Possible origin of viscosity in the Keplerian accretion disks due to
  secondary perturbation: Turbulent transport without magnetic field
  The origin of hydrodynamic turbulence in rotating shear flow is a long
standing puzzle. Resolving it is especially important in astrophysics when the
flow angular momentum profile is Keplerian which forms an accretion disk having
negligible molecular viscosity. Hence, any viscosity in such systems must be
due to turbulence, arguably governed by magnetorotational instability
especially when temperature T >~ 10^5. However, such disks around quiescent
cataclysmic variables, protoplanetary and star-forming disks, the outer regions
of disks in active galactic nuclei are practically neutral in charge because of
their low temperature, and thus expected not to be coupled with the magnetic
field appropriately to generate any transport due to the magnetorotational
instability. This flow is similar to plane Couette flow including the Coriolis
force, at least locally. What drives their turbulence and then transport, when
such flows do not exhibit any unstable mode under linear hydrodynamic
perturbation? We demonstrate that the threedimensional secondary disturbance to
the primarily perturbed flow triggering elliptical instability may generate
significant turbulent viscosity ranging 0.0001 <~ \nu_t <~ 0.1 to explain
transport in accretion flows.
