Drag reduction promoted by repetitive bubble injection in turbulent channel flows

• Effects of repetitive bubble injection for drag reduction have been examined.
• The repetitive method greatly improved drag reduction even at low void fractions.
• Reproducible bubble swarms followed by air films were main factors in improvement.
• Turbulence modification realized using the method has been measured and discussed.

To promote the efficiency of frictional drag reduction using bubbles, we designed a novel bubble control method that involves repetitive injection of bubbles rather than the conventional continuous bubble injection approach. Even if the mean void fraction of bubbles to be injected into the turbulent boundary layer is set to be low, repetitive bubble injection (RBI) maintains the frictional drag reduction by generating bubble swarms. The enhanced drag reduction mechanism and the effectiveness of the RBI approach are investigated by studying wall shear stress and the velocity vector field in the liquid phase when measured in a turbulent horizontal channel flow. The bubble swarms generated by RBI consist of bubbles of various sizes with leading large air films, which have high reproducibility. The leading air films, which are a result of the concentrated void fraction, maintain a high drag reduction effect by air lubrication and by suppression of Reynolds shear stress events in the turbulent vortical structures beneath the bubble swarm. The latter effect of RBI in particular plays a significant role at higher Reynolds numbers. Based on the combination of these effects, we confirmed that RBI provides an extra drag reduction effect when compared with continuous bubble injection.