[FLASH-USERS] Growing error in magnetic field when updating fluid variables in wind tunnel simulation
Rukmani Vijayaraghavan
rukmani at virginia.edu
Fri Feb 19 12:59:22 EST 2016
Hi Dongwook,
I'm using the HLLC Riemann solver. I've attached a recent log file from
a short run.
Thanks!
Best,
Rukmani
On 02/18/2016 10:31 PM, Dongwook Lee wrote:
> Dear Rukmani,
>
> What kind of Riemann solver are you using?
> Can you send me a log file or flash.par?
>
> Thanks,
> Dongwook
>
> On Feb 18, 2016, at 6:59 PM, Rukmani Vijayaraghavan
> <rukmani at virginia.edu <mailto:rukmani at virginia.edu>> wrote:
>
>> 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
--
Rukmani Vijayaraghavan
NSF Astronomy & Astrophysics Postdoctoral Fellow
University of Virginia
rukmani at virginia.edu
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