A rapidly scanning laser Doppler anemometer

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Abstract
A rapidly scanning directionally sensitive fringe-type laser Doppler anemometer (SLDA) which scans the measurement volume perpendicular to the optical axis of the transmitting optics is described. Scan frequencies up to 60 Hz over scan distances of 40 cm have been used, although scan frequencies up to 150 Hz are possible. The maximum scanning velocity of the measurement volume that can be used is directly proportional to the shift frequency of the Bragg cell since each signal-producing particle must cross a minimum number of fringes to produce a valid signal. Signal-averaging bias is lower with a scanning LDA than with pointwise measurements. Results obtained for a separating turbulent boundary layer show that very good mean and RMS velocity profiles can be obtained in less than 1 min of data acquisition.