Turbulent drag reduction over air-fed hydrophobic surfaces with longitudinal grooves

This study presents an experimental investigation on the drag reduction (DR) over air-fed hydrophobic surfaces (AFHS) with longitudinal grooves in a turbulent boundary layer (TBL). The AFHS, designed with longitudinal grooves and air supplement channels, enables active maintenance and reversible restoration of the plastron in TBL. The shear stress sensor, particle image velocimetry (PIV) and interfacial visualization are applied for simultaneous measurement of the skin friction drag, TBL velocity profiles and plastron coverage. The AFHS demonstrated the ability to control plastron shape and enhance its sustainability with friction Reynolds numbers up to 1723. Drag reductions ranging from 14.8–35.8 % are obtained over the AFHS. At same designed air fraction, the AFHS exhibits higher DR than the conventional hydrophobic surface. By minimizing influences of the degradation of plastron coverage and the shape, the monotonic increase in DR and slip velocity with Reynolds number is confirmed, which corroborates trends from direct numerical simulations. Turbulence statistics measured by PIV reveal an apparent decrease in near-wall viscous shear stress, and corresponding slip velocities both in the viscous sublayer and log-law region. The Reynolds shear stress and streamwise velocity fluctuations over the AFHS are larger than those over a smooth wall, where near-wall vortex cores of the AFHS are found to be shifted 10 % towards the wall. This study presents the first simultaneous experimental quantification of skin friction, plastron coverage and turbulence statistics under sustained plastron conditions in TBL. The results demonstrate the efficacy of the plastron control strategy on hydrophobic surfaces and address a critical gap in validating numerical predictions for turbulent flows in practical applications.