Turbulent Flow Near Smooth Wall

Experiments have demonstrated the existence of mean streamwise vorticity in turbulent flow near a smooth wall. Mean streamwise vorticity in uniform turbulent flow has also been shown to depend to a large extent on gradients of the difference in the traverse Reynolds normal stresses. It is suggested therefore that the large gradient of this difference in the viscous sublayer is significant and that the streamwise vortices or small-scale secondary flows created by an instability in this gradient may cause the longitudinal streaks that have been observed. An explanatory solution for the streamwise vorticity using estimated characteristics of a perturbed turbulence distribution in the viscous sublayer shows that the postulate is quantitatively feasible. The results of several experimental investigations seem to be consistent with the postulated origin of the streamwise structure.