Behavior of the Hairpin Votex in a Laminar Boundary Layr of Surfactant Solution

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In order to clarify the mechanism of drag-reduction in the turbulent flow of a surfactant solution, the behavior of hairp in vortex generated continuously or solitary in a laminar flat-plate boundary layr was investigated experimentally. The results obtainedare as follows.[continuously generated hairpin vortex](1)The frequency of hairpin generation is lower in the flow of surfactant solution than in the water flow.(2)In the flow of surfactant solution, both the kinked region of the vortex legs and the secondary votex are hard to develop.(3)At downsream location, multiple low-speed streaks are observed in the water flow. On the other hand, only single low -speed streak can be observed in the flow of surfactant solution.(4)The ratio of the convecting velocity of the vortex head to the local flow velocity is smaller in the surfactant solution thon in the water.(5)The angle between the neck of hairpin vortex and the wall is smaller in the flow of surfactant solution[single hairpin vortex](1)In order to generate a single hairpin vortex in the flow of surfactant solution, the speed of the infecting jet must be higher and the duration time must be shorter.(2)The hairpin vortex in the flow of surfactant solution does not develop if the shear rate is higher than the critical value for the shear inducing state.(3)The single hairpin vortex can be generated for the limiting conditions.