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Hi Jason,<br>
<br>
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.<br>
<br>
For the different runtime parameters -- <br>
<br>
1. I've tried cfl = 0.5 and 0.8, but nothing lower yet. I'll check
to see if that works.<br>
<br>
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.<br>
<br>
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.<br>
<br>
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?<br>
<br>
Thanks,<br>
Rukmani<br>
<br>
<div class="moz-cite-prefix">On 02/18/2016 09:38 AM, Jason Galyardt
wrote:<br>
</div>
<blockquote
cite="mid:CABdF9ufrgWEkf3KVdQRffZHDaNGgbXStkBVPn0=i_SoB=VX4gQ@mail.gmail.com"
type="cite">
<div dir="ltr">
<div>
<div>
<div>
<div>Hi Rukmani,<br>
<br>
</div>
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:<br>
<br>
#~~~~<br>
</div>
<div># Refine on the magnetic variables:<br>
</div>
<div>refine_var_1 = "dens"<br>
refine_var_2 = "magp"<br>
</div>
<div># -OR-<br>
</div>
<div># refine_var_2 = "magx"<br>
</div>
<div># refine_var_3 = "magy"<br>
</div>
<div># refine_var_4 = "magz"<br>
</div>
<div># prefer higher refinement, according to magp (default
refine_cutoff_X = 0.8)<br>
refine_cutoff_2 = 0.7<br>
# refine_cutoff_3 = 0.7<br>
# refine_cutoff_4 = 0.7<br>
<br>
</div>
<div># Lower CFL: between 0.25 and 0.5<br>
</div>
<div>cfl = 0.5<br>
<br>
</div>
<div># Use second order MUSCL-Hancock reconstruction scheme<br>
</div>
<div>order = 2<br>
</div>
<div><br>
# I've mostly used the "hybrid" slope limiter, but
occasionally I've found the "minmod" useful in
particularly difficult situations <br>
</div>
<div>slopeLimiter = "hybrid"<br>
<br>
# use flattening (dissipative) (originally for PPM)<br>
use_flattening = .true. <br>
<br>
# Use high order algorithm for E-field construction<br>
E_modification = .true.<br>
<br>
# Update magnetic energy using staggered B-fields<br>
energyFix = .true.<br>
<br>
# Prolongation method (injecton_prol, balsara_prol) --
Using Balsara's method is particularly critical, in my
experience.<br>
prolMethod = "BALSARA_PROL"<br>
<br>
</div>
<div># For the Riemann solver, I use HLLD for MHD runs, and
HLLC for pure hydro runs.<br>
</div>
<div>RiemannSolver = "HLLD"<br>
</div>
<div>#~~~~<br>
</div>
<br>
<div>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.<br>
</div>
<br>
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....<br>
<br>
</div>
Regards,<br>
</div>
Jason<br>
<br>
</div>
<div class="gmail_extra"><br>
<div class="gmail_quote">On Wed, Feb 17, 2016 at 9:22 PM,
Rukmani Vijayaraghavan <span dir="ltr"><<a
moz-do-not-send="true" href="mailto:rukmani@virginia.edu"
target="_blank"><a class="moz-txt-link-abbreviated" href="mailto:rukmani@virginia.edu">rukmani@virginia.edu</a></a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0
.8ex;border-left:1px #ccc solid;padding-left:1ex">Hi
everyone,<br>
<br>
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?<br>
<br>
Thanks,<br>
Rukmani<span class="HOEnZb"><font color="#888888"><br>
<br>
-- <br>
Rukmani Vijayaraghavan<br>
NSF Astronomy & Astrophysics Postdoctoral Fellow<br>
University of Virginia<br>
<a moz-do-not-send="true"
href="mailto:rukmani@virginia.edu" target="_blank">rukmani@virginia.edu</a><br>
<br>
</font></span></blockquote>
</div>
<br>
</div>
</blockquote>
<br>
<pre class="moz-signature" cols="72">--
Rukmani Vijayaraghavan
NSF Astronomy & Astrophysics Postdoctoral Fellow
University of Virginia
<a class="moz-txt-link-abbreviated" href="mailto:rukmani@virginia.edu">rukmani@virginia.edu</a></pre>
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