Radial mixing in protoplanetary accretion disks - VI. Mixing by large-scale radial flows

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 a-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 a-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.