The effects of drag reduction by microbubble injection were assessed using long ship models with flat bottoms. The local skin friction and the total drag of 8 m, 20 m, and 40 m long slender ship models were measured while injecting microbubblesfrom porous plate during towing, and the microbubbles were observed by CCD camera. Drag reduction was obtained by injecting microbubbles in all experimental cases, and the maximum drag reduction was about 60% in frictional drag on the part of the flatbottom covered by microbubbles. Reduction of local skin friction persisted for about 40 m downstream from the injection point, although the reduction decreased with distance.
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