Hairpin vortex trajectories in a turbulent boundary layer

Hairpin vortices within a turbulent boundary layer have been tracked in space and time resulting in their average trajectory and spreading rates with respect to the average. This information is used to predict the space-time correlation function of different flow variables, such as the individual velocity components and the invariants of the velocity gradient tensor, given their respective spatial auto-correlation functions. The predictions are in good agreement with the actual space-time correlations over convection distances up to one boundary layer thickness, which indicates that the decreasing peak correlation values over time reflect the spreading rate rather than an actual topological evolution of the individual flow structures. The results will also be discussed in relation to Taylor's hypothesis.