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1. INTRODUCTION
Drag is a mechanical force generated by a solid object moving through a fluid. For drag to be generated, the solid body must be in contact with the fluid. If there is no fluid, there is no drag. Drag is generated by the difference in velocity between the solid object and the fluid. There must be motion between the object and the fluid. If there is no motion, there is no drag. Drag acts in the direction opposite to the direction of motion of the body.
A body moving through a fluid experiences a drag force, which is usually divided into two components: frictional drag (sometimes called viscous drag) and pressure drag (sometimes called form drag or profile drag).
Frictional drag comes from friction between the fluid and the surfaces over which it is flowing. This friction is associated with the development of boundary layers. Pressure drag comes from the eddying motions that are set up in the fluid by the passage of the body. This drag is associated with the formation of a wake, which can be readily seen behind a passing boat. When the drag is dominated by viscous drag, we say the body is streamlined, and when it is dominated by pressure drag, we say the body is bluff. Whether the flow is viscous-drag dominated or pressure-drag dominated depends entirely on the shape of the body. A streamlined body looks like a fish, or an airfoil at small angles of attack, whereas a bluff body looks like a brick, a cylinder, or airfoil at large angles of attack. For streamlined bodies, frictional drag is the dominant source of air resistance. For a bluff body, the dominant source of drag is pressure drag. For a given frontal area and velocity, a streamlined body will always have a lower resistance than a bluff body. For example, the drag of a cylinder of diameter $D$ can be ten times larger than a streamlined shape with the same thickness (see figure 1).
Microbubbles Technology is the latest development to the study of the drag reduction in ships. Almost 20-80% reduction is possible using this technology.
Why microbubbles?
Microbubbles is a drag reduction device that reduces skin friction of a solid body moving in water by injecting small bubbles into the turbulent boundary layer developing on the solid body. But at the same time the energy needed for injecting bubbles at he hull bottom is not small because large ships have large water depth against which the bubbles have to be injected. Therefore it is important to reduce the amount of injected air in order to put microbubbles into practical use.
Fig.1 shows an example of its skin friction reduction effect. The data was a circulating water tunnel, where the bubbles were injected at the top flat wall and skin friction was measured by a skin friction sensor placed downstream of the injection point. The horizontal axis shows the amount of injected air and the vertical axis shows the ratio of reduced skin friction to that at non-bubble condition. This figure shows that, as the amount of injected air increases, skin friction reduction effect by microbubbles increases up to 80%.
Ships such as tankers play a major role in marine transportation. They are very large and move very slowly. They are especially suited to microbubbles. Fig.2 shows an image of the application of microbubbles to such a ship. One reason that they are suited is that their skin friction drag component occupies about 80% of the total drag. The drag of a ship that moves on the water consists of two components, i.e., wave-making drag and skin frictional drag.
The wave-making drag component of such a ship is very small because they move very slowly. Another reason that they are suited is in their shape. Their shape is like a box, except for bow and stern regions. They have a wide flat bottom, and the bubbles injected at the bottom near the bow stay close to the hull bottom by buoyancy while they are carried by flow all the way to the stern. Thus the injected bubbles can cover the whole hull bottom efficiently. In Japan, microbubbles have been studied intensively in the past few years toward its application to full-scale ships. This paper reports a review of such studies, focusing on those carried out by our group in NMRIJ.