[yt-users] Velocity dispersion profile

Wed Feb 29 13:23:47 PST 2012

Ok, I'm in for this.  Let's try meeting on Friday on IRC for some TCB.

Britton

On Wed, Feb 29, 2012 at 4:22 PM, Matthew Turk <matthewturk at gmail.com> wrote:

> Thanks, John!
>
> And Britton & John ZH, I'd be up for working on this tomorrow or
> Friday.  Here's the paper with the one-pass stddev.
>
> http://www.cs.berkeley.edu/~mhoemmen/cs194/Tutorials/variance.pdf
>
> (Formula 5)
>
> Meet up in IRC sometime soon, and try to knock it out?  I think it
> would be best if we just always calculated this and stick it in the
> profile.  We can calculate it along with the weight.
>
> -Matt
>
> On Wed, Feb 29, 2012 at 4:16 PM, John Wise <jwise at physics.gatech.edu>
> wrote:
> > Hi all,
> >
> > I did something like this a couple of weeks ago, where I did something
> > similar to Matt's description.  I ripped the relevant part of my script
> and
> > posted it here.
> >
> > http://paste.yt-project.org/show/2201/
> >
> > John
> >
> >
> > On 02/29/2012 04:14 PM, Matthew Turk 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
> >>>>> --------------------------------------------------------------
> >>>>>
> >>>>>
> >>>>> !DSPAM:10175,4f4e76d329502195409319!
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> >
> > --
> > John Wise
> > Assistant Professor of Physics
> > Center for Relativistic Astrophysics, Georgia Tech
> >
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