Scattering Devices

All the measurement techniques discussed earlier, determine the velocity by disturbing the flow. In some cases, the disturbance is very less (such as ultrasonic flow meter and thermal anemometers) while in other cases (orifice, pitot probe etc.), sufficient care to insert the measuring device to minimize the disturbances. So they are classified as intrusive based measurements. The modern instrumentation method used optical technique to measure the flow velocity at any desired location without disturbing the flow. So they are called as non-intrusive based measurement and works on the principle of scattering light and sound waves in a moving fluid. By measuring the frequency difference between scattered and un-scattered wave, particle/flow speed can be found. The Doppler frequency shift is responsible for this change in the speed and is illustrated in Fig. 7.4.6.

(7.4.8)

where, V is the particle velocity, is the wavelength of original wave before scattering, α and β are the angles shown in Fig. 7.4.6. The important aspect of the Eq. (7.4.8) is the proportionality between . If can be measured, then one can obtain the particle speed. Since, the particle moves with the flow, so the particle speed is equal to flow speed. Since, the laser light and ultrasonic waves have relatively high frequencies, they are normally used for measuring flow velocity because the Doppler shift will be only a small fraction compared to original frequency. Laser Doppler Velocimetry (LDV) and Particle Image Velocimetry (PIV) are the optical techniques that work on the principle of Doppler shift .

Fig. 7.4.6: Illustration of Doppler shift for a moving fluid particle.

 

Laser Doppler Velocimetry: It is also termed as Laser Velocimetry (LV) or Laser Doppler Anemometry (LDA). The operating principle of LDV is based on sending a highly coherent monochromatic light beam towards a fluid particle. The monochromatic light beam has same wavelengths and all the waves are in phase. The light reflected from the fluid particle wave will have different frequencies and the change in frequency of reflected radiation due to Doppler effect is the measure of fluid velocity. A basic configuration of a LDV setup is shown in Fig. 7.4.7. The laser power source is normally a helium-neon/argon-ion laser with a power output of 10mW to 20W. The laser beam is first split into two parallel beams of equal intensity by a mirror and beam-splitters. Both the beams pass through a converging lens that focuses the beams at a point in the flow. The small fluid volume where the two beams intersect is the measurement volume where the velocity is measured. Typically, it has a dimension of 0.1mm diameter and 0.5mm long. Finally, the frequency information of scattered and unscattered laser light collected through receiving lens and photo-detector, is converted to voltage signal. Subsequently, flow velocity is calculated.

Fig. 7.4.7: Schematic representation of a LDV setup.

Fig. 7.4.8: Interference of laser beams: (a) Formation of fringes; (b) Fringe lines and wavelengths.