| The focus of this study is on understanding the dynamics of a zero-pressure-gradient turbulent boundary layer over a flat plate. The primary objective is to relate production of turbulence to vortex structures and study the cause and effect relationship between vortex structures and Reynolds shear stress. Stereoscopic particle image velocimetry was employed to obtain detailed measurements of the fluid flow in a wind tunnel. The vector fields in the logarithmic layer reveal signatures of vortex packets similar to those found by Adrian and co-workers in their PIV experiments. Groups of legs of hairpin vortices appear to be coherently arranged along the streamwise direction. These regions also generate substantial Reynolds shear stress (-uw), sometimes as high as 40 U2t . An objective feature extraction algorithm was developed to automate the identification and characterization of these packets of hairpin vortices. Hairpin packets contained anywhere between 2--10 hairpin vortices and sometimes were found to span across the entire vector field (>2delta). Identified packets contribute close to 30% of the total Reynolds shear stress while occupying less than 5% of the total area in the log layer. Beyond the log layer, the spatial organization into packets breaks down. Instead, large individual vortex cores and spanwise strips of positive and negative wall-normal velocity are observed.; Dual-plane PIV experiments were performed at two wall-normal locations to obtain all components of the velocity gradient tensor. The availability of the complete gradient tensor aid improved identification of vortex cores, determination of their ori entation and their relationship to turbulence production. Inclination angles of vortex cores were computed using statistical tools (two-point correlations, joint p.d.f.) as well as instantaneous fields. The results indicate that most vortex cores are inclined in the downstream direction, however a small percentage of the cores are inclined backwards. The ratio of the number of forward to backward leaning cores decreases away from the wall; however the number density of backward leaning cores remains relatively a constant. A hypothetical model to represent the structure of the boundary layer is proposed that includes forward-leaning and backward-leaning vortex cores. |
