Digital Particle Image Velocimetry (DPIV) is a planar optical measurement technique which provides a means of measuring the velocity of seeded particles (~1 - 50 microns (in air)) in the flow of interest over a plane using a charge-coupled device (CCD) camera. The measurement area within the flow field is defined by the position and physical dimension of a fan of laser light. With the illumination of two short duration laser flashes in the measurement area, a double-exposure of the flow field is captured (through the CCD camera to the frame grabber card) on the computer. Once the spatially displaced images are stored in two separate frames, each velocity vector is extracted by performing mathematical correlation analysis on a cluster of particles within each interrogation region between the two frames (eliminating the problem of directional ambiguity). The PIV interrogation process is repeated until all the required velocity information is extracted from the captured record.   There should be a high enough seeding concentration of particle images within the interrogation window to minimise any potential bias  error arise from data drop out  (See Westerweel 2000).

In general terms, particle image velocimetry (PIV) is the low particle concentration equivalent of Laser Speckle Velocimetry (LSV) and developed from speckle interferometry which is used to measure surface displacements in solid mechanics. 

The name, Particle Image Velocimetry (PIV), first appeared in literature in the early eighties. Adrian (US) - 1984 and Pickering and Halliwell (UK) -1984 were among the first to point out that seeding densities as high as those required in the speckle mode for Laser Speckle Velocimetry (LSV) are rarely achieved in practice. They therefore argued that the low seeding density mode of LSV should be more appropriately classified as PIV. 

However, unknown to the PIV community, as early as in 1971, a paper published in a journal entitled  "An Automated Technique for Obtaining Cloud Motion from Geosynchronous Satellite Data Using Cross Correlation" by J.A. Leese et al. Journal of Applied Meteorology Vol. 10. pp.118-132, had clearly demonstrated the validity of cross-correlation techniques as a whole flow field motion tracker in a massive scale! 

DPIV is now regarded by many as the method of choice in measuring velocity data for code validation in Computational Fluid Dynamics (CFD).  The information generated is what we would call Quantitative Flow Visualisation.

The DPIV technique (Stereoscopic PIV) can be extended to measure the out-of-plane velocity component with an additional camera, frame grabber and some optics that use stereoscopic imaging techniques. 
The attractiveness of this technique is in the generation of an in-plane two-dimensional velocity vector map at signal detection angle of 90^o (side-scatter) which makes it quite user friendly.

Current trend has shown that there is a need to develop Micro-PIV for applications in micro-channel flows and biological flows.

Typical DPIV Equipments:

Laser system (CW or double-pulsed with good laser beam quality), laser sheet generation optics (high energy mirrors, lenses, posts and mounts), seeding particles generator (solid or liquid vapour), high quality CCD camera, pulse generator for equipment synchronisation (optional), high performance computer (large memory and large capacity hard disk), CD burner for data storage (optional), frame grabber card, D/A & A/D converter board and image processing software.

PIV processing software download (MATLAB)

Original Paper:

 
Adrian, R.J., 1984. Scattering particle characteristics and their effect on pulsed laser measurements of fluid flow: speckle velocimetry vs. particle image velocimetry. Appl. Optics 23, pp. 1690-91.
Pickering, C.J.D. & Halliwell, N.A., 1984. Speckle photography in fluid flows: signal recovery with two-step processing. Appl. Optics 23, pp.1128-1129.

Additional info:

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General Reference:

Adrian, R.J., 1991. Particle-imaging technique for experimental fluid mechanics. Annual Reviews in Fluid Mechanics 23, pp. 261-304.
Wernet, M. P., 2000. Application of DPIV to study both steady state and transient turbo machinery flows. Opt. & Laser Technol. 32, pp. 497-525.

Verdict and Comment:

A conceptually simple and mature planar (near instantaneous) flow measurement technology. DPIV will in the future become the standard in fluid flow measurements within industrial sectors (e.g. car manufacturers) and universities all over the world.