Laser Doppler Velocimetry

An optical method of measuring instantaneous point velocity in a flow region is described as below.: Laser Doppler Velocimetry (LDV) is a non-intrusive technique for velocity measurement though it requires seeding of flow with small non-buoyant particles. Liquids naturally contain a large number of suspensions that can scatter light; in gaseous flows a seed generator that atomizes non-volatile oil, e.g. glycerine is required. LDV is based on the Doppler effect. It measures velocity in both isothermal and non-isothermal flows and is not affected by temperature variations in the fluid. This is a major advantage over thermal transducers, e.g. a hot-wire. Fluid temperature can be measured using interferometry or by Rayleigh scattering. LDV is a linear device and is hence uniformly sensitive to low as well as high velocities. It can measure three components of velocity as well as turbulence. Monochromatic light is adequate for mean velocity measurement; however, coherence of light is required for turbulence measurement. LDV tends to be expensive and requires an exceptionally clean and stable environment for its operation.

Let be the direction of propagation of light from a monochromatic source (Figure 3.3). The point at which the velocity is being measured is denoted as . Let be the unit vector that represents the position of the observer relative to and let be the velocity vector. The Doppler effect is stated as: The frequency of radiation from a source that is scattered by a particle moving relative to it is changed by an amount that depends on the relative velocity and the positions of the source and the observer. Let be the frequency, the wavelength of the incident light, the frequency of scattered light from a particle moving with a velocity assuming that the observer is stationary. Scattering will be more or less uniform in all directions. The Doppler shift in the frequency of the scattered light is given by