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

Hi Wolfram, On Tue, Aug 8, 2017 at 10:11 AM, Wolfram Schmidt 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)

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Message: 1 Date: Sat, 5 Aug 2017 04:48:11 +0530 From: Prateek Gupta To: Discussion of the yt analysis package Subject: Re: [yt-users] problem with units when defining difference of velocity-like fields Message-ID:

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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 wrote:

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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:

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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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