velocimetry

Velocimetry is the measurement of velocity in fluids or gases. In practice, velocimetry methods determine how fast and in what direction the flow is moving, often producing spatially resolved velocity fields or time series at specific points. Velocimetry techniques are typically non-intrusive, using optical or acoustic signals and tracer particles or natural scattering to infer motion.

Common methods include particle-image velocimetry (PIV), particle-tracking velocimetry (PTV), and laser Doppler velocimetry (LDV). PIV involves seeding the flow with tracer particles, illuminating a plane with a laser sheet, and recording images with cameras. Displacements of particle patterns between image pairs are analyzed to yield velocity vectors over the plane; stereo configurations extend this to three dimensions, and tomographic variants reconstruct full three-dimensional fields. PTV follows individual particle trajectories over time to build velocity fields or statistics, requiring higher seeding density and frame rates in three dimensions.

LDV, in contrast, uses interference of two laser beams to create a measurement volume. The Doppler shift of light scattered by particles passing through the volume provides a precise velocity component along the beam, delivering high temporal resolution at a single point.

Doppler ultrasound velocimetry applies similar Doppler principles to biological and medical contexts, measuring blood or flow velocities non-invasively. Other techniques, such as hot-wire anemometry, offer point velocity measurements in laboratory settings but are more intrusive.

Applications span aerospace and automotive testing, turbulence research, wind and environmental studies, process monitoring in industry, and cardiovascular flow analysis. Each method balances spatial and temporal resolution, measurement volume, and invasiveness according to the research or engineering goal.