Dual-plane PIV investigation of structural features in a turbulent boundary layer

This is a post-print of a paper from 15th Australasian Fluid Mechanics Conference 2004 published by AFMC. This version is reproduced with the permission of AFMC.Simultaneous dual-plane PIV experiments were performed instreamwise-spanwise planes in the log region of a turbulentboundary layer at a moderate Reynolds number (Ret » 1100).The acquired datasets were used to resolve all 9 velocity gradientsfrom which the complete vorticity vector and other invariantquantities like 3-D swirl strength were computed. Thesederived quantities were used to analyze and interpret the structuralcharacteristics and features of the boundary layer. Resultsof the vorticity vector and the 2-D swirl strength from the twoneighbouring planes indicate the existence of hairpin shapedvortices inclined downstream along the streamwise direction.These vortices envelop low speed zones and generate Reynoldsshear stress that enhances turbulence production. Plots of full3D swirl strength indicate the existence of additional vorticalstructures in the middle of the low speed zones that may representheads of smaller eddies intersecting the measurementplane. This concept is in accordance with the hierarchy of structuresize in a hairpin packet proposed by Adrian et al.[2]. Computationof inclination angles of individual eddies using the vorticityvector suggests that most cores are inclined at 25± to thestreamwise-spanwise plane with a resulting projected eddy inclinationof 32±.13-17 December 200