Experimental study on turbulent structure in dragreducing surfactant solution channel flow-Reynolds number dependence

The structure of turbulence in a drag-reducing surfactant solution channel flow has been studied with particle image velocimetry. Measurements were performed for water and surfactant solution flow at three regions with different drag-reducing features: regions 2, 3 and 4 with respect to the Reynolds number, in the x-y and x-z planes for a total 500 frames for each run. The drag-reducing flows at regions 2 and 3 showed different characteristics, even though the DR levels were similar. The intensity of the wall-normal velocity fluctuations was decreased at both regions, but that of the streamwise velocity fluctuations, normalized with bulk velocity, was depressed at region 2 whereas increased at region 3 near the wall. The Reynolds shear stress was depressed more greatly at region 2. At region 4, the surfactant solution flow did not display characteristic features as in a Newtonian fluid flow.