Experimental investigation on streamwise development of turbulent structure of drag-reducing channel flow with dosed polymer solution from channel wall

Streamwise development of the turbulent structure of the drag-reducing channel flow by dosed polymer solution from a channel wall was investigated experimentally. Particle image velocimetry (PIV) was employed to investigate the turbulent structure in the x–y plane and we carried out PIV measurements downstream at three positions: 250 mm (position 1), 800 mm (position 2), and 1350 mm (position 3) from the leading edge of the dosing wall. The Reynolds number based on the channel height and the bulk mean velocity was set to 40,000. 100 ppm of weight concentration of dosing polymer solution was dosed at 10.5 L/min from the whole surface of the dosing wall. As a result of the experiments, Reynolds shear stress and root mean square (RMS) of the wall-normal velocity fluctuation gradually decreased downstream. Corresponding to this decrement, the drag reduction rate developed downstream and drag reduction rate of about 63% was obtained at position 3. In addition, the results of the analyses by Galilean decomposition and swirling strength showed that the suppression of the ejection around the vortical core became stronger downstream. These changes of the turbulent structure in the streamwise direction led to the development of drag reduction downstream. However, near the starting point of the polymer dosing (position 1), drag reduction was not obtained but drag increases. In addition, the unique turbulent structure was observed at this position.