Aspects of water velocity measurements with acoustic Doppler velocimetry

Abstract

The paper analyzes the various aspects of acoustic Doppler anemometry usage, for the measurements of instantaneous water velocity, in the context of the problems of hydraulic engineering. Physical principle of the measuring method is described. The method itself, in current experience, is proved to be extremely robust and practical for the purposes of monitoring, research, etc. Some of the examples of its application both in the laboratory and in field conditions are shown. Practical limitations in the method usage are defined, on the basis of experience in the application. Inherent flaws of the method, which are originating from the use of the Doppler principle for determining speed, are described: Doppler signal aliasing and the appearance of the Doppler noise. Signal aliasing is one of the main problems in the use of this measurement technique, since it affects the accuracy of the measured velocity components. Data contaminated by aliasing errors, can be processed in the post processing by applying some of the algorithms for the detection of peaks and their subsequent replacement. The paper briefly describes the algorithms for the detection of peaks with the use of a filter based on a projection of the phase-space as well as on the basis of correlation between fluctuating velocity components. Also, algorithm for replacement of detected erroneous data using the fitted cubic polynomial interpolation is presented.

U radu su analizirani različiti aspekti primene akustične Dopler anemometrije (merenje vektora brzine u jednoj tački) za merenje trenutne brzine vode, u kontekstu problema hidrotehnike. Opisan je fizički princip rada ove merne metode, koja se u dosadašnjem iskustvu pokazala kao izuzetno robusna i praktična za potrebe monitoringa i fundamentalnog istraživanja rasporeda brzina. U radu su prikazani neki od primera njene primene, kako u laboratorijskim tako i u terenskim uslovima. Navedena su i praktična ograničenja upotrebe ove metode, definisana na osnovu iskustva u primeni. Opisane su nasledne mane merne metode koje potiču od upotrebe Doplerovog principa za određivanje brzina: alijasing Doplerovog signala i pojava Doplerovog šuma. Alijasing signala predstavlja jedan od glavnih problema u upotrebi ove merne tehnike, budući da utiče na tačnost proračuna komponenati brzine. Podaci kontaminirani pojavom alijasinga se mogu obraditi u naknadnom procesiranju primenom nekog od algoritama za detekciju pikova i njihovu naknadnu zamenu. U radu su ukratko opisani algoritmi za detekciju pikova sa upotrebom filtra na bazi projekcija faza-prostor kao i na bazi korelacija između fluktuacija komponenata brzina. Takođe, u radu je predložen i algoritam za zamenu detektovanih podataka koji koristi interpolaciju fitovanih kubnih polinoma.