The images below represent an accretion disk around a black hole as it would appear to a distant observer. Color indicates the frequency shift of observed photons across the face of the disk, assuming that the sources are in orbit around the black hole. A model turbulent Schwarzschild disk is shown here.
Turbulence is required to help disk material lose angular momentum so it can accrete onto the hole.
In doing so it may generate the powerful radiation which we observe in quasars and active galaxies. A geometrically thin, non-turbulent disk around an extreme Kerr black hole. Same as above, except with turbulence. Note that the disk is turbulent all the way down to the inner edge, and that the overall flow is approximately circular. In the bright regions, velocity shear shifts the frequency of the surface material enough so that it cannot reduce the flux by line scattering.