Experimental investigation on drag reduction in turbulent boundary layer over superhydrophobic surface by TRPIV

This study aims at the mechanism of drag reduction in turbulent boundary layer (TBL) with superhydrophobic surface. Comparing the time-resolved particle image velocimetry (TRPIV) measurement results with that of hydrophilic surface, the drag reduction rate over a superhydrophobic surface is approximately 10%. To investigate the characteristics of coherent structure in a drag-reduced TBL with superhydrophobic surface, a modified multi-scale spatial locally-averaged structure function is proposed for detecting coherent structure. The conditional sampling and spatial phase-lock average methods are employed to obtain the topology of physical quantities like the velocity fluctuation, spanwise vorticity, and Reynolds stress during eject and sweep process. The results indicate that the suppression of coherent structure burst in the near-wall region is the key mechanism in reducing the skin friction drag for TBL over superhydrophobic surface.