A SMALL-SCALE TURBULENCE MODEL

A previously derived analytical model for the small-scale structure of turbulence is reformulated in such a way that the energy spectrum may be computed. The model is an ensemble of two-dimensional (2-D) vortices with internal spiral structure, each stretched by an axially symmetric strain flow. Stretching and differential rotation produce an energy cascade to smaller scales in which the stretching represents the effect of instabilities and the spiral structure is the source of dissipation at the end of the cascade. The energy spectrum of the resulting flow may be expressed as a time integration involving only the enstrophy spectrum of the time evolving 2-D cross-section flow, which may be obtained numerically. Examples are given in which a k-5/3 spectrum is obtained by this method. The k-5/3 inertial range spectrum is shown to be related to the existence of a self-similar enstrophy preserving range in the 2-D enstrophy spectrum. The results are found to be insensitive to time dependence of the strain rate, including even intermittent on-or-off strains.