[yt-users] yt-users Digest, Vol 114, Issue 2
Nathan Goldbaum
nathan12343 at gmail.com
Tue Aug 8 08:28:07 PDT 2017
On Tue, Aug 8, 2017 at 10:21 AM, Matthew Turk <matthewturk at gmail.com> wrote:
> Hi Wolfram,
>
> On Tue, Aug 8, 2017 at 10:11 AM, Wolfram Schmidt
> <wolfram.schmidt at 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.)
>
You can also define an alias field:
def my_ave_momentum_x(field, data):
return data.ds.arr(data['enzo', 'AveMomtX'],
'code_mass/code_length**2*code_time')
ds.add_field('average_momentum_x', function=my_ave_momentum_x,
units= 'code_mass/code_length**2*code_time')
-Nathan
> >
> > Cheers,
> >
> > wolfram
> >
> >
> > On 05.08.2017 20:14, yt-users-request at 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 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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> >>>>
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> >> ------------------------------
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> >> End of yt-users Digest, Vol 114, Issue 2
> >> ****************************************
> >
> >
> > --
> > Dr. Wolfram Schmidt
> > Head of IT, Hamburg Observatory
> > Phone +49 (0)40 42838 8584
> > http://www.hs.uni-hamburg.de
> >
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