Measurements of Flow Velocities in Gases and Liquids

For many technical problems of hydrodynamics or aerodynamics, the velocity profile v(r,t) of a flowing medium in pipes or around solid bodies is of great importance. Doppler anemometry (Sect. 12.8) is a technique of heterodyne laser spectroscopy, where these velocity profiles can be determined from the measured Doppler shifts of the scattered light [15.121-15.123]. The beam of a HeNe or Ar laser with wave vector k] passes through a volume element dV of the flowing medium. The frequency cu of light, scattered in the direction by particles with velocity v (Fig. 15.21), is Doppler shifted to 

The scattered light is imaged onto a detector, where it is superimposed with part of the laser beam. The detector output contains the difference-frequency spectrum Ao) = cjoi — (i = ( l + s) v, which is electronically monitored with a heterodyne technique. One example is an airborne CO2 laser anemometer that was developed for measuring wind velocities in the stratosphere in order to improve long-term weather forecasts [15.124]. Further examples are measurements of the velocity profiles in the exhaust of turbine engines of planes, in pipelines for gases and liquids, or even in the arteries of the human body. 

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