The skin friction reduction effect of microbubbles in various flow conditions are reviewed. At location immediately downstream from an air injection point, the reduction does not depend on the flow speed, but at further downstream, it becomes smaller at higher flow speed, and decays with downstream distance. Experiments in a towing tank using a very long flat plate show that the reduction persists up to nearly 50m. In a full scale experiment using a 105m-long ship, 5.5% drag reduction was obtained. In case the bubbles are away from the wall, the skin friction increases. An optical technique has shown that bubbles in a boundary layer increase velocity fluctuations but decrease the Reynolds shear stress. Three possible skin friction reduction mechanisms are listed.

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