[yt-users] Velocity dispersion profile

John Wise jwise at physics.gatech.edu
Wed Feb 29 13:16:14 PST 2012


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!
>>>> _______________________________________________
>>>>
>>>> yt-users mailing list
>>>> yt-users at lists.spacepope.org
>>>> http://lists.spacepope.org/listinfo.cgi/yt-users-spacepope.org
>>>>
>>>>
>>>> !DSPAM:10175,4f4e76d329502195409319!
>>>>
>>>>
>>>>
>>>> _______________________________________________
>>>> yt-users mailing list
>>>> yt-users at lists.spacepope.org
>>>> http://lists.spacepope.org/listinfo.cgi/yt-users-spacepope.org
>>>>
>>>>
>>>
>> _______________________________________________
>> yt-users mailing list
>> yt-users at lists.spacepope.org
>> http://lists.spacepope.org/listinfo.cgi/yt-users-spacepope.org
> _______________________________________________
> yt-users mailing list
> yt-users at lists.spacepope.org
> http://lists.spacepope.org/listinfo.cgi/yt-users-spacepope.org

-- 
John Wise
Assistant Professor of Physics
Center for Relativistic Astrophysics, Georgia Tech



More information about the yt-users mailing list