DNS revealing hairpin vortices in: a) the laminar boundary layer with an artificial low speed streak. b) the turbulent boundary layer. The light gray structures represent the low-speed streaks and the darker ones represent regions with low pressure and rapid tornado-like motion. The arrows point to some typical hairpin vortices. The flow is directed upward in the figures. The laminar streak breaks down due to an instability originating from an inflectional velocity profile, and a similar mechanism was found in the turbulent low-speed streaks.

Vertical planes in the spanwise (z) and normal (y) directions. Arrows represent the spanwise and normal velocity. Blue through green lines represent constant streamwise velocity. Red lines represent constant low pressure. a) From the laminar simulation. b) From the turbulent simulation.

Reference: Skote, M., Haritonidis, J.H. and Henningson, D.S. 2002 Varicose instabilities in turbulent boundary layers. Physics of Fluids. Vol 14, No 7, pp 2309-2323.