[yt-users] Default Units for new Enzo Fields

[Nathan Goldbaum]

On Tue, Feb 16, 2016 at 4:38 PM, Andrew James Emerick <
emerick at astro.columbia.edu> wrote:

> Hi all,
>
> I wasn't sure if this should go in the yt-users list or Enzo-dev, as I'm
> not sure where the solution lies.
>
> I've defined some new fields in Enzo that have units of density that yt is
> reading as having dimensionless units.
>
> Are the units for fields defined and outputted somewhere in Enzo (to be
> read by yt)? Or does yt assume units depending on the name of the field?
>

The conversions to CGS for the fields are written out by Enzo, but not the
dimensions of the units. Taking an example parameter file (IsolatedGalaxy
from yt-project.org/data), you'll see the following lines in the parameter
file:

DataLabel[0]              = Density
DataUnits[0]              = none
#DataCGSConversionFactor[0] = 2.76112e-30

>From which yt is able to infer the conversion from code density units to
CGS.  yt is *not* able to read in that Density has dimensions of
code_mass/code_length**3, that is hard-coded into yt in the enzo frontend:

https://bitbucket.org/yt_analysis/yt/src/ee29ca6d81c95d55073d4cf9db89e9486c40156d/yt/frontends/enzo/fields.py?at=yt&fileviewer=file-view-default#fields.py-75



>
> If the former, where do I tell the code that my field has density units
> when its written to file? I see "DataUnits" throughout the code, but I'm
> not sure if this is relevant or how to use it.
>
> If this is a yt-side thing, is there an easy way to just have yt read in
> my new fields with the correct units without me manually changing them once
> I've loaded a dataset?
>
>
I think the easiest way to accomplish this without modifying yt is to
define new derived fields that apply the correct units to the fields you've
added to Enzo. You could include these fields in a plugin file (
http://yt-project.org/doc/reference/configuration.html#the-plugin-file) or
just at the top of your analysis scripts. You'll need to strip the
incorrect dimensionless units from the data read in by yt from the Enzo
output files and "by hand" attach the correct units. I'll happily write up
an example if you need a hand with this.

In addition, I think it's likely we would accept a pull request adding
support for your new fields to yt's Enzo frontend - you'll need to modify
the yt/frontends/enzo/fields.py file to include new entries for your fields
in the known_other_fields list for mesh fields and known_particle_fields
list for particle fields. The only reason why we wouldn't do so is if
adding support for your fields would somehow inhibit adding support for
fields in the public distribution of Enzo in the future, which I think is
unlikely.

-Nathan


>
>
> Andrew
>
> --
> Graduate Student
> Columbia University
> Department of Astronomy
>
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