Experimental study of air lubrication with fluidic oscillators for flat plate drag reduction

• Novel air lubrication using fluidic oscillators for flat plate drag reduction. • Up to 21.2 % drag reduction achieved at low towing velocities. • Drag reduction plateaus for air layer thicknesses beyond 10 mm. • Oscillator frequency increases with air flow rate and towing velocity. The frictional drag reduction by air lubrication on a 7 m-long and 1.1 m-wide flat plate was investigated in a towing tank at four towing velocities (3 m/s–8.23 m/s). Air was injected through fluidic oscillators installed on the underside of the plate with air flow rates ranging from 0 L/s to 80 L/s, producing air-layer thicknesses of 0 m m t o 16 m m . Maximum drag reduction of 21.2 % was achieved at 3 m/s (10.6 % at 8.23 m/s). Results indicate a drop-off in further drag reduction improvement with air layer thickness beyond 10 mm. Videos of the air topology revealed an initial air layer film immediately downstream of the air injector at low velocities, which transitions to a dispersed bubble layer. The videos also showed side vortices appearing at higher velocities. The oscillator frequency of the fluidic oscillators was shown to increase with air flow rate and with higher towing velocity.