  The question of a purely hydrodynamic origin of turbulence in accretion disks
is reexamined, on the basis of a large body of experimental and numerical
evidence on various subcritical (i.e., linearly stable) hydrodynamic flows.
  One of the main points of this paper is that the length scale and velocity
fluctuation amplitude which are characteristic of turbulent transport in these
flows scale like $Re_m^{-1/2}$, where $Re_m$ is the minimal Reynolds number for
the onset of fully developed turbulence. From this scaling, a simple
explanation of the dependence of $Re_m$ with relative gap width in subcritical
Couette-Taylor flows is developed. It is also argued that flows in the shearing
sheet limit should be turbulent, and that the lack of turbulence in all such
simulations performed to date is most likely due to a lack of resolution, as a
consequence of the effect of the Coriolis force on the large scale fluctuations
of turbulent flows.
  These results imply that accretion flows should be turbulent through
hydrodynamic processes. If this is the case, the Shakura-Sunyaev $\alpha$
parameter is constrained to lie in the range $10^{-3}-10^{-1}$ in accretion
disks, depending on unknown features of the mechanism which sustains
turbulence. Whether the hydrodynamic source of turbulence is more efficient
than the MHD one where present is an open question.
