Statistical evidence of hairpin vortex packets in wall turbulence

Statistical analysis of particle image velocimetry (PIV) measurements in the streamwise, wall-normal plane of turbulent channel flow indicates a coherent and robust alignment of hairpin vortices in the outer layer. Previous experimental and computational analyses of instantaneous velocity realizations show that hairpin vortices dominate the organized structure of the outer layer of wall turbulence and appear to align in a consistent manner. The present work reveals that these hairpin packets leave clear footprints in the statistics of the flow, indicating that they are highly-organized and robust. Linear stochastic estimation is used to estimate the conditional average of the velocity field given the presence of a single vortex core centered in the logarithmic layer. Remarkably, this simple estimate provides a vivid picture of multiple swirling motions coherently aligned in the streamwise direction and inclined away from the wall. This is the first statistical evidence that the outer layer of wall turbulence is dominated by a large-scale, coherent organization of hairpin vortices.NSF, Air Force Office of Scientific Researc