Drag reduction of a zero‐pressure gradient boundary layer flow by surfactants.

To investigate drag reduction by a surfactant in a zero‐pressure gradient flat‐plate turbulent boundary layer flow, the velocities were measured by a two‐component laser Doppler velocimeter, and the buildup as well as relaxation of micelle shear‐induced structures in a simple shear flow was then analyzed by a strain‐controlled rheometer through applying and releasing a given constant shear strain rate. The surfactant was cetyltrimethyl ammonium bromide mixed with sodium salicylate as a counterion in a molar ratio of 1:2 at 60 and 100 ppm by weight. It has been found that the drag reduction rate first increases and then decreases with decreasing wall shear stress in the streamwise direction for the two concentrations of solutions, which is totally different with that in a pipe flow. In addition, the drag reduction in a lower concentration solution boundary layer flow reaches the maximum earlier. Moreover, a shorter buildup time for the micelle shear‐induced structures makes the maximal drag reduction reach earlier in surfactant solution boundary layer flows. [ABSTRACT FROM AUTHOR]