Experimental study of drag-reduction mechanism for a dilute surfactant solution flow

The quantitative characteristics of vortex structures and turbulent events in turbulent channel flows with and without drag reduction were investigated experimentally. Drag reducing aqueous solutions of CTAC (CTAC - cetyltrimethyl ammonium chloride) with concentration of 25 ppm and 75 ppm at 30 °C were tested. Particle image velocimetry (PIV) was used to measure the instantaneous velocity field in the streamwise-wall-normal plane of the flow. Through visualizing the instantaneous vector fields, contour maps of the swirling strength and instantaneous value of uv (u and v are the streamwise and wall-normal velocity fluctuations, respectively), the characteristic angle of vortex packets was quantified. It was shown that the drag-reducing CTAC additive reduced both the strength and frequency of turbulent bursts near the wall, and the characteristics of vortex structures and bursts were not only dependent on drag-reduction level but also on concentration of additive. From the quantified parameters characterizing turbulent events in a wall-bounded turbulent flow, it was obtained that f_T was linearly proportional to the product of frequency and strength of turbulent events.