摘要

An experimental study involving a space-filling square fractal grid is presented. Using time-resolved stereoscopic Particle Image Velocimetry and Taylor's hypothesis, all components of the velocity gradient tensor are determined in three regions along the centreline of this spatially developing flow: where turbulence is produced, at the turbulence intensity peak and in the decay region. Some of the acclaimed universal aspects of small-scale turbulence are shown to be the result of an evolution process over a considerable streamwise extent of the flow. The spatial evolution of the second (Q) and third (R) velocity gradient invariants is presented and the related characteristic "universal tear-drop" shape is only fully established at the most downstream location. A similar evolution is seen in the alignments between vorticity and the eigenvectors of the strain rate tensor where, in the production region, vortex stretching is only slightly favoured over compression.