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Fluid mechanics is the study of the static and dynamic behavior of fluids and the analysis of fluid flow.
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Fluid mechanics is a subcategory of mechanics and is defined as the study of the behavior of fluids at rest (fluid statics) or in motion (fluid dynamics), and the interaction of fluids with solids or other fluids at the boundaries. It is the analysis of fluids. Fluid mechanics may also be referred to as fluid dynamics by considering fluids at rest as a special case of motion with zero velocity.
Fluid mechanics is divided into several categories. The study of the motion of fluids that are practically incompressible (such as liquids, especially water, and gases at low speeds) is usually referred to as hydrodynamics. A subcategory of hydrodynamics is hydraulics, which is the study of liquid flows in pipes and open channels. Gas dynamics is the study of the flow of fluids that undergo significant density changes, such as the flow of gases through nozzles at high speeds. The category aerodynamics is the study of the flow of gases (especially air) over bodies such as aircraft, rockets, and automobiles at high or low speeds. Some other specialized categories such as meteorology, oceanography, and hydrology deal with naturally occurring flows. From fluid mechanics, one may get into computational fluid dynamics, or CFD.
Applications range from microscopic biological systems to automobiles, airplanes, and spacecraft propulsion. The number of fluid engineering applications is enormous: breathing, blood flow, swimming, pumps, fans, turbines, wind generators, airplanes, ships, rivers, icebergs, pipes, missiles, engines, filters, sprinklers, etc..
The essence of the subject of fluid flow is a judicious comporomise between theory and experiment.