[yt-users] Angular momentum sign error?

[Elizabeth Tasker]

Hi everyone,

I think there is a small error in the angular momentum calculation.

Below, L is calculated as:

L = velocity x radius

def _SpecificAngularMomentumX(field, data):
    xv, yv, zv = obtain_velocities(data)
    rv = obtain_rvec(data)
    return yv*rv[2,:] - zv*rv[1,:]
def _SpecificAngularMomentumY(field, data):
    xv, yv, zv = obtain_velocities(data)
    rv = obtain_rvec(data)
    return -(xv*rv[2,:] - zv*rv[0,:])
def _SpecificAngularMomentumZ(field, data):
    xv, yv, zv = obtain_velocities(data)
    rv = obtain_rvec(data)
    return xv*rv[1,:] - yv*rv[0,:]


but it should be:

L = radius x velocity

This leads to a sign error (and gives me a whole load of retrograde GMCs!)


def _SpecificAngularMomentumX(field, data):
    xv, yv, zv = obtain_velocities(data)
    rv = obtain_rvec(data)
    return  zv*rv[1,:] -  yv*rv[2,:]
def _SpecificAngularMomentumY(field, data):
    xv, yv, zv = obtain_velocities(data)
    rv = obtain_rvec(data)
    return -(zv*rv[0,:] - xv*rv[2,:])
def _SpecificAngularMomentumZ(field, data):
    xv, yv, zv = obtain_velocities(data)
    rv = obtain_rvec(data)
    return yv*rv[0,:] - xv*rv[1,:]


Does that look right?


Elizabeth




------------------------------------------------------
Elizabeth TASKER
Assistant Professor

Physics Department, Faculty of Science
Hokkaido University

Sapporo, Japan

tasker at astro1.sci.hokudai.ac.jp
http://astro3.sci.hokudai.ac.jp/~tasker/
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