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Unformatted text preview: one photodetector and other operating modes. Either liquids or gases can be measured as long as scattering particles are present. In liquids, normal impurities serve as scatterers, but gases
may have to be seeded. The particles may be as small as the wavelength of the light.
Although the measuring volume is not as small as with a hot wire, the LDA is capable of measuring turbulent fluctuations.
The advantages of the LDA are as follows:
5. No disturbance of the flow
High spatial resolution of the flow field
Velocity data that are independent of the fluid thermodynamic properties
An output voltage that is linear with velocity
No need for calibration The disadvantages are that both the apparatus and the fluid must be transparent to light
and that the cost is high (a basic system shown in Fig. 6.29h begins at about $50,000).
Once installed, an LDA can map the entire flow field in minutest detail. To truly
appreciate the power of the LDA, one should examine, e.g., the amazingly detailed
three-dimensional flow profiles measured by Eckardt  in a high-speed centrifugal
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- Spring '08