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

In our earlier work [Itoh , Phys. Fluids 17, 075107 (2005)], 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-theta 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 (LDV) 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 (SIS) 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.