Superhydrophobic surfaces (SHSs) have emerged as a promising technology for achieving skin-friction drag reduction. This study investigates the drag reduction effect of SHSs in combination with riblet structures through both numerical simulations and experimental investigations.

In our numerical studies, high- to intermediate-fidelity turbulence models were utilized to simulate the turbulent boundary layer on SHSs. The well-known Navier’s slip velocity method was used to model slip conditions over the SHS. To validate the numerical solutions, the slip velocity and skin friction over the SHS were compared to the experimental output. The developed numerical models were further extended to investigate the drag reduction effect of SHSs with rectangular grooves. The subsequent results showed...[ Read more ]

Superhydrophobic surfaces (SHSs) have emerged as a promising technology for achieving skin-friction drag reduction. This study investigates the drag reduction effect of SHSs in combination with riblet structures through both numerical simulations and experimental investigations.

In our numerical studies, high- to intermediate-fidelity turbulence models were utilized to simulate the turbulent boundary layer on SHSs. The well-known Navier’s slip velocity method was used to model slip conditions over the SHS. To validate the numerical solutions, the slip velocity and skin friction over the SHS were compared to the experimental output. The developed numerical models were further extended to investigate the drag reduction effect of SHSs with rectangular grooves. The subsequent results showed that the combination of superhydrophobicity and rectangular grooves led to a better performance with a maximum drag reduction of 46.1%.

In our experimental studies, we explored the drag reduction mechanism of patterned SHSs using Taylor-Couette (TC) apparatus and water tunnel flow. First, we studied the use of SHSs for frictional drag reduction in turbulent boundary layers. The reduction of frictional drag over these SHSs was measured in a water channel using a dedicated, high-resolution force measurement system. The near-wall velocity field was measured with a particle image velocimetry (PIV) system. Results revealed that the SH surface can achieve significant drag reduction if a homogeneous air film is able to form, regardless of the Reynold number. Next, we investigated the drag reduction mechanism of patterned superhydrophobic surfaces in laminar and turbulent flow (500 < Re < 1.12 x 10⁵) using a custom-made TC apparatus. The TC system experiments showed that triangular-shaped riblets without coating provided surface drag reduction in laminar flow. We also explored the combined effect of superhydrophobic coatings and triangular-shaped riblets on drag reduction. The designed surfaces maintained their superhydrophobicity and drag reduction performance under turbulent flow conditions. PIV was implemented to visualize flow patterns in TC flows, and vortex formation and flow transition were compared between smooth, flat, and grooved SHSs in laminar and turbulent regimes. Further numerical simulations in TC flow were validated with our experimental data. The flow structures and vortex behaviors of SHSs were illustrated in various flow cross-sections via RANS/LES models. the results of the research indicate that the integration of superhydrophobic coatings and V-shaped riblets holds promise for achieving substantial drag reduction across diverse flow regimes up to 48%. In addition, the use of SiO2 nanoparticles and the FAS-17/hexane solution ensures longevity and resilience to pressure-induced wetting.

When considering the findings of these two studies collectively, it becomes evident that a well-designed micro textured surface can consistently achieve a net reduction in viscous drag force in both laminar and turbulent flows. The study contributes to the knowledge of how intricate alterations in near-wall velocity profiles can lead to reduced drag force. Comprehending these changes can effectively guide the design process of internal flows, such as pipes or ducts, as well as external flows, such as ship hulls or underwater vehicles. This guidance allows for the creation of customized and optimized surfaces that lead to reduced frictional drag across the complete submerged area in both laminar and turbulent flows.