[yt-users] Velocity dispersion profile

Britton Smith brittonsmith at gmail.com
Wed Feb 29 13:16:03 PST 2012


I wouldn't mind working on this with you.  We might be able to do this with
teamwork.

On Wed, Feb 29, 2012 at 4:14 PM, Matthew Turk <matthewturk at gmail.com> wrote:

> 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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