Photon correlation Doppler-difference laser anemometry for wind-tunnel applications.

This study is concerned with the development of laser anemometers based on digital autocorrelation of the scattered-light signal and intended for wind-tunnel applications at the Royal Aircraft Establishment. The optical arrangements are of the Doppler-difference type. The theory of photodetection and photon statistics is first reviewed and the analytical relationship established between the Doppler-difference signal and its autocorrelation function for both laminar and turbulent flows. It is shown that interpretation of the data is greatly facilitated if the optical geometry meets certain simple criteria. When operating in backscatter on large facilities, integration of the signals from a number of successive scatterers is generally necessary, and it is shown how under these circumstances the data can be related by a Fourier transform to the probability distribution of the velocity in the measurement region. The design of data-reduction algorithms capable of fast and accurate extraction of this probability distribution from the composite autocorrelation function is then discussed. The properties of laser beams of particular relevance to the design of the optical system are considered and a simple technique for the measurement of beam diameter and divergence is described. Methods for the determination of fringe spacing, the basic experimental parameter in Doppler-difference arrangements, are assessed and compared. Frequency-modulation devices for use in highly turbulent flows are briefly reviewed and a technique is described by means of which large and variable frequency offsets can be obtained without deviation of the transmitted beam. A number of experiments carried out with photon correlation anemometers on subsonic and supersonic flows under a wide range of turbulence conditions are described. The results of these experiments are presented and discussed. Further improvements which may be incorporated in future systems are considered