In compressible fluid dynamics, impact pressure is the difference between total pressure (also known as pitot pressure

Pitot tube

A pitot tube is a pressure measurement instrument used to measure fluid flow velocity. The pitot tube was invented by the French engineer Henri Pitot Ulo in the early 18th century and was modified to its modern form in the mid-19th century by French scientist Henry Darcy...

 or stagnation pressure

Stagnation pressure

In fluid dynamics, stagnation pressure is the static pressure at a stagnation point in a fluid flow.At a stagnation point the fluid velocity is zero and all kinetic energy has been converted into pressure energy . Stagnation pressure is equal to the sum of the free-stream dynamic pressure and...

) and static pressure

Static pressure

In fluid mechanics the term static pressure has several uses:* In the design and operation of aircraft, static pressure is the air pressure in the aircraft’s static pressure system....

. In aerodynamics

Aerodynamics

Aerodynamics is a branch of dynamics concerned with studying the motion of air, particularly when it interacts with a moving object. Aerodynamics is a subfield of fluid dynamics and gas dynamics, with much theory shared between them. Aerodynamics is often used synonymously with gas dynamics, with...

 notation, this quantity is denoted as or .

When input to an airspeed indicator, impact pressure is used to provide a calibrated airspeed

Calibrated airspeed

Calibrated airspeed is the speed shown by a conventional airspeed indicator after correction for instrument error and position error. Most civilian EFIS displays also show CAS...

 reading. An air data computer

Air data computer

An air data computer is an essential avionics component found in modern glass cockpits. This computer, rather than individual instruments, can determine the calibrated airspeed, Mach number, altitude, and altitude trend from input data from sensors such as an aircraft's pitot-static system,...

 with inputs of pitot and static pressures is able to provide a Mach number

Mach number

Mach number is the speed of an object moving through air, or any other fluid substance, divided by the speed of sound as it is in that substance for its particular physical conditions, including those of temperature and pressure...

 and, if static temperature is known, true airspeed

True airspeed

True airspeed of an aircraft is the speed of the aircraft relative to the airmass in which it is flying. True airspeed is important information for accurate navigation of an aircraft.-Performance:...

.

Some authors in the field of compressible flows use the term dynamic pressure or compressible dynamic pressure instead of impact pressure.

Isentropic flow

In isentropic flow the ratio of total pressure

Stagnation pressure

In fluid dynamics, stagnation pressure is the static pressure at a stagnation point in a fluid flow.At a stagnation point the fluid velocity is zero and all kinetic energy has been converted into pressure energy . Stagnation pressure is equal to the sum of the free-stream dynamic pressure and...

 to static pressure

Static pressure

In fluid mechanics the term static pressure has several uses:* In the design and operation of aircraft, static pressure is the air pressure in the aircraft’s static pressure system....

  is given by:



where:

is total pressure

is static pressure

is the ratio of specific heats

Heat capacity ratio

The heat capacity ratio or adiabatic index or ratio of specific heats, is the ratio of the heat capacity at constant pressure to heat capacity at constant volume . It is sometimes also known as the isentropic expansion factor and is denoted by \gamma or \kappa . The latter symbol kappa is...

is the freestream Mach number
Taking to be 1.4, and since

Expressing the incompressible dynamic pressure as and expanding by the binomial series

Binomial series

In mathematics, the binomial series is the Taylor series at x = 0 of the function f given by f =  α, where is an arbitrary complex number...

gives:


where:

is dynamic pressure