Influence of Drag-Reducing Surfactant Additives on Vortex Structures and Turbulent Events in a Channel Flow

The characteristics of vortex structures and turbulent events of drag-reducing surfactant (CTAC) solution flows in a two-dimensional channel have been studied using particle image velocimetry (PIV) to measure the instantaneous velocity fields in a streamwise-wall-normal plane. Through visualizing the instantaneous velocity field, contour maps of the swirling strength and instantaneous value of uv, the characteristic angle of vortex packets was quantified, and it was shown that the drag-reducing CTAC additive reduced both the strength and frequency of turbulent bursts near the wall, and that the characteristics of vortex structures and bursts were not only dependent on drag-reduction level but also on concentration of additive. Based on the quantified parameters characterizing turbulent events in a wall-bounded turbulent flow, a model of turbulent contribution to the friction factor, f_T, was proposed. It was obtained that f_T was linearly proportional to the product of frequency and strength of turbulent events.