A numerical investigation is carried out to analyse the effects of the outer boundary conditions on the viscous instability of accretion discs. A one-dimensional time-dependent calculation of geometrically thin accretion discs is used. When the outer boundary is reflective, oscillations are trapped between the outer boundary and the outer edge of the inner evanescent region. When the outer boundary is non-reflective, the oscillations decay on a viscous time-scale. An analytical treatment of numerically non-reflective boundary conditions is carried out for a polytropic flow and for a gas including radiation.
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Patrick Godon (1996) studied this question.