A model for rapid stochastic distortions of small-scale turbulence
Résumé
We present a model describing evolution of the small-scale Navier-Stokes turbulence due to its stochastic distortions by much larger turbulent scales. This study is motivated by numerical findings that such interactions of separated scales play an important role in turbulence intermittency. We introduce description of turbulence in terms of the moments of the k-space quantities using a method previously developed for the kinematic dynamo problem. Working with the k-space moments allows to introduce new useful measures of intermittency such as the mean polarization and the spectral flatness. Our study of the small-scale 2D turbulence shows that the Fourier moments take their Gaussian values in the energy cascade range whereas the enstrophy cascade is intermittent. In 3D, we show that the statistics of turbulence wavepackets deviates from gaussianity toward dominance of the plane polarizations. Such turbulence is formed by ellipsoids in the k-space centered at its origin and having one large, one neutral and one small axes with the velocity field pointing parallel to the smallest axis.