11.4 Laser Doppler Velocimetry or Anemometry
245
a particle is crossing this fringe pattern, it is subjected to a dark or bright illumination, the intensity of the light scattered by the particle occurring at a frequency,
f m , inversely proportional to the fringe spacing, i, and directly proportional to the
velocity component of the particle, U, which is normal to the fringe. As the fringe
spacing is defined by the optical characteristic of the laser source, f m is captured by
a receiving optic and the velocity component can be obtained from:
U = i f m
11.4.2 Signal Analysis
The light scattered by the particles is collected and transmitted to a photomultiplier
which converts it into an electronic signal. The passage of a particle within the probe
volume produces a signal or more commonly a “burst” as shown in Fig. 11.7. This
signal is constituted of a low frequency component (the pedestal) corresponding to the
Gaussian distribution of the light in the emitted laser beam, on which the periodic
signal containing the interesting information is superimposed, together with high
Fig. 11.7 The unfiltered and filtered signal from a burst
245
a particle is crossing this fringe pattern, it is subjected to a dark or bright illumination, the intensity of the light scattered by the particle occurring at a frequency,
f m , inversely proportional to the fringe spacing, i, and directly proportional to the
velocity component of the particle, U, which is normal to the fringe. As the fringe
spacing is defined by the optical characteristic of the laser source, f m is captured by
a receiving optic and the velocity component can be obtained from:
U = i f m
11.4.2 Signal Analysis
The light scattered by the particles is collected and transmitted to a photomultiplier
which converts it into an electronic signal. The passage of a particle within the probe
volume produces a signal or more commonly a “burst” as shown in Fig. 11.7. This
signal is constituted of a low frequency component (the pedestal) corresponding to the
Gaussian distribution of the light in the emitted laser beam, on which the periodic
signal containing the interesting information is superimposed, together with high
Fig. 11.7 The unfiltered and filtered signal from a burst