In the present study, we employed stereoscopic particle image velocimetry (SPIV) 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 (cetyltrimethyl ammonium chloride)/NaSal/Water system maintained at 25°C, having a 30-ppm concentration of CTAC. SPIV measurement was performed for a water flow (Re=1.1×105) and a CTAC solution flow (RE=1.5×105 with 54% drag reduction) in both the streamwise-spanwise and wall-normal-spanwise planes, respectively. A series of wall-normal vortex cores were found to align with the low-speed streaks with opposite vorticity signals at both sides of the streaks and with the vorticity decreased averagely by about one order in CTAC solution flow compared with water flow; the spanwise spacing between the low-speed streaks in the solution flow is increased by about 42%. The streamwise vorticity of the vortex cores appearing in the wall-normal-spanwise plane was also decreased by the use of additives.
Formation of low-speed ribbons in turbulent channel flow subject to a spanwise travelling wave
