ON THE GENERATION OF TURBULENT WALL FRICTION

The formation of streaky velocity structures in the near wall region o f turbulent boundary layers is studied through a simplified two-dimens ional computational model in the plane normal to the average velocity. It is shown that the redistribution of the longitudinal velocity by s treamwise vortices produces features very similar to those observed in the experiments, and that compact streamwise vortices form naturally from more general vorticity distributions. It is also shown, both nume rically and analytically, that one effect of the formation of the stre aks is to increase the average wall friction, and it is suggested that this effect is responsible for the higher friction in turbulent bound ary layers, as opposed to laminar ones. An approximate quantitative an alysis of the process supports this assumption.