## Stagnation Pressure – Pitot Pressure

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.v^{2}) 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:

where:

- u is flow velocity to be measured in m/s,
- p
_{s }is stagnation or total pressure in Pa, - p
_{t}is static pressure in Pa, - ρ is fluid density in kg/m
^{3}.

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