Effect of finite-size probe volume upon laser Doppler anemometer measurements

Abstract

The biasing of an ensemble of individual realization laser Doppler anemometer data caused by both spatial and temporal variation of the velocity in the probe volume has been analyzed. The analysis shows that the timeand space-average velocity through the probe volume can be calculated with a weighted ensemble of data. Likewise, the root-mean-square velocity in the probe volume can also be calculated with a weighted average. However, the velocity variance in the data ensemble originates from three sources: velocity fluctuations caused by turbulence at the center of the probe volume; spatial variations in the mean velocity across the probe volume; and spatial variations in velocity fluctuations across the probe volume. Since the root-mean-square velocity at the center of probe volume is generally the parameter of interest, the other two sources of velocity variance must be subtracted from the measured value. The results of the analysis are used to reduce LDA data taken in the viscous sublayer of a channel flow of water. Without correction, the mean velocity estimates can be as much as 109/o too high while the root-mean-square estimates are as much as 100% high. The proposed correction techniques brings the LDA data into good agreement with previous hot-film data.