Chapt06

Hot wires can easily be arranged in groups to measure

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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: 1. 2. 3. 4. 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 [29] in a high-speed centrifugal compressor impeller....
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