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The engineering science concerned with the relation between the forces acting on fluids (liquids and gases) and their motions, and with the forces caused by fluids on their surroundings. It is distinct from solid mechanics by virtue of the different responses of fluids and solids to applied forces. In an ideal elastic solid, the deflection or deformation is proportional to the applied stress, whereas a fluid cannot support an applied shear stress unless it is in motion. In most fluids, called simple or newtonian fluids, it is the rate of deformation of the fluid, as opposed to the amount of deformation in a solid, that is proportional to the applied stress. See Fluid flow, Newtonian fluid
Many substances of everyday experience and of engineering importance are found naturally in the fluid state. These include water (liquid and vapor), air (gaseous and liquid), as well as other liquids and gases of natural and industrial importance. The most common fluids are newtonian under most flow conditions.
Fluid mechanics treats the fluid as a continuum, ignoring the fact that it actually consists of individual molecules that may be, in the case of gases, widely spaced compared to molecular dimensions. Nevertheless, the continuum assumption is valid for almost all applications down to the size of bacteria. An exception occurs with gases at very low densities, such as exist in the uppermost regions of the atmosphere. At extremely high altitudes the mean free paths of air molecules—that is, the distances they travel between collisions in random thermal motion—can become as large, or even larger than, the dimensions of a space vehicle, making the assumption of a continuum invalid. See Rarefied gas flow
Fluid mechanics is of fundamental importance to a number of disciplines, including aerospace, chemical, civil, environmental, mechanical, and ocean engineering, as well as to climatology, geology, meteorology, and oceanography. Applications in these fields include, but are not limited to, the study of fluid forces acting on vehicles; flows in natural rivers and artificial channels and the flow of ground water; the dispersion of pollutants in the atmosphere, lakes, rivers, and oceans; the flows in the circulatory and pulmonary systems of humans and animals; the flows in pipelines that carry crude oil and natural gas over many hundreds, or even thousands, of miles from the petroleum fields of their origin to deep-water ports or refineries; the flow of molten plastics or metals filling molds in the manufacture of numerous solid parts; the flow in pumps for water distribution systems; and both hydraulic and gas turbines for power generation and propulsion. Fluid mechanics forms the basis for much of chemistry and physics, and is sometimes applied to such apparently remote fields as cosmology. The fluid mechanical behavior of gases and liquids plays an important role in the dispersion of dissolved or entrained substances. See Aerodynamic force, Aerodynamics, Hydrodynamics