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Particle Image Velocimetry or PIV  is one of the techniques that is used to measure fluid flow velocity fields in both space and time. The technique depends two steps. First , imaging the flow field. Second, Then analyzing the grabbed images numerically. The flow imaging in PIV is different from the flow visualization techniques, which mainly acquire images for a rough qualitative results. However, PIV technique is used get accurate quantitative measurement of the velocity field and velocity directions throughout the field of view.

Simply, the flow is seeded with tiny particles so as to reflect the light introduced to the flow field and thus be able to be visualized. A thin sheet of light is then to be introduced to the flow field. The light sheet should have a sufficient power in order to be able to get high intensity light  reflected from the seeding particles. With optical and/or digital threshold to be used so as to only have the images of the illuminated particles kept in the images. Again, this emphasizes the need for a powerful light sheet so as to get images of high intensity light reflections from the particles and low intensity light reflections from the flow field which is be removed from the images.

In order to determine the velocity vector, it is necessary to know not only the particle positions but also the time between two successive positions. The particle positions is obtained from the two images by the application of image processing algorithms. The time between successive images is achieved by either the camera frame rate or by multiple laser shot. The laser beams, sources of light, send pulses of light at a certain frequency. Several arrangements of Laser beams and cameras exist. They all have the same principal but the differ in capabilities and then the cost of the system.

In the current investigation the flow is injected with seeding of certain size (50 microns diameter in this case). A Laser light sheet is introduced in the flow field to illuminate a the desired plane. The Laser sheet has a sufficient thickness so that a particle can have at least 5 exposures in a certain time interval upon crossing it. Flow images are captured with a high speed camera (CCD camera) for a certain period of time. This period is highly dependent on hardware capabilities, specifically the camera. Processing every two consecutive images with either some retail software is done to get velocity vectors of the flow field.


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