Intermittency Of Coherent Structures An Approach Using Statistical Theories Of Isotropic Turbulence

This paper contended that there is no real contradiction between the predictions of the statistical theories of turbulence and the existence of coherent structures observed in various turbulent flows. The dynamics of these flows are characterized by a competition between the organizing action of external forces or instability mechanisms which tend to create the coherent structures, and an explosive growth of small scale three-dimensionality which tends to destroy them. The latter mechanism is analogous with the inverse cascade of error in the statistical theory of the predictability of two-dimensional turbulence. It follows that the coherent structures are intermittently destroyed by three-dimensional turbulence, then reform under the action of the instability. Three-dimensional turbulence renormalizes the viscosity acting on the large scales into an eddy-viscosity proportional to delta-where delta and U are, respectively, the characteristic scale and velocity of the coherent structures. This theory is developed taking the turbulent free shear layer as a test flow, but its extension to rotating turbulence is also envisaged.