Particle Image Velocimetry
Particle Image Velocimetry (PIV) is a non-contact optical method for determining velocity fields in fluid mechanics . Particles in the fluid are photographed at short intervals . The direction and speed of movement can be approximately averaged from the particle positions in the images.
principle
Smallest particles are added to a fluid to be examined . For flow analyzes in water, microscopic glass spheres filled with air are usually used. Plastic particles made of polyamide are also suitable . In gases, for example, the particles are generated from conventional edible oil or a test fluid such as DEHS using compressed air. The diameter of the particles is between 0.5 and 50 micrometers . The density of the particles should be chosen similar to the density of the fluid to be examined. A laser beam expanded to a plane illuminates the particles in a pulsating manner. Two pictures are taken at short intervals during one pulse. Usually two CCD chips from a camera are used for this. The time interval between the release delay must be adapted to the main flow velocity. The faster the flow, the shorter the interval between the release delay must be. The particles move in the time between the two images with the local flow velocity. The light from the two pulses reflected by the particles is mapped onto the CCD sensor of a camera with a lens and then digitally processed.
Two-screen process
A separate image of the flowing particles is recorded for each light pulse. The determination of the speed components in the image plane is achieved by calculating the cross-correlation function between adjacent image areas in both images. The camera can be calibrated using the methods of photogrammetry so that the movement can be converted from the image coordinates into spatial coordinates.
Single image process
A variant of the method works with a single image for the evaluation. The intensity of the illumination pulse is varied during the exposure of the image. As a result, each particle draws a trace in the image through its movement, the brightness of which decreases between the beginning and the end. Direction and speed information can thus be assigned to the track. The following 3-D evaluation is carried out exactly as with the two-image method.