Characteristics of Low-Speed Streaks in the Flow of Drag-Reducing Surfactant Solution.

The characteristics of low-speed streaks in the turbulent channel flow of a drag-reducing surfactant solution are studied by employing the hydrogen-bubble flow visualization method. The mean velocity profiles and the distributions of streamwise turbulence intensity are also obtained. It is shown that, at a large drag reduction rate DR, the profile of the streamwise turbulence intensity has a plateau region around its peak. In the drag-reducing flow, the mean spanwise streak spacing reaches minimum at some distance from the wall. In the drag-reducing flow of high-density(1000 ppm)surfactant solution, the low-speed streaks appear intermittently at the Reynolds number smaller than those for the 'hump' of the f-Re curve(f:friction factor). The mean spanwise streak spacing very near the wall increases as DR becomes large. It is speculated that the nondimensional wall-normal distance y+ of the center of streamwise vortices is larger for the flow of large DR compared with that for the Newtonian fluid flow.