Structure Identification and Analysis in Turbulent Boundary Layers by Stereo PIV

Stereoscopic PIV was used to measure three instantaneous components of velocity in streamwise-spanwise planes of a turbulent boundary layer. The local boundary layer thickness was δ = 69 mm, and the corresponding Reynolds number based on friction velocity was 1060. Data sets were obtained in the log region at wall-normal positions of z⁺ = 92 and z⁺ = 192. The resulting velocity fields at z⁺ = 92 revealed distinctive signatures of vortex packets similar to those observed by Adrian, Meinhart, & Tomkins (2000). At this height, trains of 'necks' from hairpin vortices were observed aligned with the streamwise direction. The packets, which were frequently associated with high levels of Reynolds stress, appear to be a dominant mechanism for near-wall momentum transport. At z^- = 192, larger individual vortex cores and heads of hairpin vortices were observed, but packet structures within the plane were not as evident.