microbubbles, 2,3 air sheets, 4 riblets, 5 etc. However, none of them has proved practical as yet, because more energy is consumed than is saved. This paper introduces a new frictional drag reduction technique. The new technique makes use of a specially coated surface (a super-water-repellent surface) and air-injection. Its effectiveness is examined by carrying out four kinds of model tests.
A new concept in frictional drag reduction techniquesThis new frictional drag reduction technique makes use of the properties of a surface coated with a superwater-repellent paint (SWR surface). This paint is highly water repellent and capable of forming a thin air film over an underwater surface which stops the surface from becoming wet. The air film formed over the surface has the property of being able to take in air supplied from outside because of the surface tension of the water.The new technique uses this phenomenon to reduce frictional drag in ships. When air is supplied from the bow section to a ship's hull coated with SWR paint, it becomes attached to the SWR surface and forms an air film on it. The frictional drag can thus be reduced by an air lubricant effect (called an SWR & A technique).The shape of the air held in the water on a circular SWR-painted surface is shown in Fig. 1. Figure 2 shows a diagram of the behavior of air injected onto an immersed inclined surface with and without a SWR paint coating.Abstract This paper presents a new technique for reducing frictional drag using a super-water-repellent surface and airinjection (called an SWR & A technique). Its effectiveness was examined by carrying out pressure-loss measurements with a tube of rectangular cross section, along with resistance tests on a horizontal flat plate, a 7.2-m-long tanker model, and a 12-m-long high length-to-beam-ratio model ship. These test results showed that the new technique can significantly reduce the models' frictional drag; for example, the frictional resistance on the SWR surface was reduced by 80% at a speed of 4 m/s and 55% at 8 m/s.
SummaryIn the 1 st report, a new technique for reducing frictional drag using a super water repellent surface and air-injection (SWR & A method) was proposed, and its effectiveness for two-dimensional flow was confirmed by carrying out pressure loss tests in a rectangular tube and resistance tests on a horizontal flat plate.This paper presents the results of resistance tests applying the new technique to a tanker model of 7.2 m long and a high length-to-beam-ratio ship model of 12 m long. The results of these tests show that the new technique can significantly reduce frictional drag of the tanker model and the high lengthto-beam-ratio ship model, and that frictional drag on the SWR surface of the 12.0 m high length-tobeam-ratio ship model was reduced by 75% at a speed of 6 m/sec, which is the same reduction rate obtained with the flat plate.
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