Observation of streamwise rotation in the near-wall region of a turbulent boundary layer

Simultaneous top-end view visualization studies of the flow behavior in the near-wall region of a turbulent boundary layer have been done using hydrogen bubble wires and a dual-camera, high-speed video system. The studies show clearly that the low-speed streaks formed in the near-wall region are discrete regions of outward motion, with the degree of activity varying with distance from the wall. The visualizations also reveal the presence of significant streamwise rotation in the wall region for y+≳10, including frequent observation of counter-rotating streamwise structures. An apparent interdependency between these counter-rotating structures and low-speed streak formation is observed, and appears to support the suggestions of several previous investigators.