Turbulent transport measurements with a laser Doppler velocimeter

The power spectrum of phototube current from a laser Doppler velocimeter operating in the heterodyne mode has been computed. The spectrum is obtained in terms of the space time correlation function, G(†r,t), of the fluid. The spectral width and shape predicted by the theory is in agreement with experiment. For normal operating parameters the time average spectrum contains information only for times shorter than the Lagrangian integral time scale of the turbulence. To examine the long time behaviour one must use either extremely small scattering angles, much longer wave-length radiation or a different mode of signal analysis, e.g. F.M. detection.