Disc turbulence and viscosity.
Abstract
Three-dimensionsional simulations of hydromagnetic flows in accretion disc provide strong evidence that the turbulence in discs is driven by a magnetic instability. Some basic results of those simulations are reviewed, current shortcomings discussed, and open questions and important issues are highlighted. The main motivation behind thise simulations was imply to show tht turbulence is self-sustained. However, an important quantitative outcome has been the determination of the magnitude of the Shakura-Sunyaev viscosity parameter αSS. It is emphasized that αSS cannot be considered a constant, as it does in fact depend on a number of factors: the magnetic field strength, the height above the midplane, and the magnitude of the velocity shear - to mention just a few. Given the availability of detailed simulations, it is now possible to address specific questions, for example what are the rates of Joule and viscous heating, where is the energy deposited, what are the values of turbulent Prandtl numbers, and how efficiently does the flow disperse and mix particles? Finally, the disc simulations have significantly affected and enhanced research in dynamo theory in different fields of astrophysics, because some of the ideas (dynamo-generated turbulence) may also apply to stars and galaxies.
- Publication:
-
Theory of Black Hole Accretion Disks
- Pub Date:
- 1998
- Bibcode:
- 1998tbha.conf...61B
- Keywords:
-
- Accretion Disks: Turbulence;
- Accretion Disks: Viscosity