= Effective viscosity defined by disk stress
{title2=$\bar\nu=\frac{\int T_{r\phi}dz}{\Sigma r\Omega\prime}$}
For cylindrical rotation independent of height, write $W_{r\phi}=\int T_{r\phi}dz$. Define the effective <kinematic viscosity> by $W_{r\phi}=\bar\nu\Sigma r\Omega'$, where $\Sigma$ is the <surface density of a disk>. For Keplerian rotation, $\bar\nu=-2W_{r\phi}/(3\Sigma\Omega)$, and the outward <viscous torque in an accretion disk> is $-2\pi r^2W_{r\phi}=3\pi\bar\nu\Sigma\sqrt{GM r}$. A <Newtonian fluid stress tensor> gives the <density-weighted viscosity of a disk>; the stress definition can also parameterize an effective turbulent or magnetic transport stress.
// Target: astrophysics.bigb
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