# Time-resolved measurements of coherent structures in the turbulent boundary layer

## Abstract

Time-resolved particle image velocimetry was used to examine the structure and evolution of swirling coherent structure (SCS), one interpretation of which is a marker for a three-dimensional coherent vortex structure, in wall-parallel planes of a turbulent boundary layer with a large field of view, 4.3*δ* × 2.2*δ*. Measurements were taken at four different wall-normal locations ranging from *y*/*δ* = 0.08–0.48 at a friction Reynolds number, *Re* _{ τ } = 410. The data set yielded statistically converged results over a larger field of view than typically observed in the literature. The method for identifying and tracking swirling coherent structure is discussed, and the resulting trajectories, convection velocities, and lifespan of these structures are analyzed at each wall-normal location. The ability of a model in which the entirety of an individual SCS travels at a single convection velocity, consistent with the attached eddy hypothesis of Townsend (The structure of turbulent shear flows. Cambridge University Press, Cambridge, 1976), to describe the data is investigated. A methodology for determining whether such structures are “attached” or “detached” from the wall is also proposed and used to measure the lifespan and convection velocity distributions of these different structures. SCS were found to persist for longer periods of time further from the wall, particularly those inferred to be “detached” from the wall, which could be tracked for longer than 5 eddy turnover times.

## Keywords

Vortex Particle Image Velocimetry Coherent Structure Particle Image Velocimetry Measurement Convection Velocity## Notes

### Acknowledgments

We would like to acknowledge and thank Prof. Mory Gharib and Dr. David Jeon for their assistance and allowing us to use their free surface water tunnel facility at Caltech in which all the experiments presented were performed. We would also like to gratefully acknowledge the Air Force Office of Scientific Research (AFOSR) for their support of this research under award number #FA9550-09-1-0701. Anonymous reviewers provided recommendations that have significantly improved the quality of this manuscript.

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