[Yt-dev] spin parameter

[Matthew Turk]

Okay, after some tests, it turns out that I think I was doing this all
correctly.  So I apologize for the noise.

On my current run, enzo_anyl gives:

spin gas: 0.0251921
for the same radius, yt gives: 0.0251090534601

This is not a huge deal, I think.  I'm not sure *why* it doesn't give
the same result past four decimal places, but I am not sure that it is
a meaningful difference.

I will put in the DM spin parameter later today...

-Matt

On Sat, Jan 24, 2009 at 9:20 AM, Matthew Turk <matthewturk at gmail.com> wrote:
> Is anybody here an expert on the way enzo_anyl calculates spin
> parameter?  I was hoping to convince myself that:
>
> a) I understand how it does it in enzo_anyl
> b) That is being replicated in DerivedQuantities.py
>
> I have gone back and forth on this with Brian and Britton, but that
> was almost a year ago.  I think it needs another look.  Here is how yt
> works right now:
>
>    am = data["SpecificAngularMomentum"]*data["CellMassMsun"]
>    j_mag = am.sum(axis=1)
>    m_enc = data["CellMassMsun"].sum() + data["ParticleMassMsun"].sum()
>    e_term_pre = na.sum(data["CellMassMsun"]*data["VelocityMagnitude"]**2.0)
>    weight=data["CellMassMsun"].sum()
>
> so we get the sum of Lx, Ly, Lz, the total mass, the total kinetic
> energy in the baryons, and the total *baryon* mass.
>
> then during the combination step:
>
>    W = weight.sum()
>    M = m_enc.sum()
>    J = na.sqrt(((j_mag.sum(axis=0))**2.0).sum())/W
>    E = na.sqrt(e_term_pre.sum()/W)
>    G = 6.67e-8 # cm^3 g^-1 s^-2
>    spin = J * E / (M*1.989e33*G)
>
> What this does it combine all the weights from the individual grid or
> processors to get the total baryon mass in the entire region, the
> entire *enclosed mass* (which includes the particles), and then the
> magnitude of the angular momentum vector for all the enclosed baryons,
> which gets divided by the enclosed *baryon mass* to get the average
> specific angular momentum for the region.  E is then the total kinetic
> energy divided by the total enclosed mass, which gives a
> characteristic baryon velocity.  Finally, we take the average specific
> angular momentum, multiply that by the characteristic velocity, and
> divide by the total enclosed (baryon+particle) mass.
>
> Does this make sense?  Should the characteristic velocity and angular
> momentum include the particles?  Or does this not matter?
>
> -Matt
>