Velocimetry is the measurement of the velocity of fluids. The fluids can be either liquid or gas. This is a task often taken for granted and involves far more complex processes than one might expect. It is often used to solve fluid dynamics problems, study fluid networks, in industrial and process control applications, as well as in the creation of new kinds of fluid flow sensors. Methods of velocimetry include particle image velocimetry and particle tracking velocimetry, Molecular tagging velocimetry, laser-based interferometry, ultrasonic Doppler methods, Doppler sensors, and new signal processing methodologies.
As an illustration of how velocimetry can be done, let’s take the method of particle image velocimetry (PIV). In PIV, the fluid is seeded with tracer particles which, for sufficiently small particles, are assumed to faithfully follow the flow dynamics. The fluid with entrained particles is illuminated so that particles are visible. The motion of the seeding particles is used to calculate the speed and direction (the velocity field) of the flow being studied.
Here is a brief video explaining Particle Image Velocimetry (PIV): https://www.youtube.com/watch?v=JbuuhpQCWz8