Fluids are substances in a state that continually changes shape (flow) when force or pressure is applied, and are often synonymous with liquids, although fluids can also be plasmas or gases. Thus, fluid flow measurement is the quantification of its bulk movement and is important to many discovery, evaluative and monitoring studies. Furthermore, a wide variety of methods are available to measure fluid flow, where the suitability of each method depends on the nature of the fluid and aim of the study. A few are listed below.
Pressure Based Flow Measurement
These methods are based on Bernoulli's principle and they either measure the differential pressure within a constriction, or measure the static and stagnation pressures to derive the dynamic pressure, would directly indicate changes in fluid flow.
Ultrasound Transit-time Measurement
Liquid flow measurements can be made by using wide beam ultrasonic illumination and measuring the difference between the transit times of ultrasonic pulses propagating in and against the direction of flow. This difference in the integrated transit times is a measure of true volume flow.
Laser Doppler Flow Measurement
This method focuses a beam of laser light into an area of flowing fluid containing small particles (naturally occurring or induced). The particles will scatter the light with wavelength changes, which are proportional to the particle's speed (Doppler effect), and its quantification can be used to directly to determine the speed of the particle, and thus closely approximate the fluid velocity.