Particle Image Velocimetry (PIV) is a whole-flow-field technique providing instantaneous velocity vector measurements in a cross-section of a flow. This technique being a non-intrusive one, allows the application of PIV in high speed flows, boundary layer studies of fluids.
The technique is applicable to a range of liquid and gaseous flows. The fluid is seeded with particles which are generally assumed to faithfully follow the flow dynamics. It is the motion of these seeding particles from which the velocity information is calculated. It is done by taking two images shortly after one another and calculating the distance individual particles traveled within this time. The displacement field is determined from the motion of the seeding particles between the two images. The velocity field is obtained by dividing the displacement field by the known time separation.
A typical PIV setup consists of a CCD camera, high power laser, an optical arrangement to convert the laser output light to a light sheet, tracer particles and the synchronizer.
A special camera is utilized so that it can store the first image (frame) fast enough to be ready for the second exposure. There are different types of CCD sensors such as full frame, frame transfer, interline transfer and full frame interline transfer CCDs. A full frame interline transfer progressive scan CCD camera is used to acquire two single exposed images with a time separation of the order of micro seconds.
To avoid having blurred images while analyzing fast flows, laser pulses are to be used. They freeze any motion and also acts as a photographic flash for the digital camera. To obtain high light energy within a short duration of time, a pulsed laser is preferred. For example a double-pulsed Nd-YAG laser or a copper vapour laser.
Only laser light can be focused into a thin enough light sheet so that only particles in that plane are imaged. The light sheet is obtained by using a laser as the source of illumination. A light sheet can be formed from the laser beam by simply using spherical and cylindrical lenses in combination. A cylindrical lens is an essential element for the generation of light sheet.
The tracer particles form the basis of the velocity measurement in PIV. The particles should be as small as possible so that they are able to closely follow the flow. However on the other hand, they may not be too small, because then they will not scatter enough light, and hence produce too weak images. Any particle that follows the flow satisfactorily and scatters enough light to be captured by the camera can be used. The number of particles in the flow is of importance in obtaining a good signal peak in the cross-correlation.
The synchronization between the laser and the camera is controlled by the Synchroniser.
Limitations of PIV:
i)The time delay between the laser pulses should be long enough to capture of the displacement of the tracer particles and short enough so that the particles with an out-of-plane velocity component leaving the light sheet.
ii) With the use of high power lasers, the tracer particles size can be reduced. The accuracy of the PIV measurements will drastically improve as the particles will follow the flow more closely.
ii) The size of the interrogation area should be small such that there is no significant velocity gradient within the interrogation area.