[FLASH-USERS] Growing error in magnetic field when updating fluid variables in wind tunnel simulation

[Jason Galyardt]

Hi Rukmani,

I've had some similar issues with MHD runs. You didn't mention which
version of FLASH you're using, but I've found the latest (v4.3) to be a bit
more stable than v4.2 or v2.5. As for runtime parameters, found the
following combination to be helpful:

#~~~~
# Refine on the magnetic variables:
refine_var_1 = "dens"
refine_var_2    = "magp"
# -OR-
# refine_var_2 = "magx"
# refine_var_3 = "magy"
# refine_var_4 = "magz"
# prefer higher refinement, according to magp (default refine_cutoff_X =
0.8)
refine_cutoff_2 = 0.7
# refine_cutoff_3 = 0.7
# refine_cutoff_4 = 0.7

# Lower CFL: between 0.25 and 0.5
cfl = 0.5

# Use second order MUSCL-Hancock reconstruction scheme
order = 2

# I've mostly used the "hybrid" slope limiter, but occasionally I've found
the "minmod" useful in particularly difficult situations
slopeLimiter    = "hybrid"

# use flattening (dissipative) (originally for PPM)
use_flattening    = .true.

# Use high order algorithm for E-field construction
E_modification  = .true.

# Update magnetic energy using staggered B-fields
energyFix       = .true.

# Prolongation method (injecton_prol, balsara_prol) -- Using Balsara's
method is particularly critical, in my experience.
prolMethod      = "BALSARA_PROL"

# For the Riemann solver, I use HLLD for MHD runs, and HLLC for pure hydro
runs.
RiemannSolver    = "HLLD"
#~~~~

What sort of inflow conditions have you implemented? Small non-linearities
in the inflow can grow into large unphysical features over time (I've seen
this happen in my own simulations). So, it's worth checking your boundary
condition code for undesirable features. In any case, I hope this helps.

Sean: is the E_upwind option available for the unsplit MHD solver in FLASH
4.3? My recollection is that it caused some problems in previous
versions....

Regards,
Jason


On Wed, Feb 17, 2016 at 9:22 PM, Rukmani Vijayaraghavan <
rukmani at virginia.edu> wrote:

> Hi everyone,
>
> I've come across an error when updating fluid variables at the inflow edge
> of a wind tunnel simulation. I'm running a simulation of a galaxy (with
> active dark matter particles, gas, and passive particles) in a box, whose
> fluid is initialized to be identical to the incoming wind (with vx, vy, vz
> = 600 km/s, 0, 0). There is a small error (on the order of 1%) when
> updating grid cells near the inflow boundary (with both USM and PPM
> solvers), and this error is spatially correlated with  block boundaries.
> While this error itself is tolerable as far as the density and pressure go,
> this has bad consequences for the magnetic field which grows as the wind
> propagates through the box (see attached figure, xl_boundary). This figure
> shows slices of Bx at two timesteps (annotated with block boundaries and
> magnetic field vectors). The dynamic range of Bx in this image has been
> reduced to highlight these discontinuities. At the timesteps shown in the
> attached image, the fluctuations in Bx are ~1%, but grow with time up to
> order unity. I've tried a variety of Riemann solvers (HLLC, HLLD, Roe,
> Hybrid), slope limiters (mc, minmod, etc.), interpolation orders,
> prolongation methods, turning on and off specific USM switches, but nothing
> seems to solve this issue so far. Has anybody else dealt with and/or
> successfully solved this issue?
>
> Thanks,
> Rukmani
>
> --
> Rukmani Vijayaraghavan
> NSF Astronomy & Astrophysics Postdoctoral Fellow
> University of Virginia
> rukmani at virginia.edu
>
>
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