TRPIV experimental investigation of drag-reductionmechanism in turbulent boundary layerover superhydrophobic surfaces

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.