When the waves of two beams interfere in the measurement volume, it creates bright fringes when they are in phase and dark fringes when they are out of phase. The bright and dark fringes form lines parallel to the mid-plane between two incident laser beams as shown in Fig. 7.4.8(a). The spacing between fringe lines can be viewed as wavelength of fringes as shown in Fig. 7.4.8(b).

(7.4.9)

It is the fundamental LDV equation that shows the flow velocity proportional to the frequency.

Particle Image Velocimetry (PIV) : It is a double-pulsed laser technique used to measure instantaneous velocity distribution in a plane of flow by determining the displacement of particles in that plane during a short time interval. The all other techniques such as LDV and thermal anemometry, measure the velocity at a point while PIV provides the velocity values simultaneously throughout entire cross-section through instantaneous flow field mapping.

Fig. 7.4.9: Experimental arrangement of a PIV system .

 

The PIV technique for velocity measurement is based on flow visualization and image processing as shown in Fig. 7.4.9. The first step is to trace the flow with suitable seed particles in order to obtain the pathlines of fluid motion. A pulse of laser light illuminates certain region of flow field at any desired plane and the photographic view is recorded digitally by using a video camera positioned at right angle to the plane. After a short interval of time , the particles are illuminated again through the laser light and the new positions are recorded. Using the information of both the images, the particle displacement is determined and subsequently the magnitude of velocity of the particle in the plane is calculated.

A variety of laser sources such as argon, copper vapour and Nd-YAG can be used with PIV system, depending on the requirements for pulse duration, power and time between the pulses. Silicon carbide, titanium dioxide and polystyrene latex particles are few categories of seed particles that are used depending on the type of fluid (liquid/gas). In addition to velocity measurement, the PIV is capable of measuring other flow properties such as vorticity and strain rates. The measurements can be extended to supersonic flows, explosions, flame propagation and unsteady flows. The accuracy, flexibility and versatility are the few distinct advantages of a PIV system.