How do turbulence and viscosity cause material in an accretion disk to spiral inward?

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six quasar host galaxies — Quasar Six quasar host galaxies, as observed by the Hubble Space Telescope.
Shown are apparently normal, solitary galaxies (left), colliding galaxies (centre), and merging galaxies (right).

In an accretion disk, material orbiting a central object—such as the black hole at the heart of a quasar—must lose energy and angular momentum to spiral inward. Without a mechanism to dissipate this momentum, the material would simply remain in stable circular orbits.1

While standard viscosity is generally too low to drive this process, turbulence within the disk acts as an effective source of friction. This turbulence is thought to arise from the disk's rotation amplifying existing magnetic fields. This process serves two critical functions: it provides the effective viscosity necessary to drive the inward drift of matter and simultaneously transports excess angular momentum outward, allowing the material to move closer to the central accretor.1