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Drag Reduction in Ships Seminar Report

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). Micro-bubbles Technology is the latest development to the study of the drag reduction in ships. Almost 20-80% reduction is possible using this technology.

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