The drag-reducing property of a superhydrophobic surface is investigated along with its mechanism. A superhydrophobic surface with micro-nanotextures is fabricated and tested using SEM and contact angle measurement. Velocity distributions in the turbulent boundary layer with a superhydrophobic surface and a smooth surface are measured by particle image velocimetry at Re-theta = 810, 990, and 1220. An upward lift effect on the velocity profile caused by the rugged air layer on the superhydrophobic surface is observed, which indicates drag reduction. Estimated by the wall shear stress, a drag reduction of 10.1, 20.7, and 24.1 % is observed for Re-theta equal to 810, 990, and 1220, respectively. The drag reduction is caused mainly by slip on the interface and modifications in the turbulent structures, and the latter plays a more important role as Re-theta increases. Suppressions are observed in turbulence intensities, and reductions in the total Reynolds shear stress T-turb(+) are 2.5, 18.5, and 23.1 % for Re-theta = 810, 990, and 1220, respectively. Vortex fields above the superhydrophobic and smooth surfaces at Re-theta = 990 are investigated. Vortexes are weakened and lifted upward by the superhydrophobic surface, and the position of the maximum swirling strength is lifted 0.17 delta (delta is the boundary layer thickness) upward in the wall-normal direction. This modification in turbulence structures contributes significantly to the drag reduction in the turbulent boundary layer flow.
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