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observer, a positive sign is taken in front of V 0 . It can be seen from the above
formula that when the observer and the sound source are close to each other,
f > f 0 ; when the observer and the sound source are distant from each other,
f < f 0 .
(2) Laser velocimetry technique
The spectral technique of laser velocimetry relies on the absorption line
frequency of the measured flow field medium component or the Doppler
frequency shift in the fluorescence emission or scattering spectrum. Since
this method directly acquires the velocity from the molecular motion, so as to
avoid the disadvantages caused on the particle, velocity is not only real and of
high precision, but is suitable for high-speed flow measurement (as shown in
Fig. 5.16). Based on a variety of optical velocimetry techniques, such as laserinduced Doppler integrated velocimetry (DGV), filtered Rayleigh scattering
(FRS), coherent Raman spectroscopy (including coherent anti-Stoke Raman
spectroscopy (CARS),), inverse Raman spectroscopy (IRS), and stimulated
Raman gain spectroscopy (SRGS) have been developed.
Fig. 5.16 Laser velocimetry technique