  Generation of the large-scale coherent vortical structurs in homogeneous
shear flow couples dynamical processes of energy and enstrophy production. In
the large rate of strain limit, the simple estimates of the contributions to
the energy and enstrophy equations result in a dynamical system, describing
experimentally and numerically observed self-sustained non-linear oscillations
of energy and enstrophy. It is shown that the period of these oscilaltions is
independent upon the box size and the energy and enstrophy fluctuations are
strongly correlated.
