Perhaps through spiral waves?

  Tidally-induced spiral waves may efficiently
  transport angular momentum out of the disk
  so that gas can fall into the black hole.




We use time-dependent hydrodynamical simulations of an isothermal accretion disk to measure the effects of binary tidal forces on transport within an accretion disk. Our initial 3D simulation was run for half a million timesteps on a grid of 200 x 120 x 120 zones, requiring 100 hours on 120 processors using the IBM SP.

Parallelization was achieved by breaking up the 3D disk model into 2D planes, with each plane computed on a different processor. Evolution in the direction around the disk is computed locally on individual processors after the data is transposed across processors with a single MPI_ALLTOALL call.