The phenomenology of turbulence involves fluid motion at a broad range of length scales, where the small scales of motion are classically assumed to be universal, isotropic and independent of the large-scales. Some evidence supporting small-scale universality is given by point statistics of vorticity alignment with the directions of principal strain, and the invariants of the strain rate tensor. It suggests that universality is associated with flow properties evaluated with respect to the directions of principal strain, i.e. the eigenvectors of the strain rate tensor. The classical theory is, however, being challenged by most experimental and numerical data showing evidence of small-scale anisotropy and significant direct energy transfer between the large- and the small-scales. A revised conceptual picture of the turbulent motions in physical space is thus needed.

This talk discusses how the eigenvectors of the strain rate tensor can be used to (i) examine the universal aspects of the coherent motions, and (ii) explore small-scale anisotropy and scale interactions. The analysis of the mean flow in the strain eigenframe reveals a shear layer containing vortical motions inside. The layer appears universally in many different turbulent flows (including wall-bounded and homogenous isotropic turbulence), and captures many features of the underlying instantaneous turbulent flow field. The layer structure is found to contain both the large and the small turbulent length scales simultaneously. Moreover, the shear layer reveals the expected -5/3 power scaling of the energy spectrum in nearly all directions. This flow structure thus reconciles the -5/3 power spectrum with small-scale universality, small-scale anisotropy and direct scale interactions.