Air Bubble Techniques in Reducing Friction Drag in Turbulent Boundary Layer

Superhydrophobic surfaces (SHSs) have emerged as a promising passive method for reducing turbulent drag in fluid systems, and therefore substantially improving the energy efficiency of engineered systems, yet their effects remain not fully understood. SHSs passively maintain a layer of air-films that are both free-slip and dynamically deformable. This study investigates the free-slip and dynamic effects of patterned SHSs on transition and turbulent boundary layer flows. Three SHS configurations - uniform coating, streamwise strip pattern, and transverse strip pattern - at three Reynolds numbers were considered. A smooth, no-slip boundary surface was used as the baseline control case. The boundary layer profiles and turbulent stresses are substantially modified by the dynamic free-slip boundaries. At low flow speeds, particularly in the laminar-turbulent transition regime, the SHS air films behaved as steady free-slip boundary conditions, resulting in increased near-wall velocity for uniform and streamwise strip configurations. In contrast, the transverse strip pattern yielded reduced near-wall velocity. At higher Reynolds numbers, the air films dynamically deformed through coupling interactions with unsteady turbulent flows, which generated distinct turbulence manipulation effects depending on the orientation of SHS patterns. The streamwise strips stabilized turbulent flows and reduced the Reynolds shear stress (RSS), whereas the transverse strips energized turbulence and enhanced RSS, although both patterns exhibited similar mean velocity profiles. Anomalous flow behavior was observed in the uniform SHS at the lowest Reynolds number, in which surface capillary wave motions were excited. In this case the RSS was significantly amplified. These findings emphasize the dual role of SHS air-films: while the free-slip effect usually reduce shear stresses, the dynamic interaction may be beneficial or detrimental depending on the geometry and orientations of air-film with turbulent flows. The basic understanding discovered through this research has applications in engineering transport systems, where reducing frictional drag can lead to improved energy efficiency and performance.