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

[Jason Galyardt]

Hi Rukmani,

I used a spatially varying wind; the velocity of the wind varies along the
boundary, but it has a well-defined, time-independent form. I've also seen
problems with more realistic B-field geometries which (to my horror)
included step functions in the domain interior. I had to smooth these out
to avoid unphysical evolution in those regions.

I've also seen some modest increase in B-field magnitude for the cells
adjacent to a refinement boundary. I haven't reported the latter previously
because I haven't had time to figure out what's going on there. You might
try setting lrefine_min = lrefine_max to get uniform refinement and see
whether that helps (some of our group's simulations do this).

The block by block variation does seem strange. I would expect this kind of
variation to be correlated with variation in another variable. How do the
other variables look in the problem region?

Another idea: could this variation be tied to the equation of state? If
you're using one of the supported FLASH EOS units, you're probably fine.

Regards,
Jason


On Thu, Feb 18, 2016 at 11:07 AM, Rukmani Vijayaraghavan <
rukmani at virginia.edu> wrote:

> Hi Jason,
>
> Thanks! I'm using FLASH 4.2, I'll try using 4.3 to see if that makes a
> difference. I haven't tried refining on the magnetic variables yet.
>
> For the different runtime parameters --
>
> 1. I've tried cfl = 0.5 and 0.8, but nothing lower yet. I'll check to see
> if that works.
>
> 2. For the Riemann Solver, I've found HLLC to be a bit more dissipative
> than HLLD, and therefore marginally better at smoothing out the magnetic
> field at the edges. Ditto with second order MUSCL-Hancock over third order
> PPM.
>
> 3. All the other runtime parameters are mostly the same. I don't refine on
> the magnetic variables, but I tried higher overall lrefine_min (to make
> sure the outer edges get further refined) and it didn't help -- the same
> block-based discontinuity persists.
>
> 4. I'm using a constant wind inflow for this particular run. One thing I
> checked to see was if there was a round off error in reading my input
> variables into double precision arrays, and this tiny "seed" instability
> might grow, but it doesn't seem to be an issue. What is strange is that the
> value (and sign) of the initial instability varies block-by-block. In your
> simulations, did you use a constant wind?
>
> Thanks,
> Rukmani
>
>
> On 02/18/2016 09:38 AM, Jason Galyardt wrote:
>
> 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>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
>>
>>
>
> --
> Rukmani Vijayaraghavan
> NSF Astronomy & Astrophysics Postdoctoral Fellow
> University of Virginiarukmani at virginia.edu
>
>
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