velocimeter

A velocimeter is an instrument for measuring velocity. It can determine the speed of a moving object, the velocity of a fluid at a point, or the velocity field within a flow. The term encompasses devices used in engineering, physics, meteorology, oceanography, and astronomy, among other fields.

In fluid dynamics, velocimeters can be categorized as point measurements or flow-field instruments. Point velocimeters include Pitot tubes, which relate stagnation pressure to flow speed, and hot-wire or hot-film anemometers, which infer velocity from changes in heat transfer. Ultrasonic velocimeters use the Doppler effect or transit-time measurements to determine flow speed in pipes or open channels. Laser-based methods such as Laser Doppler Velocimetry (LDV) and Particle Image Velocimetry (PIV) provide more detailed information: LDV measures the velocity of seed particles at a point by interference of laser beams, while PIV captures velocity fields by tracking particle displacements in successive images.

In addition to laboratory and industrial applications, acoustic Doppler and ultrasonic techniques are widely used to study ocean currents and river streams, as well as to monitor blood flow in medical contexts. In astronomy and astrophysics, a radial-velocity velocimeter is a spectroscopic instrument that measures the line-of-sight velocity of celestial objects by detecting Doppler shifts in their spectra, a method central to detecting exoplanets and studying stellar motions.

Key considerations for velocimetry include velocity range, spatial and temporal resolution, the need for seeding particles or reflective features, calibration requirements, and susceptibility to flow disturbances or turbulence. Each method offers trade-offs between invasiveness, accuracy, and complexity.