Local measurement of velocity in a fluid

Fluid velocity measurement is found in both industrial and environmental applications. Instruments are used to record fluid displacements in confined conditions, in pipes, or in the open field. Other instruments are used to check that fluid velocity does not exceed or fall short of a critical safety threshold. The choice of instrument must therefore be made wisely, all the more so as velocity measurement is a delicate operation insofar as the introduction of an instrument into a flow generates a disturbance that can lead to an error in velocity estimation.

This means that measurement conditions can vary widely. For measurements in ducts, for example, velocities of the order of 10 m.s^-1 are easily encountered for air that may be polluted and at high temperature in chimney ducts, clean in ventilation ducts, or even contaminated in nuclear environments. In the case of open-field measurements, very low velocities of less than 1 m.s^-1 may be encountered when qualifying an office working environment, or monitoring a critical threshold in a paint booth or extractor hood. High speeds, in excess of 20 m.s^-1, can also be encountered when qualifying the aeraulic field in a wind farm, the flow of a river or the speed of an aircraft.

This article presents the principle of the main instruments used to measure fluid velocity, along with their characteristics, including field of application and influencing factors. Some technologies, such as pressure probes and thermal anemometers, are relatively old. Pressure probes were the first instruments to measure fluid velocity. Thermal anemometers enabled the first research into turbulence phenomena. Other technologies, such as ultrasonic anemometers and laser Doppler velocimeters, are much more recent and have benefited from advances in all technical fields. The specific features of each type of instrument enable us to provide the best possible response to each industrial problem.