Turbulent boundary layer modified by local polymer solution injection in low- and high-drag reduction states

The effect of local polymer injection on a flat-plate turbulent boundary layer was investigated experimentally, with a particular focus on the distinct mechanisms of drag reduction in the low-drag reduction (LDR) and high-drag reduction (HDR) states. High-resolution measurements of the near-wall flow field were achieved using particle tracking velocimetry (PTV), while large-field measurements covering the entire boundary layer were obtained through particle image velocimetry (PIV). It is found that in the LDR state, the mean shear and turbulence intensity in the near-wall region are mildly suppressed. The influence of polymer solution is limited to the near-wall region and does not affect the self-sustaining cycle of wall-bounded turbulence. Conversely, in the HDR state, the polymer solution effectively disrupts the self-sustaining process of wall-bounded turbulence by suppressing the lifting of the low-speed streaks. Consequently, the turbulence production is attenuated. The buffer layer is extended, and the slope of the log-law region also becomes larger when polymer concentration or the Weissenberg number increases.