Measurement of turbulent velocities from the Doppler shift in scattered laser light

The local mean values and fluctuations of the velocity of water in turbulent flow through a tube have been determined from the Doppler shifts in the frequency of laser light scattered by small particles in the water. An optical hetrodyne technique was used to measure the shift in frequencies. The limited lifetime of the Doppler shifted signals, due to the finite time of passage of the scattering particles through the laser beam, causes broadening of the frequency spectra representing the fluctuations in turbulent velocity. This broadening, which is analogous to the Doppler radar ambiguity, was investigated with laminar flow and was allowed for in interpreting the spectra observed with turbulent flow. The optics were chosen so that the coherence limits were at all times greater than the beam diameter and did not further shorten the lifetimes of the Doppler signals. The root mean squares of the fluctuations in velocity, calculated from the observed spectra, are in agreement with corresponding data obtained by hot wire anemometry.