Abstract
Dilute surfactant solution of 25 ppm in the two-dimensional channel is investigated experimentally compared with water flow. Particle image velocimetry (PIV) system is used to take 2D velocity frames in the streamwise and wall-normal plane. Based on the frames of instantaneous vectors and statistical results, the phenomenon of zero Reynolds shear stress appearing in the drag-reducing flow is discussed. It is found that 25 ppm CTAC solution exhibits the highest drag reduction at Re = 25000 and loses drag reduction completely at Re = 40000. When drag reduction lies in the highest, Reynolds shear stress disappears and reaches zero although the RMS of the velocity fluctuations is not zero. By the categorization in four quadrants, the fluctuations of 25 ppm CTAC solution are distributed in all four quadrants equally at Re = 25000, which indicates that turnaround transportation happens in drag-reducing flow besides Reynolds shear stress transportation. Moreover, the contour distribution of streamwise velocity and the fluctuations suggests that turbulence transportation is depressed in drag-reducing flow. The viscoelasticity is possible to decrease the turbulence transportation and cause the turnaround transportation.
Highlights
If small amounts of polymer or surfactant are added into the water, the friction will decrease in a large degree in turbulent flow
Reynolds shear stress is important to the turbulence transportation in wall-bounded flow [7], but it was strongly reduced in drag-reducing flow and some researchers ascribed the phenomenon to the less correlation between the two velocity fluctuations
Water and CTAC solution with the weight concentration of 25 ppm are compared under the Reynolds number from 10000 to 40000
Summary
If small amounts of polymer or surfactant are added into the water, the friction will decrease in a large degree in turbulent flow. This phenomenon of drag reduction is named “Toms effect” because it was firstly reported by Toms in 1948 [1]. Li et al found that the inclination angle of the lowmomentum region below the hairpin vortices decreased and the frequency of bursts was reduced, which indicated the inhibition of bursting events by surfactant additives [11, 12] All these studies have not given the reasonable explanation to the appearance of zero Reynolds shear stress. The dynamic characteristics of the surfactant solution flow are studied based on the statistic results and instantaneous velocity distribution
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