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Optical Flow Path

Planar Velocity Techniques
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Particle Imaging Velocimetry (PIV)

PIV is a planar measurement technique wherein a pulsed laser light sheet is used to illuminate a flow field seeded with tracer particles small enough to accurately follow the flow. The positions of the particles are recorded on either photographic film or digital CCD cameras at each instant the light sheet is pulsed. The data processing consists of either determining the average displacement of the particles over a small interrogation region in the image or the individual particle displacements between pulses of the light sheet. Knowledge of the time interval between light sheet pulses then permits computation of the flow velocity. The figure below shows the configuration of a typical PIV experiment where a pair of pulsed Nd:YAG lasers are used to provide the pulsed light sheet illumination. The positions of particles entrained in the flow is recorded by a CCD camera, which is oriented 90 degrees to the plane of the light sheet. Depending on the type of CCD camera used and the particle concentration either particle tracking or correlation processing can be used to produce the processed velocity vector map. In low particle concentration cases the individual particle displacement can be determined. In high particle concentration cases, correlation processing is usually the technique of choice. In some instances, particle tracking can be used after correlation processing to provide "super resolution" particle velocity maps.


PIV Animation

The animation below shows a seeded flow entering the test section and the pulsed laser firing. A pair of single exposure images are acquired, and then processed via cross-correlation. The resulting vector map is displayed as the processing progresses.

Frame Straddling

The ability to acquire a pair of single exposure image frames is the critical element in any PIV system, since it facilitates the use of cross-correlation PIV data processing. Therefore, one of the key elements of a Digital PIV system, is the cross-correlation camera. Acquisition of a pair of single exposure image frames using a single CCD camera would normally not be possible were it not for the development of the “frame-straddling” technique first demonstrated by Wernet, 1991. Refinement of the technique has led to the development of commercial PIV cameras which permit a pair of image frames to be acquired with a very small inter-frame period (<1 msec).  The cameras employed are standard RS-170, 60 fields/sec video cameras or high resolution full frame CCD imagers running at 30 frames/sec. Full frame CCD cameras have both a light sensitive frame integration area and also a frame storage area. After frame integration (light detection from the pulsed laser), the entire image is shifted into the frame storage area in a very short time interval (<1 msec).  The information in the storage area is readout while a new frame is being collected in the light sensitive region of the CCD. Using the “frame-straddling” technique, full frame camera systems offer inter-frame exposure intervals of 1/30 sec down to < 1 msec.  These inter-frame exposure values are obtained by adjusting the laser pulse timing to occur such that two successive single exposure image frames are obtained from the CCD camera. The first laser pulse is made to occur just at the end of one video frame integration period and the second laser pulse at the beginning of the next frame integration period. The minimum time separation between laser pulses is limited by the frame transfer period (the time it takes to shift a video frame down to the camera’s storage register) of the camera. Using this technique, single exposure image frames with time separations < 1 msec  have been obtained


Wernet, M. P., “Particle Displacement Tracking Technique Applied to Air Flows”, 4th International Conference on Laser Anemometry, Cleveland, Ohio, August 5-9, 1991.

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Last Updated: 12/20/2005