2007 Fiscal Year Final Research Report Summary
Skin friction reduction by microbubbles at very high speeds
| Project/Area Number |
17360423
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| Research Category |
Grant-in-Aid for Scientific Research (B)
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| Allocation Type | Single-year Grants |
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| Section | 一般 |
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| Research Field |
Naval and maritime engineering
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| Research Institution | National Maritime Research Institute |
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Principal Investigator |
KODAMA Yoshiaki National Maritime Research Institute, National Maritime Research Institute, Fluids Engineering Department, Director (50373410)
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| Co-Investigator(Kenkyū-buntansha) |
HISHIDA Koichi Keio University, Faculty of Science and Technology, Department of System Design Engineering, Professor (40156592)
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| Project Period (FY) |
2005 – 2007
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| Keywords | microbubble / super high-speed range / skin friction reduction / energy saving / naval architecture |
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| Research Abstract |
Injection of small bubbles into the turbulent boundary layer developing on a solid body advancing in water is known to have significant skin faction reduction effect up to 80%. The bubbles are called microbubbles and are considered as a promising drag reduction device for ships. So far considerable research effort has been made in the low speed area, aiming at low-speed ships. However, there has been little research effort in high speed areas, and therefore this research, consisting of three approaches, has been conducted.
In the first approach, detailed mechanisms of the skin friction reduction effect of bubbles were investigated by measuring bubble deformation/movement and turbulence dynamics simultaneously in a small circulating water channel. Concerning the phenomenon that skin friction reduction effect increases with increasing Reynolds number, it has been clarified that, in case of large bubbles, the bubble deformation and its movement influences the Reynolds stresses and that, as
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the size of bubbles decreases, the Reynolds stress decreases, leading to the increase of the skin friction reduction effect.
In the second approach, skin friction reduction effect in large scales was studied a 22m-long flat plate was towed in a 400m-long towing tank at the maximum speed of 10m/s. Air was injected at the bottom at 3m from the bow end, and the local skin friction reduction effect and the reduction in the total drag was measured. There is only one another experimental data in such a large scale range, i. e., Sanders, et. al. (2007) measured local skin friction reduction on the 13m -long flat plate at the maximum speed of 18m/s in a cavitation tunnel with the 3m x 3m test section. The present result and Sanders' result compensate each other in terms of the speed range and the scale, thus giving a valuable data set on the skin friction reduction effect of air bubbles at high-speed and large scale.
In the third approach, effects of surface curvature and pressure gradient were investigated. Bubble experiments were carried out by injecting air at the bottom of a 1.8m-long flat plate with a half wing type convex body installed in a cavitation tunnel. The skin friction reduction effect by bubbles on the wing surface was significantly larger than that on the flat plate, thus clarifying significant effect of surface curvature and pressure gradient. Less
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