Identification of Lagrangian coherent structures in a turbulent boundary layer

Lagrangian coherent structures (LCS) of a turbulent boundary layer at Re theta of 9800 are identified in a plane parallel to the wall at y(+) = 50. Three-component high-speed stereo particle image velocimetry measurements on a two-dimensional rectangular plane are used for the analysis. The velocity field is extended in the streamwise direction, using Taylor's frozen field hypothesis. A computational approach utilizing the variational theory of hyperbolic Lagrangian coherent structures is applied to the domain and trajectories are computed using the extended field. The method identified both attracting and repelling Lagrangian coherent structures. There are no apparent differences in distribution of size, orientation and location of attracting and repelling structures. Hyperbolic behaviour appeared in the fluid at and around points of intersection between the attracting and repelling Lagrangian coherent structures. The network of curves identifying distinct regions of coherent flow patterns is displayed in observed relationship between the arrangement of Lagrangian coherent structures and various Eulerian fields.