  Turbulent viscosity in cold accretion disks is likely to be hydrodynamic in
origin. We investigate the growth of hydrodynamic perturbations in a small
region of a disk, which we model as a linear shear flow with Coriolis force,
between two parallel walls. Although there are no exponentially growing
eigenmodes in this system, because of the non-normal nature of the modes, it is
possible to have a large transient growth in the energy of certain
perturbations. For a constant angular momentum disk, the energy grows by more
than a factor of 1000 for a Reynolds number of only 1000, and so turbulence is
easily excited. For a Keplerian disk, the growth is more modest, and energy
growth by a factor of 1000 requires a Reynolds number of nearly a million.
Accretion disks have even larger Reynolds numbers than this. Therefore,
transient growth of perturbations could seed turbulence in such disks.
