observed a rapid growth
> of instabilities at the gas-shell interface and initially considered that
> this might be a (potentially physical) result of azimuthal asymmetry
> (mode-8 is particularly expected) due to the beam imprint pattern projected
> in 2D. However, the asymmetries were far more pronounced than anticipated.
>
> To rule out this effect, I simplified the simulations by switching to a 2D
> irradiation model using an azimuthally averaged "gaussian1D" function.
> Surprisingly, the implosions remained highly unstable, indicating that the
> projected azimuthal asymmetries were not the cause this time. I
> experimented with varying the number of rays, ticks, and grid resolution,
> but the instability persisted across all configurations.
>
> At this point, I am uncertain about how to resolve this issue, and would
> greatly appreciate your advice and insights.
>
> I have attached the flash.par and setup_call files for the simulation,
> which was conducted using FLASH version 4.8. Below, I include mass density
> plots ("dens" in g/cm³) at several time steps (2.5 ns, 5 ns, and 7.5 ns out
> of the 10 ns pulse duration) to illustrate the instabilities observed:
>
> [image: image.png][image: image.png][image: image.png]
>
> Additionally, I enabled the MHD capability with an axial B-field, but it
> is currently set to B = 0 T in this simulation.
>
> Thank you in advance for your time and assistance. I look forward to
> hearing your thoughts and potential suggestions.
> Best regards,
> Mathieu Bailly-Grandvaux
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>
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