[yt-users] YT analysis questions: cylindrical profile and RMS velocity

Britton Smith brittonsmith at gmail.com
Mon Feb 6 04:39:22 PST 2012


Hi Junhwan,

For your second question, you can calculate the mean velocity within that
sphere and use that to set the bulk velocity for your disk object.  You can
do something like:
sphere = pf.h.sphere(...)
bulk_velocity = sphere.quantities['BulkVelocity']()

disk = pf.h.disk(...)
disk.set_field_parameter('bulk_velocity', bulk_velocity)

If you look at the source for the field VelocityMagnitude, it subtracts off
this bulk velocity, so I think that would do what you need.
The source for that is YT_DEST/yt/data_objects/universal_fields.py

Britton

On Sat, Feb 4, 2012 at 1:52 PM, Nathan Goldbaum <goldbaum at ucolick.org>wrote:

> Hi Junhwan,
>
> You will need to define a cylindrical radius field.  Here's how I've done
> it in the past (copied from some old messages on the newsgroup from last
> September):
>
> def _CylinderRadius(field, data):
>   center = data.get_field_parameter("center")
>   coords = na.array([data['x'] - center[0],
>                      data['y'] - center[1],
>                      data['z'] - center[2]])
>   J = na.zeros(coords.shape)
>   if len(coords.shape) == 4:
>       J[0,:,:,:] = -1
>   else:
>       J[0,:] = -1
>   JCrossR = na.cross(J,coords,axis=0)
>   return na.sqrt(na.sum(na.square(JCrossR),0))
> def _ConvertCylinderRadiusCGS(data):
>   return data.convert("cm")
> add_field("CylinderRadius", function=_CylinderRadius,
>         validators=[ValidateParameter("center")],
>         convert_function = _ConvertCylinderRadiusCGS, units=r"\rm{cm}")
>
> Note that you need to set the field parameter 'center' before you try to
> use this field.
>
> I'm not sure if there is a good way to answer your second question.  It's
> a good bet Kitsuk and Wise did not use yt as it did not exist yet ;)
>
> -Nathan
>
> On Feb 4, 2012, at 10:46 AM, Jun-Hwan Choi wrote:
>
> > Hi YT users,
> >
> > I have two questions on the YT analysis:
> >
> > First, I would like to make profiles (mass, angular momentum) in the
> cylindrical coordinate.
> > In other world, the profiles depend on r_xy (x-y is the cylinder plane).
> > I define disk object and compute Profile as follow:
> > > disk = pf.h.disk(center, [0, 0, -1], (100., 'pc'), (1., 'pc'))
> > > profile = BinnedProfile1D(disk, 50, 'Radius', smallest_bin,
> largest_bin, lazy_reader=True)
> > ,but I find that Radius is for x,y, and z distance.
> > Can I get the cylindrical profile, if I simply make derived field for
> RadiusXY and replace Radius?
> > And does there any RadiusXY kind of field exist?
> >
> > Second, I would like to compute the RMS velocity in my disk.
> > However, I would like to compute RMS velocity according to local mean
> velocity (e.g. mean velocity inside 0.1 pc sphere center on a given
> position), instead of compute according to global mean velocity.
> > Is there a routine (or way) to compute the local mean velocity in YT and
> compute RMS of it?
> > I found that some papers with Enzo simulation compute the RMS velocity
> (such as Kitsuk et al 2007 and Wise et al 2008 although I am not sure
> whether they use YT or not).
> >
> > Thank you in advance,
> > Junhwan Choi
> >
> > --
> > --------------------------------------------------------------
> > 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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