On interfacial deformation and resistance characteristics over superhydrophobic surfaces: A numerical study

The air/water interface, known as the plastron, entrapped in submerged superhydrophobic surfaces (SHSs), plays a crucial role in underwater drag reduction. However, the plastron can be easily deformed or collapsed by turbulent flow, leading to decreased drag reduction or even an increase in drag. Most previous numerical studies have simplified these interfaces as idealized flat or curved rigid boundaries. To thoroughly investigate the interfacial behavior of SHSs, this study presents a numerical comparison between ideal and dynamic interfaces. The plastron undergoes regular oscillations after a brief adaptation period, transitioning between convex, nearly flat, and concave shapes. During the oscillatory decay, the dynamic properties of the interface modify the surface drag of the SHSs by both affecting the viscous drag and introducing pressure drag. The viscous drag is affected in two main ways. First, the momentum exchange across the dynamic interface is enhanced due to the roughness-like effect, leading to an increased viscous drag in groove region. Second, the trailing interfaces induce step flows and secondary flows downstream of the grooves, creating regions of nonuniform shear stresses. Consequently, the viscous drag of the downstream walls is slightly reduced. Overall, for convex and nearly flat interfaces, the drag increase within the groove region outweighs the drag reduction at the downstream walls, resulting in a total drag increase in 47.3% and 29.8%, respectively. Conversely, for concave interfaces, the drag increase within the groove region is smaller than the drag reduction at the downstream walls, leading to a 9.8% decrease in total drag.