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EVOLUTION OF TURBULENT FLOW STRUCTURES GENERATED BY FRACTAL GRIDS

Rafael Gomes Fernandes
Turbulence, Mixing and Flow Control Group Department of Aeronautics, Imperial College London London, SW7 2AZ, United Kingdom

Bharathram Ganapathisubramani
Engineering and the Environment University of Southampton Tizard, Highfield, Southampton, SO17 1BJ, UK

John Christos Vassilicos
Turbulence, Mixing and Flow Control Group, Department of Aeronautics Imperial College London, London SW7 2AZ, United Kingdom

Abstract

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.