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
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@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 / (M1.989e33G)
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
Done with particle spin parameter. Caught some other bugs -- particle velocities not getting turned to CGS, mainly.
As a note, I think that all particle fields using velocities should carefully choose between using bulk_velocity and particle_bulk_velocity or something similarly named. That's another new quantity that should be added. Furthermore, I believe the minor differences between spin parameters in YT and in enzo_anyl is due to using actual enclosed mass versus rho_vir * 4/3 pi r_vir^3.
This raises a new point, though -- perhaps it'd be nice if we had a routine to set up bulk_velocity and whatnot. It'd be good if we did this so that you could specify the sub-radius for a smaller bulk_velocity calculation.
-Matt
On Sat, Jan 24, 2009 at 10:41 AM, Matthew Turk matthewturk@gmail.com wrote:
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@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 / (M1.989e33G)
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
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Matthew Turk