Transient velocity profiles and drag reduction due to air-filled superhydrophobic grooves

This paper presents an experimental study of horizontal channel flow with air-filled superhydrophobic grooves. Air-water interfaces in the grooves are visualized in a range of the channel Reynolds number, Re, (2000 <= Re <= 5000) while flow characteristics are evaluated using particle tracking velocimetry measurements at Re=3000 and 4000. Near the air-water interface in the superhydrophobic groove, turbulent eddies and hence the Reynolds shear stress appreciably attenuate owing to a lack of energy supply through the interface, and it takes a notable distance for the Reynolds shear stress to recover downstream of the groove. Additionally, a secondary cross-flow from the solid surface region between two grooves towards the air-water interface appears and sweeps eddies between the grooves towards the interface. The decay of turbulent eddies and the sweeping of eddies from the solid surface to the air-water interface reduce friction drag both in and immediately downstream of the grooved region.