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.

• Time-dependent ideal hydrodynamics: VH-1
• 3D spherical coordinates
• Isothermal equation of state
• simulation in rotating reference frame of binary system
• input parameters: sound speed, mass ratio, initial density profile
• periodic boundary conditions in phi, outflow in radius and theta
• typical grid of 200 radial zones by 120 azimuthal zones
• radial span of grid is typically 20 (e.g., .02 to .4)
• numerical code CONSERVES ANGULAR MOMENTUM
• radial mass flux is monitored in order to compute and effective alpha
• grid is wiggled slightly in radial direction to suppress instabilities

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.