[FLASH-USERS] Magnetic fields at refinement boundaries in USM

[Feyisso Sado]

Dear all
please help me on FLASH 4.4 I am a beginner.

On Tue, Mar 26, 2019 at 8:07 PM John Zuhone <jzuhone at gmail.com> wrote:

> Hi Yi-Hao,
>
> This is a known problem, and you are likely seeing the same issue that you
> saw on the mailing list earlier.
>
> Sometimes it helps to use the HLLC solver instead of HLLD, though it does
> not remove this artifact entirely in my experience.
>
> There is also balsara interpolation for magnetic fields, but I have found
> that neither this or the straight injection interpolation helps either.
>
> Best,
>
> John
>
> On Mar 26, 2019, at 1:01 PM, Yi-Hao Chen <ychen at astro.wisc.edu> wrote:
>
> Dear FLASH community,
>
> I have been trying to find the cause of a refinement boundary problem when
> using USM solver in cartesian 3D simulations. The problem appears as the
> accumulation of magnetic fields at the finer edge at the refinement
> boundary. I am not sure how to reproduce the problem in a simple setting,
> but this seems to be prevalent in the simulations that I have been
> running. Usually, the anomalies dissipated after many timesteps and went
> unnoticed. However, it occasionally causes a few cells to drop to extremely
> small or even negative density and crashes the simulation. I am wondering
> if anyone has seen similar behaviors.
>
> Here is a snapshot showing the By field in x-y plane. The quiver arrows
> indicate the fluid velocity.
> <Group_L438_hdf5_plt_cnt_0339_Slice_z_magnetic_field_y.png>
>
> Some relavent parameters I used: RiemannSolver="Hybrid", order=3,
> slopelimiter="mc", CFL=0.4, energfix = .true.
>
> A few solutions that I have tried:
> 1. Lower CFL to 0.2
>
> -> does not help
>
> 2. Use E_upwind = .true.
>
> -> Enabling upwind scheme for E fields does seem to lower the frequency
> that the anomaly happens, but does not get rid of the problem completely.
>
> 3. Refine or derefine the neighboring block to make the interface not at
> the coarse-fine block boundary.
>
> -> This does prevent or dissipate the anomaly but does not seem to be a
> good general solution and I don't know how to identify the problem block on
> the fly.
>
>
> A similar problem was discussed on the mailing list in 2016, although I am
> not sure the problem is the same.
> http://flash.uchicago.edu/pipermail/flash-users/2016-May/001962.html
> Following the discussion there, I found that the interpolation method for
> face-center variables in guardcells is set by interp_mask_face[x,y,z].
> They are initialized to be 1 (linear) in amr_initialize.F90. During the
> creation of new child blocks, they are temporarily set to 0 (if
> prolMethod="injection_prol" as default) in Simulation_customizeProlong.F90,
> and are reverted to the old value after the prolongation.
>
> I have two questions here:
> 1. Why does FLASH use two different interpolation methods during (a)
> prolongation, or the creation of child blocks, and (b) guardcell filling
> from coarse to fine block? In the discussion linked above, it was mentioned
> that using different treatments was based on experience with applications.
> I would like to learn more about this if possible.
> 2. It appears to me that the direct injection (0th order) is used in
> prolongation because it simply preserves the divergence-free nature of the
> B fields. I am not sure, but I suspect linear interpolation does not always
> preserve the divergence-free fields. Thus, wouldn't it be a problem using
> linear interpolation for guardcell filling?
>
> Any suggestions or possible directions to look into are very much
> appreciated.
>
> Best Regards,
> Yi-Hao
>
>
>

-- 

*With Best Regards!!!*
*-------------------------------------------------------------------*
*Feyiso **Sado*
*  (PhD Student)*
*Addis Ababa University, Department of Physics*
*Cell Phone: +251910626090 <%2B251911089463>*
*E-mail: bsfeyiso at gmail.com <bsfeyiso at gmail.com>*
*P.O.Box: 1176*
*Ethiopia*
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