[yt-users] derived field with Grackle?

Brian O'Shea bwoshea at gmail.com
Mon Oct 20 17:58:26 PDT 2014


Hi Matt,

> I am attempting to create a derived field from a cosmological simulation
> > dataset using Grackle.  Specifically, I have a simulation that was run
> with
> > Grackle, but which unfortunately did not output the cooling time.  Ergo,
> I'd
> > like to create that as a derived field.  Does anybody have an example of
> a
> > Grackle-based derived field in yt that I can use as a model?  I would
> note
> > that I looked at the "run_from_yt.py" example in the Grackle distro, but
> it
> > isn't quite what I need.
>
> You'll probably need to use bits of that, and actually call Grackle on
> your data object.  If you can set up Grackle outside of the derived
> field you can minimize the overhead cost.  But, calling grackle just
> like it would be otherwise, but inside the derived field definition,
> is going to be the way to go I think.
>

As a very high-level answer, that makes sense.  Implementation seems to be
a bit harder - I've been trying to reconcile the way that grackle is called
from its python wrapper with the way that yt generates derived fields.  In
grid_to_grackle(), you feed in individual grids and each grid's fields
(including other derived fields) are accessed by stepping through a grid in
slabs, with a great deal of information about the grid itself being
required.  When I look at derived fields in yt, everything works by calling
data[], possibly using adjacent slabs/cells in various ways, but never
referring to individual grids.  Could you point me toward an example of a
derived field where grids are manipulated more directly?  I don't really
have any idea about how to actually *do* what you've suggested, at least
given the grackle python interface as it stands in grackle 2.

Thank you!

--Brian
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