BSA Flow Software Installation and User's Guide - CSI

7.2 Theory of Phase Doppler Anemometry
7.2.1 Basic principles of phase Doppler anemometry
Phase Doppler anemometry (PDA), which is used in Dantec’s conventional
PDA and DualPDA systems, is an extension of Laser Doppler Anemometry
(LDA, see chapter 7.1).
This extension to conventional Phase Doppler Anemometry and further to
the concept of the DualPDA is discussed in this chapter.
In the LDA system (see chapter 7.1) there was only one photo-detector. If
we consider the situation in Figure 7-14, we see two photo-detectors
receiving light scattered from the surface of a reflecting spherical particle
similar to that we observe in LDA
The point to note here is that the difference in optical path length for the
reflections from the two incident beams changes with the position of the
photo-detector. This means that, when the particle passes through the
measuring volume, both photo-detectors receive a Doppler burst of the same
frequency, but the phases of the two bursts vary with the angular position of
the detectors.
This phenomenon was first utilized as an indication of the size of a particle
by Durst & Zaré, who presented their paper Laser Doppler Measurements in
Two-Phase Flows in 1975.
Again it is convenient to introduce the fringe model as a first order of
approximation.
Figure 7-14 The interference patterns differ at the two photo-detector surfaces
Figure 7-15 illustrates the intensity fluctuation in each of the photodetectors
and the time lag, ∆t, separating the wave fronts reaching the two photodetectors.
The corresponding phase difference would be:
BSA Flow Software:Reference guide 7-21