Evaluating acoustic Doppler velocimetry pulse-pair spacing/velocity range setting for turbulent flow measurements

Abstract

The velocity range, which is inversely proportional to acoustic pulse-pair spacing, is one of the most important user-set parameters of acoustic Doppler velocimeters (ADVs) and is expected to influence their flow measurements. An experimental study of the effect of the ADV velocity range on the mean and turbulence statistics measured in stagnant water, a turbulent channel flow, and a turbulent jet was undertaken. The results show that as long as the instantaneous velocities are within the user-set velocity range, increasing the velocity range does not noticeably influence the mean velocities, whereas it increases the velocity variances due to the increased noise variance. If the instantaneous velocities exceed the velocity range, phase wrapping occurs, resulting in underestimated mean velocities and overestimated velocity variances. The rate of increase in noise variance with the velocity range increases drastically as the turbulence level rises. From this, it can be inferred that in turbulence measurements, the contribution to the total noise made by Doppler noise is much more substantial than that of the sampling error. Furthermore, it is observed that for highly turbulent flows the ADV correlation significantly drops and signal quality reduces. Increasing the velocity range solves this problem at the expense of higher Doppler noise. Post-processing of the data effectively improves the statistics, even when the velocity range was set to overly high values. Finally, Doppler noise is found to be linearly related to velocity variances (at a constant velocity range), while it is nonlinearly proportional to the velocity range in measurements of turbulent flows.