Drag-reduction in turbulent boundary layer (TBL) over superhydrophobic surfaces was investigated by the time-resolved particle image velocimetry (TRPIV). Time series of velocity vector fields in TBL over superhydrophobic surfaces and hydrophilic surfaces were measured in a cyclical water channel. The distributions of mean velocity profile

Reynolds shear stress and turbulence intensity along wall-normal direction were acquired at the same free-stream velocity (0.17m/s). The center of the head of hairpin vortex in wall turbulence was detected with the multi-scale spatial locally-averaged vorticity. Conditional sampling and phase average methods were utilized to extract the spatial topologies of spanwise vortices at different normal positions. Streamwise fluctuation velocity and the pitching angle of spanwise vortex in both cases were compared. The features of streamline topologies around spanwise vortex were analyzed from the saddle-focus dynamical system. Results revealed that a drag reduction of 10.1% is acquired if Reynolds numbers are about 13500. For superhydrophobic surfaces

the average velocity seems to be increased

Reynolds shear stress is decreased

streamwise turbulence intensity is weakened

the pitching angle of spanwise vortex is smaller

and the development of coherent structure near wall region is suppressed.