Dual-plane particle image velocimetry experiments were performed in a turbulent boundary layer with Reτ=1160 to obtain all components of the velocity gradient tensor. Wall-normal locations in the logarithmic and wake region were examined. The availability of the complete gradient tensor facilitates improved identification of vortex cores and determination of their orientation and size. Inclination angles of vortex cores were computed using statistical tools such as two-point correlations and joint probability density functions. Also, a vortex identification technique was employed to identify individual cores and to compute inclination angles directly from instantaneous fields. The results reveal broad distributions of inclination angles at both locations. The results are consistent with the presence of many hairpin vortices which are most frequently inclined downstream at an angle of 45° with the wall. According to the probability density functions, a relatively small percentage of cores are inclined upstream. The number density of forward leaning cores decreases from the logarithmic to the outer region while the number density of backward-leaning cores remains relatively constant. These trends, together with the correlation statistics, suggest that the backward-leaning cores are part of smaller, weaker structures that have been distorted and convected by larger, predominantly forward-leaning eddies associated with the local shear.
Bibliographical noteFunding Information:
The authors express sincere thanks to Professor Victoria Interrante and Matt Heinzen for visualization of the channel flow dataset. The authors also thank Professor Robert Moser for providing the channel flow dataset. We are indebted to Dr. Nicholas Hutchins, William Hambleton, and Aizaz Bhuiyan for their help in data acquisition and many discussions during the course of this study. Financial support from the National Science Foundation through Grants No. ACI-9982274, No. CTS-9983933, and No. CTS-0324898, the Graduate School of University of Minnesota, and the David and Lucile Packard Foundation is gratefully acknowledged.