Stagnation pressure


Stagnation pressure

In fluid dynamics, stagnation pressure is the pressure at a stagnation point in a fluid flow, where the kinetic energy is converted into pressure energy. It is the sum of the dynamic pressure and static pressure at the stagnation point. [ [http://scienceworld.wolfram.com/physics/StagnationPressure.html Stagnation Pressure] at Eric Weisstein's World of Physics (Wolfram Research)]

Pitot tubes are used to measure stagnation (or total) pressure. A combined pitot-static tube is used on aircraft to determine flight speed. Stagnation quantities (e.g., stagnation temperature, stagnation pressure) are also frequently used in jet engine performance calculations.

Definition

The definition for stagnation pressure can be derived from the Bernoulli Equation. [ [http://www.engineeringtoolbox.com/bernouilli-equation-d_183.html Equation 4] , Bernoulli Equation - The Engineering Toolbox] For incompressible flow,

Stagnation (Total) Pressure = Dynamic Pressure + Static Pressure

or

P_ ext{stagnation}=frac{1}{2} ho v^2 + P_ ext{static}

(Only if Bernoulli's conditions are met.)

This definition is not valid for transonic or supersonic flow. The stagnation pressure may still be defined;see below. For many purposes, in transonic flow, the stagnation enthalpy or stagnation temperature plays a role similar to the stagnation pressure in incompressible flow.

Thermal definition

It is the pressure a fluid retains when brought to rest isentropically from Mach number "M". [ [http://www.grc.nasa.gov/WWW/K-12/airplane/Images/naca1135.pdf Equations 35,44] , Equations, Tables and Charts for Compressible Flow]

frac{p_t}{p} = left(1 + frac{gamma -1}{2} M^2 ight)^{frac{gamma}{gamma-1,

or, assuming an isentropic process, the stagnation pressure can be calculated from the ratio of stagnation temperature to static temperature:

frac{p_t}{p} = left(frac{T_t}{T} ight)^{frac{gamma}{gamma-1,

where:

p_t =, stagnation (or total) pressure

p =, static pressure

T_t =, stagnation (or total) temperature in kelvins

T =, static temperature in kelvins

gamma =, ratio of specific heats

The above derivation holds only for the case when the fluid is assumed to be calorically perfect. For such fluids, specific heats and gamma are assumed to be constant and invariant with temperature (See also, a thermally perfect fluid).

See also

*Stagnation point
*Dynamic Pressure
*Static Pressure
*Pitot tube
*Hydraulic Ram

References

External links

* [http://www.spaceagecontrol.com/pm/uploads/Main.Freepubs/ADA320216.pdf Pitot-Statics and the Standard Atmosphere]
* [http://www.spaceagecontrol.com/pm/uploads/Main.Litroom2/naca-tn-616.pdf The Measurement of Air Speed in Airplanes]


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