  Thin viscous Keplerian accretion disks are considered asymptotically stable,
even though they can show significant dynamic activity on short timescales. In
this paper the dynamics of non-axisymmetric hydrodynamical disturbances of
disks are investigated analytically building upon the steady state
three-dimensional structure and evolution of axisymmetric perturbations
explored in previous work. Assuming a polytropic equation of state solutions
are found by means of an asymptotic expansion in the small parameter measuring
the ratio of the disk thickness to characteristic radius. In-depth analysis
shows that every perturbation that disturbs the radial velocity induces
significant transient growth in the (acoustic) energy of the evolving
disturbance. This effect is most evident in the density and vertical velocity.
The transient growth observed is tied to the non-separable nature of the
solutions where, in particular, pattern evolution is controlled by a similarity
variable composed of the radial coordinate and time. This leads to growing
winding perturbations that display successive radial peaks and troughs. We
argue that these transient non-axisymmetric structures may precipitate
secondary instabilities which, consequently, may be a critical element for a
new alternative picture of turbulence arousal in non-magnetized astrophysical
disks.
