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In physics, physical chemistry and engineering, **fluid dynamics** is a subdiscipline of fluid mechanics that describes the **flow** of fluids—liquids and gases. It has several subdisciplines, including *aerodynamics* (the study of air and other gases in motion) and **hydrodynamics** (the study of liquids in motion). Fluid dynamics has a wide range of applications, including calculating forces and moments on aircraft, determining the mass flow rate of petroleum through pipelines, predicting weather patterns, understanding nebulae in interstellar space and modelling fission weapon detonation.

Fluid dynamics offers a systematic structure—which underlies these practical disciplines—that embraces empirical and semi-empirical laws derived from flow measurement and used to solve practical problems. The solution to a fluid dynamics problem typically involves the calculation of various properties of the fluid, such as flow velocity, pressure, density, and temperature, as functions of space and time.

Before the twentieth century, *hydrodynamics* was synonymous with fluid dynamics. This is still reflected in names of some fluid dynamics topics, like magnetohydrodynamics and hydrodynamic stability, both of which can also be applied to gases.^{[1]}

**^**Eckert, Michael (2006).*The Dawn of Fluid Dynamics: A Discipline Between Science and Technology*. Wiley. p. ix. ISBN 3-527-40513-5.