The nature of near-wall convection velocity in turbulent channel flow

A novel notion of turbulent structure—the local cascade structure—is introduced to study the convection phenomenon in a turbulent channel flow. A space–time cross-correlation method is used to calculate the convection velocity. It is found that there are two characteristic convection speeds near the wall, one associated with small-scale streaks of a lower speed and another with streamwise vortices and hairpin vortices of a higher speed. The new concept of turbulent structure is powerful to illustrate the dominant role of coherent structures in the near-wall convection, and to reveal also the nature of the convection—the propagation of patterns of velocity fluctuations—which is scale-dependent.