FARGO-2D1D is an extension of the standard version of FARGO, in which the 2D grid is surrounded by a simplified 1D grid, made of elementary rings non azimuthally resolved (see figure below). It aims at studying and taking into account the global evolution of the disk in the simulations of planet-disk interactions. This is particularly important for type II migration.
Far enough from the planets, the disk can be considered as axisymmetric, and thus can be modelled by a 1D grid. The computation in a 1D grid requires a negligible time with respect to the 2D grid. The 1D grid can be extended over all the physical extension of the disk for a negligible computing cost.
The grids communicate at their interfaces (the bold circles on the figure), in order to ensure a smooth evolution of the disk over all its radial extent. Mass and angular momentum conservation are considered with care.
Therefore, the planet-disk interactions can be accurately computed in the 2D grid, being aware of the global disk evolution outside of the 2D grid (accretion rate on the star, for instance). Symmetrically, the global evolution of the disk can be affected by the presence of a planet (opening of cavity for instance).
In fact, the 1D grid provides the 2D grid with time-evolving, realistic boundary conditions, and provides the user with the global disk profile evolution.