  The origin of hydrodynamic turbulence, and in particular of an anomalously
enhanced angular momentum transport, in accretion disks is still an unsolved
problem. This is especially important for cold disk systems which are
practically neutral in charge and therefore turbulence can not be of
magnetohydrodynamic origin. While the flow must exhibit some instability and
then turbulence in support of the transfer of mass inward and angular momentum
outward, according to the linear perturbation theory, in absence of
magnetohydrodynamic effects, it should always be stable. We demonstrate that
the three-dimensional secondary disturbance to the primarily perturbed disk,
consisting of elliptical vortices, gives significantly large hydrodynamic
growth in such a system and hence may suggest a transition to an ultimately
turbulent state. This result is essentially applicable to accretion disks
around quiescent cataclysmic variables, in proto-planetary and star-forming
disks, the outer region of disks in active galactic nuclei, where the gas is
significantly cold and thus the magnetic Reynolds number is smaller than 10^4.
