Investigation of turbulence structures in a drag-reduced turbulent channel flow with surfactant additive by stereoscopic particle image velocimetry

In the present study, we employed stereoscopic particle image velocimetry (PIV) to investigate the characteristics of turbulence structures in a drag-reduced turbulent channel flow with addition of surfactant. The tested drag-reducing fluid was a CTAC/NaSal/Water (CTAC: cetyltrimethyl ammonium chloride; NaSal: sodium salicylate) system at 25°C. The weight concentration of CTAC was 30 ppm. Stereoscopic PIV measurement was performed for a water flow (Re=1.1×10⁴) and a CTAC solution flow (Re=1.5×10⁴ with 54% drag reduction) in both the streamwise–spanwise and wall-normal-spanwise planes, respectively. The three-dimensionality of hairpin vortex structures in the near-wall region for wall-bounded turbulent flow was reproduced by conditionally averaging the stereoscopic two-dimensional-three-component velocity fields. A series of wall-normal vortex cores were found to align with the near-wall low-speed streaks with opposite vorticity signals at both sides of the streaks and with the vorticity decreased on average by about one order of magnitude in CTAC solution flow compared with water flow; the spanwise spacing between the near-wall low-speed streaks in the solution flow is increased by about 46%. The streamwise vorticity of the vortex cores appearing in the wall-normal-spanwise plane was also decreased by the use of drag-reducing surfactant additives.