<html><head><meta http-equiv="content-type" content="text/html; charset=utf-8"></head><body dir="auto"><div></div><div>Dear Rukmani,</div><div><br></div><div>What kind of Riemann solver are you using?</div><div>Can you send me a log file or flash.par?</div><div><br></div><div>Thanks,</div><div>Dongwook</div><div><br>On Feb 18, 2016, at 6:59 PM, Rukmani Vijayaraghavan <<a href="mailto:rukmani@virginia.edu">rukmani@virginia.edu</a>> wrote:<br><br></div><blockquote type="cite"><div>
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Hi Jason, Klaus,<br>
<br>
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. <br>
<br>
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. <br>
<br>
Any other suggestions would be great!<br>
<br>
Thanks,<br>
Rukmani<br>
<br>
<br>
<br>
<div class="moz-cite-prefix">On 02/18/2016 01:30 PM, Jason Galyardt
wrote:<br>
</div>
<blockquote cite="mid:CABdF9uevM2PyrdYd_1Lz1Ofx11u0mTKEjCRHxboEA9aB6_YFKA@mail.gmail.com" type="cite">
<div dir="ltr">
<div>
<div>
<div>
<div>
<div>Hi Rukmani,<br>
<br>
</div>
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. <br>
<br>
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).<br>
<br>
</div>
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?<br>
<br>
</div>
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.<br>
<br>
</div>
Regards,<br>
</div>
Jason<br>
<br>
</div>
<div class="gmail_extra"><br>
<div class="gmail_quote">On Thu, Feb 18, 2016 at 11:07 AM,
Rukmani Vijayaraghavan <span dir="ltr"><<a class="moz-txt-link-abbreviated" href="mailto:rukmani@virginia.edu"></a><a class="moz-txt-link-abbreviated" href="mailto:rukmani@virginia.edu">rukmani@virginia.edu</a>></span>
wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0
.8ex;border-left:1px #ccc solid;padding-left:1ex">
<div text="#000000" bgcolor="#FFFFFF"> 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
<div>
<div class="h5"><br>
<br>
<div>On 02/18/2016 09:38 AM, Jason Galyardt wrote:<br>
</div>
<blockquote type="cite">
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<div>
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<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 class="moz-txt-link-abbreviated" href="mailto:rukmani@virginia.edu"></a><a class="moz-txt-link-abbreviated" href="mailto:rukmani@virginia.edu">rukmani@virginia.edu</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><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>
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<pre cols="72">--
Rukmani Vijayaraghavan
NSF Astronomy & Astrophysics Postdoctoral Fellow
University of Virginia
<a moz-do-not-send="true" href="mailto:rukmani@virginia.edu" target="_blank">rukmani@virginia.edu</a></pre>
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<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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