[yt-users] yt-users Digest, Vol 114, Issue 2

Tue Aug 8 08:11:31 PDT 2017

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)?

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?

Cheers,

wolfram

On 05.08.2017 20:14, yt-users-request at lists.spacepope.org wrote:
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> Today's Topics:
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>     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 at gmail.com>
> To: Discussion of the yt analysis package
> 	<yt-users at lists.spacepope.org>
> Subject: Re: [yt-users] problem with units when defining difference of
> 	velocity-like fields
> Message-ID:
> 	<CAJcogBoDbsRseqcbuEVdKP0Z9p9LvhO_TkjHYeOgykTbQt0Phg at 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 at 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 at 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