Effect of hierarchical structured superhydrophobic surfaces on coherent structures in turbulent channel flow

In this study, the coherent structures, which were effected by superhydrophobic (SH) surfaces in turbulent channel flow, were investigated by time-resolved particle image velocimetry (TRPIV). Hierarchical structured SH surfaces which have micro-nano secondary structures were applied in the experiment. Drag reduction rate is approximately 11% at Re-0 congruent to 1300, which is based on momentum thickness. Coherent structures (low-speed streak, hairpin vortex and hairpin vortex packets) with scale information were obtained by conditional sampling and averaging method. Compared to the hydrophilic (PH) surfaces, the conditional averaged hairpin vortex structures of SH surfaces have a lager spacing of vortex legs, a smaller angle of inclination along streamwise and spanwise directions. Accordingly, a smaller-scale hairpin vortex packet with three swirling motions was emerged. Through the comparative research on angle of inclination, the lifting movement of vortex in wall-normal direction was also weaker than PH case. These shows that the existence of SH surfaces restricts the development of coherent structures and induces drag reduction effects. (C) 2015 Elsevier Inc. All rights reserved.