Abstract. The presence of crystalline dust materials in the outer, cold regions of protoplanetary accretion discs requires conditions for their formation, which are typical for the inner, warm regions of the disc. This suggests the existence of a mechanism that allows an efficient, outward-directed radial transport of material in accretion discs. Higher order analytical calculations, as well as numerical simulations reveal meridional flow structures in α-discs, which exhibit outflow of matter in regions near the disc midplane and may play a significant role in radial mixing in protoplanetary accretion discs. We present an analytical, isothermal model for the large-scale meridional flow pattern in an α-disc, verifying the approximations which it is based on by means of a 2-D numerical computation. The solution for the flow structure obtained is used for calculating the transport of a tracer in accretion discs in combination with diffusional mixing. The impact of radial flows on mixing of tracers is compared to the height-averaged inflow solution of the standard one-zone approximation.

We consider time-dependent models for rotating accretion flows onto black holes, where a transition takes place from an outer cooling-dominated disc to a radiatively inefficient flow in the inner region. In order to allow for a transition of this type we solve the energy equation, both, for the gas and for the radiation field, including a radiative cooling flux and a turbulent convective heat flux directed along the negative entropy gradient. The transition region is found to be highly variable, and a corresponding variation expected for the associated total luminosity. In particular, rapid oscillations of the transition radius are present on a timescale comparable with the local Keplerian rotation time. These oscillations are accompanied by a quasi-periodic modulation of the mass flux at the outer edge of the advection-dominated accretion flow (ADAF). We speculate about the relevance for the high-frequency QPO phenomenon.Comment: 6 pages, 3 figures. Revised version, added system of equations. Accepted for publication in MNRA