buoyancy(redirected from buoyancies)
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buoyancy(boi`ənsē, bo͞o`yən–), upward force exerted by a fluid on any body immersed in it. Buoyant force can be explained in terms of Archimedes' principleArchimedes' principle,
principle that states that a body immersed in a fluid is buoyed up by a force equal to the weight of the displaced fluid. The principle applies to both floating and submerged bodies and to all fluids, i.e., liquids and gases.
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The resultant vertical force exerted on a body by a static fluid in which it is submerged or floating. The buoyant force FB acts vertically upward, in opposition to the gravitational force that causes it. Its magnitude is equal to the weight of fluid displaced, and its line of action is through the centroid of the displaced volume, which is known as the center of buoyancy. See Aerostatics, Hydrostatics
By weighing an object when it is suspended in two different fluids of known specific weight, the volume and weight of the solid may be determined. See Archimedes' principle
Another form of buoyancy, called horizontal buoyancy, is experienced by models tested in wind or water tunnels. Horizontal buoyancy results from variations in static pressure along the test section, producing a drag in closed test sections and a thrust force in open sections. These extraneous forces must be subtracted from data as a boundary correction. Wind tunnel test sections usually diverge slightly in a downstream direction to provide some correction for horizontal buoyancy.
A body floating on a static fluid has vertical stability. A small upward displacement decreases the volume of fluid displaced, hence decreasing the buoyant force and leaving an unbalanced force tending to return the body to its original position. Similarly, a small downward displacement results in a greater buoyant force, which causes an unbalanced upward force.
A body has rotational stability when a small angular displacement sets up a restoring couple that tends to return the body to its original position. When the center of gravity of the floating body is lower than its center of buoyancy, it will always have rotational stability. Many a floating body, such as a ship, has its center of gravity above its center of buoyancy. Whether such an object is rotationally stable depends upon the shape of the body.
of a ship, the ability of a loaded ship to float in a designated position relative to the water’s surface; one of the most important features of a ship’s seaworthiness. To ensure safe operation, every vessel must have reserve buoyancy, defined as the additional weight a ship can carry and still remain afloat. Reserve buoyancy is determined by the amount of freeboard. Standards for required freeboard are established by classification societies and depend on the design of the ship and the region and season of navigation.