[yt-users] Velocity dispersion profile

[Matthew Turk]

Yes, adding a direct stddev would be nice (and would not be too much
overhead) but I think this can be done in two passes.

Pass one: calculate the average value.  This would be a standard profile.
Pass two: set_field_parameter of the average value as a function of
whatever.  Inside your new derived field, calculate the delta between
the average and the actual field value.  Take average of this new
derived field.

It's probably a better idea to just add stddev directly.  There is an
extant algorithm for doing a one-pass stddev that is robust against
round-off errors; I would be happy to help someone out to add that to
the code.

-Matt

On Wed, Feb 29, 2012 at 4:11 PM, John ZuHone
<jzuhone at head.cfa.harvard.edu> wrote:
> I do not believe this can be done with the current code. It's something
> I'd like to see, however.
>
> I would be willing to code this up, but it will probably be weeks before I
> have time.
>
> John Z
>
> On Wed, 29 Feb 2012, Britton Smith wrote:
>
>> I think what Junhwan is asking is whether it's possible to do a profile of
>> velocity dispersion.  If I understand correctly, the problem is that this
>> would require, for every spherical shell in a radial profile, the
>> calculation of the standard deviation of the VelocityMagnitude field.  I'm
>> not sure if this is possible within the existing profile framework.  Can
>> anyone else comment on this?
>>
>> On Wed, Feb 29, 2012 at 2:25 PM, Nathan Goldbaum <goldbaum at ucolick.org>wrote:
>>
>> > Hi Junhwan,
>> >
>> > Can you paste a script and a traceback that shows the error you're seeing?
>> >  You should be able to pass the BulkVelocity to the derived field via
>> > set_field_parameters.
>> >
>> > You can paste a script from the command line with the following command:
>> >
>> > yt pastebin name_of_script.py
>> >
>> > Nathan Goldbaum
>> > Graduate Student
>> > Astronomy & Astrophysics, UCSC
>> > goldbaum at ucolick.org
>> > http://www.ucolick.org/~goldbaum
>> >
>> > On Feb 29, 2012, at 8:31 AM, Jun-Hwan Choi wrote:
>> >
>> > Hi YT users,
>> >
>> > I would like to make a (spherical and/or cylindrical) profile of the
>> > velocity dispersion.
>> > In order to do so, I need to compute the mean velocity (BulkVelocity) for
>> > a given shell (or ring), and pass this value to newly defined velocity
>> > dispersion field, and then make a profile for the velocity dispersion field.
>> > Is there a particular way in yt to handle this?
>> >
>> > Junhwan
>> >
>> > P.S.: A few weeks ago I posted a similar question and Matt gave me a
>> > following hint.
>> > @derived_field(name = "deltaLocalCircularVelocity")
>> > def delta_LocalCircularVelocity(fi
>> > eld, data):
>> >  fn, x, cv = data.get_field_parameter("local_circular_velocity")
>> >  cv0 = na.interp(data[fn].ravel(), x, cv)
>> >  cv0 = cv0.reshape(data[fn].shape)
>> >  delta_cv = cv0 - data["LocalCircularVelocity"]
>> >  return delta_cv
>> >
>> > You'll need to do something like this on the data object from which
>> > you are getting your data:
>> >
>> >  sp.set_field_parameter("local_circular_velocity",
>> >      (fn, prof[fn], prof["LocalCircularVelocity"]))
>> >
>> > However, it does not work for me and it is also complicate to understand
>> > and modify it.
>> >
>> >
>> >
>> > --
>> > --------------------------------------------------------------
>> > Jun-Hwan Choi, Ph.D.
>> > Department of Physics and Astronomy, University of Kentucky
>> > Tel: (859) 897-6737        Fax: (859) 323-2846
>> > Email: jhchoi at pa.uky.edu   URL: http://www.pa.uky.edu/~jhchoi
>> > --------------------------------------------------------------
>> >
>> >
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