Near-Transducer Errors in ADCP Measurements: Experimental Findings
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Volume 136, Issue 5
Abstract
Acoustic Doppler current profilers (ADCPs) are not able to accurately determine velocity near their transducers and near the bed. These limitations have restricted the use of ADCPs to flow depths that are large enough to allow acquisition of few directly measured velocity data that can be subsequently used to accurately estimate vertical velocity profiles and flow discharge in cross sections. While the causes that make ADCPs unable to collect data in the near-bed region are relatively well documented, the causes of near-transducer errors have not yet been fully understood and are only partly documented. We present results from an experimental study aimed at characterizing the systematic errors due to the combined effect of acoustic interference and instrument-induced flow disturbance near a Janus-configured ADCP. The study comprises: (1) concurrent measurements with an ADCP and an acoustic Doppler velocimeter (ADV) under the ADCP; (2) measurements of the flow disturbance produced by the ADCP in the vertical and horizontal planes; and (3) ADV measurements along the path of the acoustic beams ensonified by the ADCP during a measurement. Results suggest that ADCPs bias low the velocity profiles with respect to the undisturbed velocity profiles, mostly because of the flow disturbance induced by the ADCP, with acoustic effects playing a secondary role. For the range of flows we studied, both undisturbed and disturbed profiles exhibit similar shapes when plotted in dimensionless form, with the bulk flow velocity and the ADCP diameter as characteristic scales. The differences between the undisturbed and the ADCP-disturbed profiles extend up to a distance of about from the ADCP, except for the profiles measured at locations where the flow depth is close to for which the boundary layer induced by the ADCP interacts with the one induced by the flume bed.
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Acknowledgments
This work is the result of the cooperative efforts by the South Florida Water Management District and the IIHR-Hydroscience and Engineering, The University of Iowa led by the the first and third writers. Special thanks are due to Zachariah Brown and Aaron Burkhardt, former students at the University of Iowa for their involvement in laboratory work, and to Earl Childress from Teledyne-RDI for lending the ZedHead ADCP used for this study. The third writer gratefully acknowledges Robb Startzman and Matahel Ansar from SFWMD for their support to work related to ADCP measurement uncertainty, and to his colleagues from the District’s Operations and Hydro Data Management Division for their technical input. The comments from Kevin Oberg and David Mueller from the USGS on the interpretation of the partition of near-transducer errors are gratefully appreciated. The three anonymous reviewers, the associate editor, and the editor of the Journal (Dennis Lyn) provided insightful comments that helped the writers’ interpret the data more rigorously and improve the final form of the manuscript. Use of trade, product, or commercial names does not imply endorsement by the South Florida Water Management District, The University of Iowa, or the present writers.
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Information & Authors
Information
Published In
Journal of Hydraulic Engineering
Volume 136 • Issue 5 • May 2010
Pages: 275 - 289
Copyright
© 2010 ASCE.
History
Received: Oct 25, 2008
Accepted: Oct 27, 2009
Published online: Oct 29, 2009
Published in print: May 2010
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