A velocimeter measures the velocity of fluids and gases using either particles or light reflections. Particle velocimetry uses a laser beam to take pictures of particles in motion, while Doppler velocimetry measures reflections of light. Velocimetry has practical applications in various industries and medical procedures.
A velocimeter is a device used to measure the velocity of fluids and gases. Some velocimeters measure the motions of particles that have been suspended in the fluid, while others use only light reflected from lasers. The study of the velocity of fluids and gases is called velocimetry.
A velocimeter can be used to measure two different types of fluid dynamics. A “Lagrangian flow-field specification” is when particles are dropped into the fluid and their motions are observed. This is similar to watching a stick carried by the current or caught in a whirlpool. A “Eulerian flow field specification” observes a specific point in the flow over a period of time. This is analogous to sitting on the bank and watching a bend in a river.
Particle velocimetry is when particles are introduced into water and their velocity is measured. The particles could be tiny droplets of water or oil or pellets of metal and glass. A velocimeter measures the speed of these particles by taking a picture of them at one point of their movement in the stream. Then another photo is scheduled to be taken moments later. The two photos will be compared and the overall velocity of the fluid will be determined.
The “camera” used to take pictures of the particles is a laser beam. Two lenses flatten and spread the beam to create a short flash, which is then analyzed to create a two-dimensional, or 2D, image. Lasers are used because the particles move fast and are microscopic. A regular camera lens can’t open and close fast enough to capture their movements.
Another type of velocimeter is called a Doppler velocimeter. In this type of velocimetry, reflections of light are measured instead of particles. The velocimeter directs a laser beam onto a patient’s arteries, for example, and how the light reflects off moving blood will tell the technician how well the blood is circulating. This type of measurement results from the Doppler effect, when the speed of light can be estimated by measuring the frequency of a wave relative to the observer.
Velometry has many practical applications. A velocimeter can determine the “swirl” of gases and fluids in combustion engines, furnaces, atomizers and chemical processing plants. Knowledge of how fluids move in machines helps designers plan against wear and diagnose problems and overall system serviceability. Because laser velocimetry is non-invasive, it can also be used as part of medical procedures.
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