Turbulence statistics and structures of drag-reducing turbulent boundary layer in homogeneous aqueous surfactant solutions

Abstract

In our earlier work ͓Itoh et al., Phys. Fluids 17, 075107 ͑2005͔͒, the additional maximum of the streamwise turbulence intensity near the center of the drag-reducing turbulent boundary layer was found in the homogeneous dilute aqueous surfactant solution which was a mixture of cetyltrimethyl ammonium chloride with sodium salicylate as counterion. In this work, we systematically investigated the influence of the drag-reducing surfactant on the velocity fields of the turbulent boundary layer at various Reynolds numbers Re from 301 to 1437 and the drag reduction ratio DR from 8% to 74% under different streamwise locations and concentration and temperature of solutions using a two-component laser-Doppler velocimetry ͑LDV͒ system. It was revealed that all data on DR versus the wall-shear rate obtained here were collapsed on a single curve. We verified the existence of the additional maximum of the streamwise turbulence intensity near the center of the boundary layer which appeared at relatively large drag reduction ratios and small Reynolds numbers. It was found that the additional maximum of streamwise turbulence intensity and its wall-normal location were independent of the streamwise location, wall-shear rate, Reynolds number, and drag reduction ratio. The additional maximum could be explained by the bilayered structure model proposed, in which the flow in the near-wall region is in shear-induced structure ͑SIS͒ and viscoelastic, whereas the flow in the region away from the wall is in non-SIS and nonviscoelastic. This model was based on measurements of the shear viscosity. We also performed particle image velocimetry measurements, which revealed that the fluctuating velocity vector fields showed two situations, with low and high activity. In low activity, the velocity fluctuations were attenuated largely across the turbulent boundary layer. In high activity, fluctuating velocity vectors were almost parallel to the wall and relatively large in both regions near the wall and the center of the boundary layer, which seemed to be a bilayered structure and supported the bilayered structure model.

Comparison of oleyl and elaidyl isomer surfactant–counterion systems in drag reduction, rheological properties and nanostructure

“…The time averaged product of these two velocity fluctuations at any point in the turbulent flow are called the Reynolds stress which is the major component of the local stress in the turbulent region away from the wall. In strongly drag reducing flows these two velocity fluctuations are nearly completely decoupled [6,7] and the resultant lowered Reynolds stress in the turbulent core, along with lower velocity gradient in the near wall region result in large decreases in drag (drag reduction). It was hypothesized by Gyr and Bewerdorff [5] and by Hu and Matthys [8] that for threadlike micelles aligned with the flow, elongational flow is more effective in producing micelle growth than shear flow as it favors side to side collision between micelles causing the micelles to grow in size after collision through fusion.…”

Experimental Investigation on Zonal Structure in Drag-Reducing Channel Flow with Surfactant Additives

“…A detailed discussion of this bilayer structure will be given in another section. In the case of turbulent boundary flow, a characteristic distribution of the streamwise velocity fluctuation was also observed in the drag-reducing flow [19,20]. However, the detailed structure of these peaks has not been discussed.…”

“…They also categorized Reynolds number dependence on the drag-reduction into four flow regimes considering that the drag-reduction rate and turbulent statistics were different in each regime, despite similar drag-reduction rates. Itoh and Tamano [19] and Tamano et al [20] investigated the influence of the drag-reducing flow with surfactant additives on the velocity fields of the turbulent boundary layer using a two-component LDV and PIV. They found the existence of the additional maximum of the streamwise turbulent intensity near the center of the boundary layer.…”

Turbulent drag reduction of boundary layer flow with non-ionic surfactant injection

“…In our recent work (Tamano et al 2010), we investigated not only the drag reduction ratio, which was obtained by measuring the pressure drop and flow rate in the pipe flow of non-ionic surfactant solutions, but also the drag-reducing effect on the development of a turbulent boundary layer using two-component laser-Doppler velocimetry (LDV) and particle image velocimetry (PIV) systems. Turbulence statistics and structures for non-ionic surfactant solutions showed the behaviour of typical drag-reducing flow, such as suppression of turbulence and modification of near-wall vortices, but they were different from those of drag-reducing cationic surfactant solutions (see Tamano et al 2009bTamano et al , 2010. They showed that turbulence statistics for turbulent boundary layers of non-ionic surfactant solutions were similar to those for polymer solutions, rather than cationic solutions, in which bilayered structures of the fluctuating velocity vectors were observed in high activity.…”