NON-REFLECTIVE BOUNDARY-CONDITIONS AND THE VISCOUS INSTABILITY IN ACCRETION DISCS

A numerical investigation is carried out to analyse the effects of the outer boundary conditions on the viscous instability of accretion dis cs. A one-dimensional time-dependent calculation of geometrically thin accretion discs is used. When the outer boundary is reflective, oscil lations are trapped between the outer boundary and the outer edge of t he inner evanescent region. When the outer boundary is nonreflective, the oscillations decay on a viscous time-scale. An analytical treatmen t of numerically non-reflective boundary conditions is carried out for a polytropic flow and for a gas including radiation.