ON THE TURBULENT VISCOSITY PRESCRIPTION IN ACCRETION DISCS

The validity of the standard alpha viscosity prescription in turbulent accretion discs is briefly analysed. It is found that the alpha visco sity prescription is no longer valid when the rotation law of the disc is not Keplerian. The analysis is based on simple scaling arguments f or fully developed turbulences in accretion discs. An analytical treat ment is carried out for geometrically thin non-Keplerian discs in vert ical hydrostatic equilibrium. The analytical expression of the viscosi ty is compared with standard values of the viscosity in boundary layer regions and with the viscosity prescription derived from a general ca usal equation for the evolution of the stress tenser.