Very-large-scale motions in wall-bounded turbulent flows

Very-large-scale motions in wall-bounded turbulent flows are investigated experimentally. Data sets acquired in the atmospheric boundary layer and in turbulent pipe flow are used in conjunction with previously available data from turbulent channel flow and a laboratory boundary layer.; Flow visualization in the first 3 m of the atmospheric boundary layer at very high Reynolds number reveals ramp-like structures strikingly similar in statistics and appearance to those observed in laboratory flows at much lower Reynolds numbers. A structural similarity in these structures with increasing distance from the wall is observed and explained in terms of the hairpin-vortex-packet paradigm.; A template matching technique is developed to allow the direct comparison between data sets obtained in different wall-bounded flows across a range of Reynolds numbers. Results indicate that the temporal signature of hairpin vortices and packets is, in many ways, universal with respect to flow type and Reynolds number. A small collection of templates is identified which are capable of reconstructing a velocity-time series. A collection of only eight of these templates, which resemble hairpin vortex packets, is capable of reconstructing 50% of a time signal with a correlation above 0.65.; Velocity spectra from turbulent pipe flow are presented. Structures as long as 8–16 pipe radii are shown to contain a significant fraction of the energy in the streamwise velocity fluctuations. More specifically, near the wall, half the energy is contained within structures larger than four pipe radii in length. Velocity co-spectra indicate that one half of the Reynolds shear stress is due to structures larger than the pipe radius. Beyond the log-region of the velocity profile, half the Reynolds shear stress is due to structures larger than three pipe radii.; Overall, results indicate that the hairpin vortex and its organization into packets is a robust and significant feature of wall-bounded turbulence over a wide range of Reynolds numbers. The hairpin-vortex-packet model of turbulence provides a consistent explanation for the distribution of energy among the largest scales of the flow and underscores the importance of the very-large scales.