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Turbulent drag reduction via interactions of deformable air films over super-hydrophobic surfaces
Journal article   Peer reviewed

Turbulent drag reduction via interactions of deformable air films over super-hydrophobic surfaces

Minh Gia Nguyen, Mohammad Mohammadzadeh Sanandaji, Skinder Ali Dar, Mohammad Ikram Haider, Hongtao Ding and Cong Wang
Experimental thermal and fluid science, Vol.175, 111722
05/2026
DOI: 10.1016/j.expthermflusci.2026.111722

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Abstract

•Investigated drag reduction of deformable air-films over superhydrophobic surfaces in turbulent boundary layers.•Used novel Lagrangian-type pathline visualization and high-fidelity PIV to characterize non-canonical turbulent flows.•Streamwise strips achieved 28% drag reduction, while transverse strips caused a 71% drag increase.•Developed a wall-shear estimation method using total shear extrapolation as the traditional log-law proved unreliable.•Shown that air-film deformation suppresses or enhances turbulence based on surface pattern and orientation. Turbulent drag reduction remains a fundamental challenge for both aerial and ocean transportation. This study investigates the drag reduction effect of passively sustained deformable air-films or plastrons over patterned superhydrophobic surfaces (SHSs) in turbulent boundary layers (TBLs). SHSs with varied surface patterns and orientations, including uniform coating, streamwise strips, and transverse strips, were tested in TBLs over a Reynolds number range of 3.2×105 to 8.6×105. These compliant air-films, characterized by high Weber numbers, exhibited dynamic deformations and strong coupling interactions with turbulent shear flows. These distinct effects are first qualitatively shown by a novel Lagrangian-type flow pathline visualization, which depicts suppressed or enhanced turbulence depending on the air-film pattern and orientations. High-fidelity time-resolved particle image velocimetry (PIV) was used to characterize such non-canonical TBLs and evaluate the associated drag reduction effects. The presence of deformable air-films led to reduced near-wall velocity and suppressed Reynolds shear stress (RSS), indicating effective drag reduction. Traditional wall shear stress estimation methods like the Charted Clauser method proved unreliable for these free-slip, deformable interfaces, due to the absence of log layer of canonical TBLs. The wall shear stress was instead estimated by extrapolating the total shear stress near the wall. The highest estimated drag reduction percentage was 28% for the streamwise strip pattern, 12% for the uniform surface, and −71% (drag increase) for transverse strip pattern, consistent with the flow visualization and PIV results. Additionally, an analysis of the total momentum flux across entire TBLs demonstrates a similar drag reduction trend, though the effect weakened at higher Reynolds numbers. The reported results contribute to a deeper understanding of turbulence control using non-canonical deformable boundaries, enabling sustainable drag reduction applications in hydrodynamic environments.
 Air–water interface Drag reduction Superhydrophobic surfaces Turbulent flow

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