An experimental study on drag reduction of superhydrophobic rotating disk with air plastron

Drag reduction performance of superhydrophobic disks in a Von Kármán swirling flow with Re~O(10⁵) was experimentally studied. Two superhydrophobic disks, which have different microstructures, i.e., one with micron-scale homogeneous roughness (abbreviated as SHS#1) and the other with additional millimeter-scale nonhomogeneous grid pattern (abbreviated as SHS#2), have been tested. Both SHS#1 and SHS#2 are prepared by the method of physically spraying nano-scale hydrophobic particles onto an acrylic-plate substrate. The grid pattern on SHS#2 is obtained by applying a mask of wire mesh during the spraying procedure. The mean skin-friction drag on the rotating disk was measured by a torquemeter. It is shown that for the superhydrophobic surface to reduce drag in Von Kármán swirling flow, there is a critical Reynolds number Re_c. When Re < Re_c, the superhydrophobic surface has a stable long-term drag reduction effect, with drag reduction ratio up to 30%; but when Re>Re_c, the drag reduction effect is rapidly lost with the increase of Re. Compared to SHS#1, SHS#2 can effectively improve the dynamic stability of the air plastron attached on the surface. Additionally, the air plastron on the superhydrophobic surface can be effectively restored by pulse air injection, and so can the drag reduction effect. This observation indicates a promising strategy for reliable and sustainable drag reduction via superhydrophobic surface.