Laser-Doppler system for rapid scanning of flow fields

A plane mirror mounted on an optical scanner was used to deflect the outgoing beams of a laser-Doppler anemometer operated in backscatter mode, thus permitting, through movement of the mirror, rapid scanning of the measuring control volume throughout the flowfield. Furthermore, the use of a zoom lens element in the transmitting optics enabled the depth range of measurement to be extended without the need to mechanically traverse the system. The signal analysis for a scanning LDA is considered for the case of mean and fluctuating velocity measurements and the effects of the inherent spatial averaging is examined. The analysis yields results that are confirmed by measurements made in a simple coaxial air jet using a transient recorder-based signal processing system. Several improvements to the present system are suggested and the applicability of the scanning LDA in turbulence research is discussed.