Scale dependence of the coarse-grained velocity derivative tensor: Influence of large-scale shear on small-scale turbulence

We discuss the effect of large-scale anisotropy of the shear type on the small-scale structure of turbulence. Our analysis is based on numerical solutions of the Lagrangian tetrad model of Chertkov M., Pumir A. and Shraiman B.I. (1999, Physics of Fluids, 11, 2394) adapted to the model case with large-scale anisotropy. The model, formulated in terms of a set of stochastic differential equations for the coarse-grained velocity gradient and tensor of inertia of a typical shape, naturally connects Lagrangian and Eulerian parameterizations of turbulence. We use diagnostics of Chertkov et al. (1999) which allows us to analyse and interpret different correlation functions at the resolved scale in terms of the flow geometry. Our main conclusion, concerning the issue of anisotropy, is that even though overall the local isotropy is restored with the scale decrease, the particular pace of the isotropy restoration depends very much on the object analysed. We found that the vorticity-dominated objects, such as enstrophy, tend to restore the isotropy much faster than their strain-dominated counterparts, e.g. energy flux and strain variance.