Superhydrophobic drag reduction in high-speed towing tank

• Published in: Journal of fluid mechanics Vol. 908
• Main Authors: Xu, Muchen, Yu, Ning, Kim, John, Kim, Chang-Jin “CJ”
• Format: Journal Article
• Language: English
• Published: Cambridge, UK Cambridge University Press 10.02.2021
• Subjects: Atmospheric pressure; Boats; Boundary layer flow; Computational fluid dynamics; Drag; Drag reduction; Experiments; Fluid flow; Friction; Friction drag; Friction reduction; High Reynolds number; High speed; Hydrophobic surfaces; Hydrophobicity; JFM Papers; Laboratories; Length; Microelectromechanical systems; Oceanic trenches; Reynolds number; Seawater; Silicon wafers; Skin; Skin friction; Towing; Towing tanks; Turbulent flow; Water flow; United States > US; MEMS/NEMS; turbulence control; drag reduction
• ISSN: 0022-1120; 1469-7645
• DOI: 10.1017/jfm.2020.872

As far as plastron is sustained, superhydrophobic (SHPo) surfaces are expected to reduce skin-friction drag in any flow conditions including large-scale turbulent boundary-layer flows of marine vessels. However, despite many successful drag reductions reported using laboratory facilities, the plastron on SHPo surfaces was persistently lost in high-Reynolds-number flows on open water, and no reduction has been reported until a recent study using certain microtrench SHPo surfaces underneath a boat (Xu et al., Phys. Rev. Appl., vol. 13, no. 3, 2020, 034056). Since scientific studies with controlled flows are difficult with a boat on ocean water, in this paper we test similar SHPo surfaces in a high-speed towing tank, which provides well-controlled open-water flows, by developing a novel $0.7\ \textrm {m} \times 1.4\ \textrm {m}$ towing plate, which subjects a $4\ \textrm {cm} \times 7\ \textrm {cm}$ sample to the high-Reynolds-number flows of the plate. In addition to the 7 cm long microtrenches, trenches divided into two in length are also tested and reveal an improvement. The skin-friction drag ratio relative to a smooth surface is found to be decreasing with increasing Reynolds number, down to 73 % (i.e. 27 % drag reduction) at $Re_x\sim 8\times 10^6$, before starting to increase at higher speeds. For a given gas fraction, the trench width non-dimensionalized to the viscous length scale is found to govern the drag reduction, in agreement with previous numerical results.