  We study the hydrodynamical evolution of massive accretion disks around black
holes, formed when a neutron star is disrupted by a black hole in a binary
system. Initial conditions are taken from 3D calculations of coalescing
binaries. Assuming azimuthal symmetry, we follow the time dependence of the
disk structure for 0.2 seconds. We use an ideal gas e.o.s., and assume that all
the dissipated energy is radiated away. The disks evolve due to viscous
stresses, modeled with an alpha law. We study the disk structure, and the
strong meridional circulations that are established and persist throughout our
calculations. These consist of strong outflows along the equatorial plane that
reverse direction close to the surface of the disk and converge on the
accretor. In the context of GRBs, we estimate the energy released from the
system in neutrinos and through magnetic-dominated mechanisms, and find it can
be as high as 10^52 erg and 10^51 erg respectively, during an estimated
timescale of 0.1-0.2 seconds. neutrino-anti neutrino annihilation is likely to
produce bursts from only an impulsive energy input (the annihilation luminosity
scales as t^(-5/2)) and so would be unable to account for a large fraction of
bursts with complicated light curves. However a gas mass ~0.1-0.25 Msun
survives in the orbiting debris, enabling strong magnetic fields (~10^16 Gauss)
to be anchored in the dense matter long enough to power short GRBs. We also
investigate the continuous energy injection that arises as the black hole
slowly swallows the rest of the disk and discuss its consequences on the GRB
afterglow emission.
