Gravitoturbulent viscosity

A mixing-length estimate for gravitational turbulence takes the largest unstable wavelength $\ell$ and orbital growth rate $\Omega$, giving turbulent velocity $u_{\mathrm{turb}}\sim \Omega \ell$ and kinematic viscosity $\nu \sim u_{\mathrm{turb}}\ell \sim \Omega {\ell }^2$.