  This paper deals with the problem of hydrodynamic shear turbulence in
non-magnetized Keplerian disks. We wish to draw attention to a route to
hydrodynamic turbulence which seems to be little known by the astrophysical
community, but which has been intensively discussed among fluid dynamicists
during the past decade. In this so-called `bypass' concept for the onset of
turbulence, perturbations undergo a transient growth, and they may reach an
amplitude that is sufficiently large to allow positive feedback through
nonlinear interactions. This transient growth is linear in nature, and thus it
differs in principle from the well-known nonlinear instability. We describe the
type of perturbations that according to this process are the most likely to
lead to turbulence, namely non-axisymmetric vortex mode perturbations in the
two dimensional limit. We show that the apparently inhibiting action of the
Coriolis force on the dynamics of such vortical perturbations is substantially
diminished due to the pressure perturbations, contrary to current opinion. We
stress the similarity of the turbulent processes in Keplerian disks and in
Cartesian flows and conclude that the prevalent skepticism of the astrophysical
community on the occurrence of hydrodynamic shear turbulence in such disks is
not founded.
