The PIV is a quantitative flow visualization technique, which can be used to determine the instantaneous whole-field fluid velocities by recording and processing the multiply-exposed particle image pattern of small traces suspended in the fluid. The PIV particle image is first obtained by illuminating the seeded flow field with a thin laser sheet. The light scattered by the seed particles generates a particle image pattern. This pattern is recorded using a digital CCD video technique (see figure 1). The local fluid velocity is then obtained using digital signal processing techniques.
Figure 1. Schematic arrangement of the PIV system
Figure 2 is a sample PIV image pattern of the flow past a circular cylinder. The multiple illumination is provided using a rotating multi-faceted mirror. A total of four images are recorded for each particle in successive instants. Fast Fourier Transform (FFT) algorithm and autocorrelation scheme are used to determine the separation between particle pairs. The whole-field velocity vectors can be determined by time differentiation and the result is shown in figure 3.
Figure 2 Multiply-exposed PIV particle image field of the flow past a circular cylinder
Figure 3 Corresponding PIV velocity vector field
