Measurement of periodic flows using laser Doppler correlation techniques

The application of photon correlation methods to the measurement of sinusoidally fluctuating flows is described. It is shown that with a laser Doppler optical configuration the correlation function for the Doppler signal is periodic with zero order Bessel function damping and that the amplitude of the velocity perturbation can easily be determined by locating the position of zeros of the Bessel function. For a two beam configuration, the cross-correlation function is a distorted form of the velocity probability density function. Experiments were performed in a low turbulence wind tunnel in the wake of a circular cylinder using photon counting techniques for the Doppler signal analysis. The results of these experiments show the characteristic forms taken on by the correlation functions in the different locations. Measurements of velocity perturbation amplitudes obtained by visual inspection of the correlograms agree well with hot wire results. A curve fitting procedure for more accurate evaluation of the flow parameters is described.