EVALUATION OF THE ACOUSTIC DOPPLER-VELOCIMETER (ADV) FOR TURBULENCE MEASUREMENTS

Accuracy of the acoustic Doppler velocimeter (ADV) is evaluated in thi s paper. Simultaneous measurements of open-channel how were undertaken in a 17-m flume using an ADV and a laser Doppler velocimeter. Flow ve locity records obtained by both instruments are used for estimating th e true (''ground truth'') flow characteristics and the noise variances encountered during the experimental runs. The measured values are com pared with estimates of the true how characteristics and values of var iance ([u'(2)], [w'(2)]),, and,,,,,,,,, covariance ([u'w']) predicted by semiempirical models for open-channel flow. The analysis showed tha t the ADV sensor can measure mean velocity and Reynolds stress within 1% of the estimated true value. Mean velocities can be obtained at dis tances less than 1 cm from the boundary, whereas Reynolds stress value s obtained at elevations greater than 3 cm above the bottom exhibit a variation that is in agreement with the predictions of the semiempiric al models. Closer to the boundary, the measured Reynolds stresses devi ate from those predicted by the model, probably due to the size of the ADV sample volume. Turbulence spectra computed using the ADV records agree with theoretical spectra after corrections are applied for the s patial averaging due to the size of the sample volume and a noise floo r The noise variance in ADV velocity records consists of two terms. On e is related to the electronic circuitry of the sensor and its ability to resolve phase differences, whereas the second is flow related. The latter noise component dominates at rapid flows. The error in how mea surements due to the former noise term depends on sensor velocity rang e setting and ranges from +/-0.95 to +/-3.0 mm s(-1). Noise due to she ar within the sample volume and to Doppler broadening is primarily a f unction of the turbulence dissipation parameter. Noise variances calcu lated using spectral analysis and the results of the ground truthing t echnique are compared with theoretical estimates of noise.