Hi Wolfram,

On Tue, Aug 8, 2017 at 10:11 AM, Wolfram Schmidt
<wolfram.schmidt@uni-hamburg.de> wrote:
> Hi Nathan, hi Prateek,
>
> before I look into the frontend and try to modify it, I wonder:
>
> 1. Is possible to define units for a new field in the Enzo output (there are
> units conversion factors for certain quantities in Enzo, but I don't now if
> this somehow enters the data dumps)?

If you add them, it at leased *used* to be the case that yt would
parse them.  I think this is no longer the case, however.

>
> 2. Could I "teach" yt to assume certain units for a field which it
> interprets as dimensionless on the fly, i.e. in a python session? If so,
> how?

You can do this in a couple ways; if it's in the output file on disk,
you can do it by appending to the object EnzoFieldInfo (which is
accessible through the yt.frontends.enzo.api module)'s attribute
_known_fluid_fields.  This takes a tuple.  Do this before you
instantiate the dataset, and it should work.

(This is clumsy, and something that's being worked on sometime in the
medium term.)

>
> Cheers,
>
> wolfram
>
>
> On 05.08.2017 20:14, yt-users-request@lists.spacepope.org wrote:
>>
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>> Today's Topics:
>>
>>     1. Re: problem with units when defining difference of
>>        velocity-like fields (Prateek Gupta)
>>
>>
>> ----------------------------------------------------------------------
>>
>> Message: 1
>> Date: Sat, 5 Aug 2017 04:48:11 +0530
>> From: Prateek Gupta <prateekgidolia@gmail.com>
>> To: Discussion of the yt analysis package
>>         <yt-users@lists.spacepope.org>
>> Subject: Re: [yt-users] problem with units when defining difference of
>>         velocity-like fields
>> Message-ID:
>>
>> <CAJcogBoDbsRseqcbuEVdKP0Z9p9LvhO_TkjHYeOgykTbQt0Phg@mail.gmail.com>
>> Content-Type: text/plain; charset="utf-8"
>>
>> I think just applying units to AveMomtX field while defining will work if
>> you derived it in yt. May be while defining a new field (AveMomtX), it
>> doesn't get units of momentum, it seems as "dimensionless" quantity.
>> But if you are taking AveMomtX as a new field from Enzo itself, after
>> modification of Enzo code, then yt required to know about this that
>> this(AveMomtX) is also the field in Enzo.
>>
>>
>> Best Regards
>> -Prateek Gupta
>>
>> On Thu, Aug 3, 2017 at 6:07 PM, Nathan Goldbaum <nathan12343@gmail.com>
>> wrote:
>>
>>> What are the units of AveMomtX right now? It seems it's getting read in
>>> as
>>> dimensionless, probably because it's not a field yt knows about. To fix
>>> this error you will need to apply units to the AveMomtX field in your
>>> field
>>> definition. If you've modified the Enzo code to add this new field to
>>> your
>>> Enzo outputs, you could also modify yt's Enzo frontend to "teach" yt
>>> about
>>> the new field you've added. See yt/frontends/enzo/fields.py, specifically
>>> the known_other_fields tuple.
>>>
>>> On Thu, Aug 3, 2017 at 3:28 AM Wolfram Schmidt <
>>> wolfram.schmidt@uni-hamburg.de> wrote:
>>>
>>>> Dear all,
>>>>
>>>> I encountered a problem when defining a the fluctuation of the velocity
>>>> with respect to a smoothed velocity as derived field in yt:
>>>>
>>>> def _fluc_velocity_x(field, data):
>>>>       return data["x-velocity"] - data["AveMomtX"]/data["density"]
>>>>
>>>> ds.add_field("fluc_velocity_x", function=_fluc_velocity_x,
>>>> force_override=True)
>>>>
>>>> Here x-velocity and density are standard baryon fields defined in Enzo
>>>> and AveMomtX is an additional baryon field for smoothed momentum.
>>>>
>>>> When I load a data dump and execute the above definition, I get the
>>>> following error:
>>>>
>>>> /lrz/sys/tools/python/2.7_anaconda_nompi/lib/python2.7/
>>>> site-packages/yt/units/yt_array.py
>>>> in sanitize_units_add(this_object, other_object, op_string)
>>>>       126     if isinstance(ret, YTArray):
>>>>       127         if not inp.units.same_dimensions_as(ret.units):
>>>> --> 128             raise YTUnitOperationError(op_string, inp.units,
>>>> ret.units)
>>>>       129         ret = ret.in_units(inp.units)
>>>>       130     # If the other object is not a YTArray, the only valid
>>>> case
>>>> is adding
>>>>
>>>> YTUnitOperationError: The subtraction operator for YTArrays with units
>>>> (code_velocity) and (code_length**3/code_mass) is not well defined.
>>>>
>>>> After looking into
>>>>
>>>> http://yt-project.org/doc/analyzing/units/fields_and_unit_conversion.html
>>>> ,
>>>> I tried
>>>>
>>>> ds.add_field("fluc_velocity_x", units="cm/s", function=_fluc_velocity_x,
>>>> force_override=True)
>>>>
>>>> but this results in the same error as before. Apparently, the problem is
>>>> that yt does not recognize that momentum divided by density is a
>>>> velocity and therefore cannot match the units to the first term
>>>> (x-velocity).
>>>>
>>>> There are unit conversion functions such as .in_cgs(), but they are not
>>>> applicable to the data objects in the above definition.
>>>>
>>>> So can anyone tell me how to fix this?
>>>>
>>>> Cheers,
>>>>
>>>> Wolfram
>>>>
>>>>
>>>>
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>
>
> --
> Dr. Wolfram Schmidt
> Head of IT, Hamburg Observatory
> Phone +49 (0)40 42838 8584
> http://www.hs.uni-hamburg.de
>
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