In general, pressure
is a measure of the force exerted
per unit area on the boundaries of a substance. In fluid dynamics and aerodynamics, stagnation pressure
(or pitot pressure
or total 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 (isentropically). This effect is widely used in aerodynamics (velocity measurement or ram-air intake).
Stagnation pressure is equal to the sum of the free-stream dynamic pressure and free-stream static pressure.
Static pressure and dynamic pressure are terms of Bernoulli’s equation:
The Bernoulli’s effect causes the lowering of fluid pressure (static pressure – p) in regions where the flow velocity is increased. This lowering of pressure in a constriction of a flow path may seem counterintuitive, but seems less so when you consider pressure to be energy density. In the high velocity flow through the constriction, kinetic energy (dynamic pressure – ½.ρ.v2) must increase at the expense of pressure energy (static pressure – p).
The simplified form of Bernoulli’s equation can be summarized in the following memorable word equation:
static pressure + dynamic pressure = total pressure (stagnation pressure)
Total and dynamic pressure are not pressures in the usual sense – they cannot be measured using an aneroid, Bourdon tube or mercury column.
Stagnation pressure is sometimes referred to as pitot pressure because it is measured using a pitot tube. A Pitot tube is a pressure measurement instrument used to measure fluid flow velocity. Velocity can be determined using the following formula:
- u is flow velocity to be measured in m/s,
- ps is stagnation or total pressure in Pa,
- pt is static pressure in Pa,
- ρ is fluid density in kg/m3.