Reynolds-number dependence of turbulence structures in a drag-reducing surfactant solution channel flow investigated by particle image velocimetry

The Reynolds-number dependence of turbulence structures in a drag-reducing surfactant solution flow is attributed to the Reynolds-number effect on rheology of the solution flowing in the passage and the rheological properties are associated with the shear-induced structure (SIS) formed in a sheared surfactant solution. Through analysis of turbulence statistics of the two-dimensional velocity field, measured by particle image velocimetry, for a drag-reducing flow with addition of cetyltrimethyl ammonium chloride (CTAC) in different flow regimes characterized by the drag-reducing effectiveness dependent on the Reynolds number, we studied the characteristics of turbulence structures for different Reynolds-number-dependent flow states and indirectly revealed the dynamic processes of SIS across the flow passage. A 25ppm (ppm - parts per million) CTAC solution at 30°C was tested. Based on the relationship between the Reynolds number and the drag-reduction levels, the CTAC solution flow was categorized into four regimes: Regime I is laminar and transitional flow without drag-reducing ability; in regime II drag-reduction level increased with the Reynolds number; in regime III drag-reduction level decreased with the Reynolds number; and in regime IV drag-reducing ability disappeared. Five cases of CTAC solution flow - two in regime II, two in regime III, and one in regime IV - were measured. Five cases of water flow at similar Reynolds numbers corresponding to those of CTAC solution flows were also measured for comparison. It was found that the flows in regimes II and III behave differently in turbulence statistics, including mean streamwise velocity, turbulence intensities of both streamwise and wall-normal velocity components, Reynolds shear stress and contour map of spectral density functions of velocity fluctuations in a coordinate plane of wave number and wall-normal position, regardless of similar drag-reduction levels. In regimes III and IV, multilayer structures associated with different flow states were obtained from the analysis of profiles of the Reynolds shear stress and the viscoelastic shear stress, which characterize the dynamic processes of SIS in the solution across the flow passage at different Reynolds numbers.