PIV

Particle image velocimetry is an optical measurement technique that captures the movement of tiny tracer particles suspended in a fluid. A pulsed laser illuminates a thin plane through the flow, and a high-speed camera records the particle positions in successive frames separated by a known time interval, typically 15 to 100 microseconds apart. Software then compares particle locations between frames to compute local fluid velocity at thousands of points simultaneously.

The method relies entirely on seeding the flow with neutrally buoyant particles, usually 1 to 100 micrometers in diameter, made from materials like polystyrene spheres or silver-coated glass. These tracers must follow the fluid faithfully without settling or clustering. The illumination sheet, formed by passing the laser beam through cylindrical optics, is kept thin, often just 1 to 2 millimeters, to ensure particles remain in focus and to isolate a two-dimensional velocity field.

PIV excels at resolving complex flow patterns: vortex cores in mixing jets, boundary layer separation near aircraft wings, or turbulence statistics in pipe flow. It provides whole-field velocity data rather than point measurements, making it invaluable for validation of computational fluid dynamics models. However, the technique struggles in opaque fluids, in flows with very high particle concentrations that scatter light unpredictably, and in situations where tracer particles cannot be introduced or cannot match the fluid density.

Variants and setup considerations

Stereo PIV uses two cameras at different angles to capture three velocity components from a single measurement plane. Volumetric PIV, or tomographic PIV, reconstructs velocity in a three-dimensional region using multiple camera views and tomographic reconstruction. Time-resolved PIV acquires images at kilohertz rates to capture unsteady flow dynamics frame-by-frame, though this demands extreme computing power for processing. In contrast, standard PIV captures two frames per trigger pulse and produces statistical results from many trigger events.

The name reflects the core physics: particle trajectories encode fluid motion. The velocimetry part refers to the velocity measurement; image indicates the optical detection method. PIV originated in the 1980s in research laboratories and is now standard in aerodynamics, hydraulics, and thermal-hydraulic testing, though its cost and setup complexity keep it primarily within experimental research rather than on-site industrial diagnostics.