Contribution of Superhydrophobic Surfaces and Polymer Additives to Drag Reduction

Drag reduction has constantly received great attention due to its extensive range of applications in fluid transportation and vehicle industries. The vital role of two different additive and non-additive techniques (polymer additives and superhydrophobic surfaces) to reduce the drag force experienced by underwater vehicles, fluid flow through pipes, ducts, open or closed channels, and other wall-bounded laminar and turbulent flows is highlighted. Reducing the drag resistance can significantly enhance the performance of immersed vehicles and results in saving the energy consumed on a large scale. The progress in theoretical modeling, experimental and computational studies of both techniques are reviewed, together with the surface design, wettability, and influence of the roughness factor of superhydrophobic surfaces and the effect of polymer drag-reducing agents for wall-bounded flows and multiphase flows. General formulations, potential applications, and major issues involved in the aforementioned approaches are summarized.