Drag reduction and hairpin packets of the turbulent boundary layer over the superhydrophobic-riblets surface

This paper presents experimental measurements by the time-resolved particle image velocimetry (TR-PIV) for the turbulent boundary layer (TBL) over the smooth surface, the superhydrophobic (SH) surface, and the superhydrophobic-riblets (SR) surface in an open-surface recirculating water channel. The Reynolds number based on the wall friction velocity and the thickness of the TBL over the smooth surface is 702. The SH surface and the SR surface are manufactured by the laser texturing method on the smooth surface and the riblets surface, respectively. By employing the (modified) Clauser method, a superior efficacy of the drag reduction of about 22.1% is obtained on the SR surface, while the drag reduction rate for the SH surface is about 18.7%. Comparing with the SH surface, the declining 2-order statistics in the near-wall region also indicates a significant drag reduction over the SR surface. The large-scale structure components extracted by the proper orthogonal decomposition are found to generate a majority of the Reynolds shear stress in the region y(+) > 40. The strength of the large-scale features over the rough surfaces (SH and SR surfaces) at disparate wall-normal positions is visualized by the Quadrant splitting method and the conditional averaging. The appearance of the large-scale structures such as the hairpin packets characterized by the two-point correlation shows an excellent consistency with the statistics profile. The hairpin packets over the SH surface seem always smaller and weaker than those over the smooth surface. Over the SR surface, the hairpin packets in the region 0.1 <= y delta < 0.3 are the smallest and weakest among those over the three surfaces, while the scale and the strength of the hairpin packets exceed those over the smooth surface in the region 0.3 < y delta < 0.6.