Vortical structures were identified and characterized using velocity fields of turbulent wall-bounded flows. Two direct numerical simulation data sets of fully developed channel flow at Reτ = 934 obtained by del Álamo et al. (J. Fluid Mech., vol. 500, 2004, p. 135) and Re τ = 590 obtained by Moser, Kim & Mansour (Phys. Fluids, vol. 11, 1999, p. 943) as well as dual-plane particle image velocimetry data at z+ = 110 in a zero-pressure-gradient turbulent boundary layer at Reτ = 1160 obtained by Ganapathisubramani, Longmire & Marusic (Phys. Fluids, vol. 18, 2006, 055105) were employed. The three-dimensional swirling strength based on the local velocity gradient tensor was employed to identify vortex core locations. The real eigenvector of the tensor was used both to refine the identification algorithm and to determine the orientation of each vortex core. The identification method allowed cores of nearly all orientations to be analysed. Circulation of each vortical structure was calculated using the vorticity vector projected onto the real eigenvector direction. Various population distributions were then computed at different wall-normal locations including core size, orientation, circulation and propagation velocity. The mean radius of the cores identified was found to increase with increasing wall-normal distance, and the mean circulation increases approximately quadratically with eddy radius. Orientations of cores with stronger circulation were distributed over a much narrower range than those for vortices with weaker circulation and were consistent with legs, necks and heads of forward-leaning hairpin structures.
Bibliographical noteFunding Information:
This study was supported by the National Science Foundation under grant number CTS-0324898. The Graduate School of the University of Minnesota is gratefully acknowledged for providing a Doctoral Dissertation Fellowship for the first author. We would like to thank Professor R. Moser for providing the DNS channel flow data. Also, we are indebted to Drs. B. Ganapathisubramani and N. Saikrishnan for their help in data analysis and many discussions during this study.
- Turbulent boundary layers
- boundary layer structure
- vortex dynamics