Intermittency, The Entrainment Problem.

The mechanism of entrainment in turbulent wake flows has been analytically and experimentally examined. It is argued that the development of the wake is dominated by the vortices shed from the recirculation region. These vortices cause the rapid diffusion of the mean flow and are responsible for the appearance of waves on the turbulent interface. An explanation for the persistence of the vortices is found in terms of a strain energy stored in the anisotropy of the vortices on the mean flow is described by relating the size and strength of the vortices to an additional eddy viscosity.