Effects of the slip wall on the drag and coherent structures of turbulent boundary layer

A comparative experiment by time-resolved particle image velocimetry (TRPIV) of the turbulent boundary layer (TBL) over a smooth surface and an anisotropy superhydrophobic (SH) surface was carried out in an open-surface recirculating water channel at Re-tau,Re- smooth = 650. Thewall friction velocity is fittedwell from the velocity of the viscous sublayer calculated by the Single-pixel resolution ensemble correlation (SPEC). After that, a drag reduction rate of 17%, a slip velocity of 0.0119 m/s, and a slip length of 90.8 mu m are obtained over the SH surface. In the main modes of the reduced-order flow fields, the wave packet structures over the SH surface become "upright". Such large-scale structures in motion are also found in the instantaneous field. According to the statistical results of the correlation, it is found that the slip wall leads to the change of the convection velocity at different positions of the structure, which leads to the change of structure morphology and the distortion of the shear layer.