[FLASH-USERS] Growing error in magnetic field when updating fluid variables in wind tunnel simulation
Rukmani Vijayaraghavan
rukmani at virginia.edu
Thu Feb 18 21:59:40 EST 2016
Hi Jason, Klaus,
This block-by-block variation is correlated with similar variation in
other fluid variables (density, pressure), and this persists even where
there is a zero velocity inflow, as well as with a uniform grid, and
with both USM and PPM (pure hydro) solvers. Modifying the gravity solver
from Multigrid to Multipole doesn't make a difference either. I'm using
the FLASH Gamma EOS unit. As far as I've seen, there is no variation in
B-field across grid cells adjacent to block / refinement boundaries,
this only happens at the inflow edge.
I also update the magnetic field face variables (MAG_FACE_VAR and/or
MAGI_FACE_VAR), with no effect. Div(B) still seems to be 0.
Any other suggestions would be great!
Thanks,
Rukmani
On 02/18/2016 01:30 PM, Jason Galyardt wrote:
> 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> 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 <mailto:rukmani at virginia.edu>
>>
>>
>
> --
> Rukmani Vijayaraghavan
> NSF Astronomy & Astrophysics Postdoctoral Fellow
> University of Virginia
> rukmani at virginia.edu <mailto:rukmani at virginia.edu>
>
>
--
Rukmani Vijayaraghavan
NSF Astronomy & Astrophysics Postdoctoral Fellow
University of Virginia
rukmani at virginia.edu
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