[yt-users] units

[gso at physics.ucsd.edu]

Yeah I was just confused by the comment

>         """
>         Returns the mass-weighted RMS velocity for the halo
>         particles in code units.
>         """

because I got values and thought it was in code units so multiplying by
the velocity conversion factor gave speeds exceeding c by orders of
magnitude.  So it is indeed already in CGS then.

From
G.S.

> Hi Geoffrey,
>
> Here's the code for calculating RMS:
>
>     def rms_velocity(self):
>         """
>         Returns the mass-weighted RMS velocity for the halo
>         particles in code units.
>         """
>         bv = self.bulk_velocity()
>         pm = self["ParticleMassMsun"]
>         sm = pm.sum()
>         vx = (self["particle_velocity_x"] - bv[0]) * pm/sm
>         vy = (self["particle_velocity_y"] - bv[1]) * pm/sm
>         vz = (self["particle_velocity_z"] - bv[2]) * pm/sm
>         s = vx**2. + vy**2. + vz**2.
>         ms = na.mean(s)
>         return na.sqrt(ms) * pm.size
>
> particle_velocity_[xyz] is returned in cgs (I just checked.)  Bulk
> velocity is also cgs, and ParticleMassMsun is the weighting factor, so
> it doesn't need to be in any units.  Looking at this code, it could be
> substantially more memory efficient, but as-is it should be correct
> and returning in cgs.  Why do you think it might be wrong?  Did you
> get bizarre answers?
>
> -Matt
>
> On Wed, Mar 10, 2010 at 6:32 PM,  <gso at physics.ucsd.edu> wrote:
>> When the line
>> self["particle_velocity_x"]
>> is called in HaloFinding.py, does it automatically give the number in
>> CGS
>> units already or is it in code units?  It's used in calculating the rms
>> velocity for each of the dark matter haloes, but it's subtracting
>> centers
>> and multiplying other values in code units I believe, just want to make
>> sure the calculations are done in the same units.
>>
>> From
>> G.S.
>>
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