Tube of Pitot

A tube of Pitot is a measuring device of Speed fluid .

History

The tube of Pitot owes his name with the physicist French Henri Pitot (1695 - 1771) which was the first in 1732 to propose a “machine to measure the speed of running water and the wake of the vessels”. The concept is taken again and improved by Henry Darcy then by Ludwig Prandtl which thinks of using the tube in a drain to measure local speeds of flow of the fluids.

In Aeronautical from the technological point of view, he is the successor of the system Etévé. The tube of Pitot is a component of the anemobarometric system.

Principle

Use in aeronautics

It consists of two concentric bent tubes of which the openings, in communication with the fluid of which one wants to measure speed, are laid out in a particular way.

One, placed orthogonally, has a relative speed v equal at the speed of the fluid and a static pressure p_s equal to the environmental pressure.
the other, placed in the direction of the flow, has a null relative speed and a total pressure p_t , sum of the dynamic Pression and static pressure .

The difference between these pressures gives the Speed air which one can deduce the Ground speed, these two parameters being paramount information for the pilot who needs some to calculate his displacement in space (Navigation) and the consumption of his vehicle.

It is then enough to apply the Théorème of Bernoulli by neglecting term Z to have a direct relationship between speed and the dynamic pressure p_t - p_s which one measures with a pressure pick-up or simple a Manomètre.

${v^2 \ over 2} + {p_s \ over \ rho} = 0 + {p_t \ over \ rho}$

from where

{v^2} = {2 (p_t - p_s) \ over \ rho}

v = speed

p = Pressure in control (p_s is the static pressure, p_t is the total pressure)

ρ = Density of the fluid

Remarks

Note that the dynamic pressure is not a real pressure like the east the static pressure. Indeed, the force which the dynamic pressure can exert on a surface depends on the flow of the fluid compared to this one. The fact of naming the product $1/2 \ rho v^2$ a pressure is only one abuse language which led to many errors always conveyed nowadays. This product corresponds in fact to the kinetic energy of the particle of unit fluid of volume animated speed v.

In conclusion, it is important to retain that the pressure pick-up does not measure the difference between a total pressure and a static pressure but well between two static pressures which let us recall it once again is the only one to exert a force on surfaces.

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