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 h ydrogen-bubble flow visualization method. The mean velocity profiles a nd the distributions of streamwise turbulence intensity are also obtai ned. 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 rea ches 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 t he 'hump' of the f-Re curve (f : friction factor). The mean spanwise s treak spacing very near the wall increases as DR becomes large. It is speculated that the nondimensional wall-normal distance y(+) of the ce nter of streamwise vortices is larger for the flow of large DR compare d with that for the Newtonian fluid flow.