Editing Fluid dynamics (section)

== Terminology ==
The concept of pressure is central to the study of both fluid statics and fluid dynamics. A pressure can be identified for every point in a body of fluid, regardless of whether the fluid is in motion or not. Pressure can be [[Pressure measurement|measured]] using an aneroid, Bourdon tube, mercury column, or various other methods.

Some of the terminology that is necessary in the study of fluid dynamics is not found in other similar areas of study. In particular, some of the terminology used in fluid dynamics is not used in [[fluid statics]].

=== Characteristic numbers ===
{{excerpt|Dimensionless numbers in fluid mechanics}}

=== Terminology in incompressible fluid dynamics ===
The concepts of total pressure and [[dynamic pressure]] arise from [[Bernoulli's equation]] and are significant in the study of all fluid flows. (These two pressures are not pressures in the usual sense—they cannot be measured using an aneroid, Bourdon tube or mercury column.) To avoid potential ambiguity when referring to pressure in fluid dynamics, many authors use the term [[static pressure]] to distinguish it from total pressure and dynamic pressure. [[Static pressure]] is identical to pressure and can be identified for every point in a fluid flow field.

A point in a fluid flow where the flow has come to rest (that is to say, speed is equal to zero adjacent to some solid body immersed in the fluid flow) is of special significance. It is of such importance that it is given a special name—a [[stagnation point]]. The static pressure at the stagnation point is of special significance and is given its own name—[[stagnation pressure]]. In incompressible flows, the stagnation pressure at a stagnation point is equal to the total pressure throughout the flow field.

=== Terminology in compressible fluid dynamics ===
In a compressible fluid, it is convenient to define the total conditions (also called stagnation conditions) for all thermodynamic state properties (such as total temperature, total enthalpy, total speed of sound). These total flow conditions are a function of the fluid velocity and have different values in frames of reference with different motion.

To avoid potential ambiguity when referring to the properties of the fluid associated with the state of the fluid rather than its motion, the prefix "static" is commonly used (such as static temperature and static enthalpy). Where there is no prefix, the fluid property is the static condition (so "density" and "static density" mean the same thing). The static conditions are independent of the frame of reference.

Because the total flow conditions are defined by [[isentropic]]ally bringing the fluid to rest, there is no need to distinguish between total entropy and static entropy as they are always equal by definition. As such, entropy is most commonly referred to as simply "entropy".