[yt-users] problem with units when defining difference of velocity-like fields

[Wolfram Schmidt]

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