To clarify the drag-reducing behaviors of a surfactant solution in turbulent boundary layers, the turbulence structures of cetyltrimethylammonium bromide(CTAB) surfactant solution mixed with NaSal which provide counter ions in a zero pressure gradient boundary layer on flat plates were investigated by a two dimensional laser doppler velocimetry(LDV). The results were found as follows: The peak of the streamwise velocity turbulence intensity for surfactant solution appeared away farther from the wall than that for water, and the second peak occured near the center of the boundary layer where the flow was not in SIS. The drag-reducing surfactant solution depressed the motions associated with ejections of low momentum fluid away from the wall and sweeps of high momentum fluid toward the wall so that Reynolds shear stress was almost zero throughout the thickness of the boundary layer. In the power spectrum of the streamwise velocity fluctuation, the power intensity at low frequencies was larger and the power intensity at high frequencies was smaller than that for water. These showed that the surfactant additives can weaken the correlation between the two components of velocity fluctuation, and enlarge the turbulent structure scales in the boundary layers.
