[yt-users] yt's treatment of enzo velocity data: Zeus vs PPM

Cameron Hummels chummels at gmail.com
Tue Sep 10 15:16:04 PDT 2013


Hi everyone,

This is a pretty technical question about the differences in how yt deals
with enzo velocity data, both those created using the Zeus or the PPM
backends.

As I understand it, PPM creates all fields, including velocity fields, as
cell-centered quantities.  On the other hand, Zeus creates all fields as
cell-centered quantities, except velocity fields, which are defined as the
left-face of each corresponding cell.  If I'm wrong about this, then stop
me here.

Since for the most part, yt treats PPM and Zeus datafiles identically
(although there are a few exceptions to this, which I'll talk about below),
it seems that analyzing or visualizing a velocity field from a Zeus dataset
is actually wrong, albeit only by half a cell size.  So if one makes a
slice of "x-velocity" or something like that, to get the *true* velocity
slice, everything should be shifted over to the left by one-half of one
cell width, yes?  I'm not grouching about this; I'm just curious if this is
true.

The problem I see is when one starts to deal with derived fields which rely
upon both spatial and velocity fields, such as DivV or Shear, which require
use of "dx" or "soundspeed" value for a cell (cell centered), for combining
with velocity data for a cell (edge-centered).  Yes, these are small
precision problems, but as I'm finishing up the Shear derived field, I want
to make sure it's correct as opposed to just approximate.

I guess one solution to this would be to provide a corrected version of the
velocity fields in yt (when reading in Zeus data), which trilinearly
interpolates edge-centered data so as to get cell-centered data?  Or have
people already thought about this and done it?  Or is it too small of an
effect and people just haven't worried about it?  Elizabeth, I saw you had
talked about this in the past for Zeus-based fields.

Just curious to bounce this off some more heads before I barrel through
with this interpolation, since it may break old results made from enzo zeus
data.

Cameron

-- 
Cameron Hummels
Postdoctoral Researcher
Steward Observatory
University of Arizona
http://chummels.org
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