[FLASH-USERS] refinement problems with FLASH 4.2

[Klaus Weide]

On Thu, 22 May 2014, Thomas Peters wrote:

> Dear Kevin, Christoph and John,
> 
> the attached slice shows that the density field is smooth across refinement
> boundaries. Since the velocity field jumps across the boundary, the momentum
> must jump as well.
> 
> I have also attached slices at exactly z = 0, as well as at 10 and 20 times
> the finest grid size off the midplane. The effect is still clearly visible,
> there are just fewer jumps in refinement in that region.
> 
> I would not call this "noise" since in these slices, the z-velocity is of the
> same size as the x- and y-velocities were in the previous slices.

I just finished looking (with Sean Couch) at a different 3D problem, but 
with the same kind of symmetry - a central plane z = 0 where z-velocities
should vanish. We saw the same kinds of behavior in z-slices of velz taken
at close to z=0 with Visit.

We came to the conclusion that the apparent jumps in velz are exactly 
what one would expect, given the following assumptions:
  (1) slices are just showing cell values, without interpolation.
  (2) requesting a slice at exactly z=0.0 actually gives you the same 
      slice as either z=0.0+epsilon or 0.0-epsilon (since z=0.0 is a cell
      boundary).
  (3) Cell values represent volume averages.
      (The cell value of velz should actually be interpreted, strictly
      speaking, as <mom_z> / <dens>, where <*> means volume average, but
      I don't think it matters here that this is different from <velz>.)

Basically we think that Christoph's explanation is correct and FLASH 4.2
is behaving as it should, for a finite volume representation of data on
an AMR grid.

I would expect slices to look substantially different when using a tool 
that actually does a (correct) linear interpolation of the cell values.

> Not sure if this helps, but I have not seen this effect in any of the collapse
> simulations I have run with FLASH 2.5.

It would be really surprising if these kinds of apparent jumps were
absent from FLASH2.5 checkpoint or plot files (at the same resolution and 
otherwise comparable, viewed with the same kind of tool). Can you confirm 
that that is the case?

Klaus