Effects of Concentration of Surfactant Solutions on Drag-Reducing Turbulent Boundary Layer

Abstract

In this study, the influence of a drag-reducing surfactant on the turbulent boundary layer was extensively investigated under different solution concentrations using a two-component laser-Doppler velocimetry system. The surfactant solution used here was a mixture of cetyltrimethyl ammonium chloride (CTAC) with sodium salicylate as counterion, which was dissolved in tap water. The concentrations tested were 100 and 150 ppm, and the previous data on surfactant solution of 65 ppm were also used.The temperature of surfactant solution was T=20°C.It is found that the drag reduction ratio DR becomes larger downstream, and decreases with the increase of concentration from C=65 to 150 ppm. Regardless of solution concentrations, the value of the mean velocity scaled by the friction velocity increases with increasing the amount of drag reduction, while the mean velocity scaled by the free-stream velocity, which are collapsed for the different Reynolds number and drag reduction ratio, are about in the middle between the mean velocity profile of water and the Blasius laminar profile. For the present experimental conditions tested, the maximum of streamwise turbulence intensity near the wall is not related to the amount of DR directly and seems to be affected by the low Reynolds number effect strongly, while the location of the maximum becomes more distant from the wall with the increase of DR. The additional maximum of streamwise turbulence intensity near the center of the boundary layer, which is observed for all the concentration tested, and the wall-normal location of additional maximum are almost constant and are independent of the amount of drag reduction.