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

[Rukmani Vijayaraghavan]

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 <mailto: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

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