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A velocimeter is a device used to measure the speed of fluids and gases. Some velocimeters measure the movements of particles that have been suspended in the fluid, while others use only the reflected light from lasers. The study of the speed of fluids and gases is called velocimetry.
A velocimeter can be used to measure two different kinds of fluid dynamics. A "Lagrangian specification of the flow field" is when particles are dropped in the fluid and their movements are observed. This is similar to watching a stick being carried by the current or caught in an eddy. A "Eulerian specification of the flow field" observes a specific point in the flow over a period of time. This is analogous to sitting on the bank and watching one bend of a river.
Particle velocimetry is when particles are introduced into the water and their speed is measured. The particles might be minute droplets of water or oil, or pellets of metal and glass. A velocimeter measures the speed of these particles by taking a photo of them at one point in their movement in the stream. Then another photo is timed to be taken a few moments later. The two photos will be compared, and the overall speed of the fluid will be determined.
The "camera" used to take the particles' photos is a laser beam. Two lenses flatten and widen the beam in order to create a short flash, which is then analyzed to create a two-dimensional, or 2D, image. Lasers are used because the particles are fast-moving and microscopic. A normal camera lens cannot open and shut quickly enough to capture their movements.
Another type of velocimeter is called the Doppler velocimeter. In this type of velocimetry, the reflections of light are measured instead of particles. The velocimeter directs a laser beam over a patient's arteries, for example, and the way the light reflects off the moving blood will tell the technician how well the blood is circulating. This type of measurement comes from the Doppler effect, when light's velocity can be judged by measuring the frequency of a wave relative to the observer.
Velocimetry has many practical applications. A velocimeter can determine the "swirl" of gases and fluids in combustion engines, kilns, spray driers, and chemical processing plants. Knowledge of how the fluids move in machines helps designers plan against wear and tear and diagnose problems and the overall practicality of the system. Because laser velocimetry is non-invasive, it can also be used as part of medical procedures.