Intrinsic and extrinsic vortex nucleation mechanisms in the flow
  We propose very general vortex nucleation mechanisms analogous to a hydrodynamic instability and calculate associated critical velocity in agreement with experiments.   The creation of vortices via extrinsic mechanism is driven by a formation of the surface vorticity sheet created by the flow, which reaches a critical size. Such a sheet screens an attraction of a half-vortex ring to the wall, the barrier for the vortex nucleation disappears and the vortex nucleation is started. In the intrinsic mechanism the creation of a big vortex ring, which transforms into the vortex, is driven by a fluctuative generation of small vortex rings 
F.V. Kusmartsev (NORDITA, Copenhagen, Denmark and Department of   Physics, Loughboro' University Loughboro', Leicestershire, UK)