[yt-users] derived field with Grackle?

[Brian O'Shea]

>
> >> > 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.
> >
>
> Sure. You'll want to either split up the data container into chunks of N
> elements of data using an iterator, or use a spatial field as described in
> the docs and iterate over slabs.
>
> To do the former, you'll want something like:
>
> start = 0
> N = 1024
> rv = np.zeroes(data["ones "])
> while start < rv.size:
>     grackle(data["density"][start:start+N], ...)
>     start += N
>
> inside your derived field.
>

Ah, excellent.  I'll give this a shot - thanks!

--Brian
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