Vortex packets and the structure of wall turbulence

Experimental evidence in low to moderate Reynolds number wall flows shows that hairpin vortices (including asymmetric inclined vortices) occur in groups that propagate as a whole with relatively slow dispersion. These groups, or "packets", grow upwards from the buffer layer to about one-half of the thickness of the boundary layer. Direct numerical simulations of the growth of a single hairpin eddy in a clean background flow show how these packets may be formed in the near wall (low Reynolds number) region by a viscous autogeneration mechanism that is similar in many regards to the mechanism proposed by Smith et al. [Philos. Trans. R. Soc. London, Ser. A 336 (1991) 131]. The organization of hairpin eddies into packets and the interactions of those packets is an important feature of wall turbulence that provides a new paradigm by which many seemingly unconnected aspects of wall turbulence can be explained. These include the inordinately large amount of streamwise kinetic energy that resides in very long streamwise wavelengths, the occurrence of multiple Q2 events per turbulent burst, the formation of new streamwise vorticity, and the characteristic angles of inclination of fronts. The autogeneration process may also explain the formation of long quasi-streamwise vortices in the buffer layer and the associated low-speed streaks.