Plastron morphology and drag of a superhydrophobic surface in turbulent regime

The relationship between the state of the plastron, slip velocity, and drag of a superhydrophobic surface in contact with a turbulent boundary layer was investigated. The experiments were carried out using a body of revolution which was spray coated with a superhydrophobic layer and tested in a water tunnel at velocities ranging from 0.98 to 2.92 m/s (Reynolds number from 5.0 x 10(5) to 1.5 x 10(6) based on the length of the model). Visualization of the plastron and particle-tracking velocimetry (PTV) of the near-wall boundary layer was carried out using a long-range microscope with backlight illumination. The model was also equipped with a load cell to simultaneously measure the drag force. The load measurement showed a 36.4% reduction in drag at the lowest Re of 5.0 x 10(5), which decreased to 5.6% at the highest Re of 1.5 x 10(6). The microscopic PTV showed an increase in the slip velocity from 0.131 to 0.602 m/s, and a relatively constant slip length (85-66 mu m) over the superhydrophobic surface (SHS) as Re increased from 5.0 x 10(5) to 1.5 x 10(6). The wall-normal gradient of mean velocity in the linear viscous sublayer showed smaller viscous wall shear stress over the SHS compared with the smooth baseline surface for Re smaller than 1.0 x 10(6). At a larger Re of 1.2 x 10(6), drag reduction diminished with an increase of relative roughness. The visualizations of the surface showed frequent appearance of a full air plastron with average thickness of similar to 10 mu m at the two lowest Re of 5.0 x 10(5) and 7.0 x 10(5). The air-water interface of the plastron had a transient behavior due to low-wave-number ripples, which leads to thickness variation. This full plastron was essential for obtaining a considerable drag reduction (>16%). At the three higher Re with smaller drag reduction (<8%), the plastron demonstrated isolated menisci of air, pinned between the tip and the valley of large roughness protrusions.