Effect of Acoustic Doppler Velocimetry Sampling Frequency on Statistical Measurements of Turbulent Axisymmetric Jets

Abstract

Acoustic Doppler velocimeters (ADVs) are used extensively in various field and laboratory studies of hydraulic engineering. However, their accuracy in predicting statistics of turbulence quantities has been questioned. Two fundamental limitations of this type of velocimeter are Doppler noise and the damping of fluctuations due to the temporal averaging performed by the instrument. An important factor that may affect both error sources is the sampling frequency of the ADV. An experimental investigation of the effect of the ADV sampling frequency on the measurement of both the mean and the high-order statistics of the flow in a turbulent jet was conducted. The experiments were carried out in the self-similar zone of an axisymmetric nonbuoyant jet at a Reynolds number of 10,000 released into quiescent water. Measurements of the mean and RMS velocities, spectra, and Reynolds shear stresses at different sampling frequencies are presented. Results were compared with those of other measurement techniques and interpreted using a novel analytical model quantifying the noise and the damping effect on the basis of their nonvanishing statistical correlation in the postaveraging domain, as well as the ratio of the flow’s integral timescale to the sampling interval. The damping effect at high sampling frequencies was eliminated using a hypothesis of proportionality of a relative change in the correlation coefficient to a change in the noise variance, provided that the integral timescale is adequately larger than the sampling interval. The proposed precision-enhancement technique (referred to herein as denoising and reverse-damping transformation) was shown to improve the accuracy of velocity variances. The results and model offer an opportunity to improve the precision of ADV measurements in turbulent flows.