Abstract
The surfactant used in this study is well known as an additive for drag reduction in straight (non-swirling) pipe flow. This paper deals with swirling flow characteristics of a surfactant solution. We investigated the effects of surfactant concentration and Reynolds number on flow characteristics. It is shown that the drag reducing effect is less than in straight pipe flow, and that the surfactant solution must be highly concentrated with great elasticity in order to produce a drag reduction. As the results of velocity measurement by LDV and wall pressure measurement in the axial direction, it is expected that the swirl intensity of the drag reducing swirling flow with a surfactant decays more quickly as the flow progresses downstream. And we propose that this mechanism helps create the drag reduction in swirling flow. For the velocity profiles of drag reducing swirling flow, only the forced-vortex type is observed in this experiment. This suggests that the velocity profiles of drag reducing swirling flow change more quickly to a forced-vortex type from a Rankin's combined vortex type. It is also reported that turbulence intensity of drag reducing swirling flow is smaller than turbulence intensity of a solvent.
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