Limited accuracy of turbulence measurement in laser Doppler anemometry

Laser Doppler anemometry is in wide-spread use in research laboratories of industry and university. Based on innumerable applications in the past, there is no doubt that laser Doppler anemometry is one of the most accurate flow measuring techniques. However, recent investigations have shown that the period lengths of LDA signal bursts are not as constant as one might expect within the individual burst. Since a finite number of cycles within each burst is evaluated for frequency determination, this induces an additional scatter in the measurement of flow velocity. The paper describes experimental investigations which show quantitatively that the particle passage through the laser beams in the vicinity of the point of superposition, i.e. the LDA measuring volume, yields a distorted LDA fringe pattern which results in a scatter of the measured velocity data for those particles, passing the center of the measuring volume at the same time. As a consequence, turbulence is determined slightly higher than in reality.