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Using a Broadband ADCP in a Tidal Channel. Part I: Mean Flow and Shear

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  • 1 School of Earth and Ocean Sciences, University of Victoria, Victoria, British Columbia, Canada
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Abstract

This paper discusses the principles of measuring the mean velocity and its vertical shear in a turbulent flow using an acoustic Doppler current profiler (ADCP), and presents an analysis of data gathered in a tidal channel. The assumption of horizontal homogeneity of the first moments is fundamental to the derivation of the mean velocity vector because the velocity is never homogeneous over the span of the beams in a turbulent flow. Two tests of this assumption are developed—a comparison of the mean error velocity against its standard deviation and against the mean speed. The fraction of the samples that pass these tests increases with increasing spatial averaging and exceeds 95% for distances longer than 55 beam separations. The statistical uncertainty of the velocity and shear vector, averaged over 10 min and longer, stems from turbulent fluctuations rather than Doppler noise. Estimation of the vertical velocity requires a correction for the bias in the measured tilt.

The mean velocity and shear estimates from this natural tidal channel show more complex depth–time variations than found in idealized one-dimensional channel flow, which seldom occurs in nature. The ADCP measurements reveal the secondary circulation, bursts of up- and downwelling, shear reversals, and transverse velocity shear.

* Current affiliation: Department of Oceanography, Dalhousie University, Halifax, Nova Scotia, Canada.

Corresponding author address: Dr. Rolf G. Lueck, School of Earth and Ocean Sciences, University of Victoria, P.O. Box 1700, 3800 Finnerty Rd., Victoria, BC V8W 2Y2, Canada.

Email: rlueck@uvic.ca

Abstract

This paper discusses the principles of measuring the mean velocity and its vertical shear in a turbulent flow using an acoustic Doppler current profiler (ADCP), and presents an analysis of data gathered in a tidal channel. The assumption of horizontal homogeneity of the first moments is fundamental to the derivation of the mean velocity vector because the velocity is never homogeneous over the span of the beams in a turbulent flow. Two tests of this assumption are developed—a comparison of the mean error velocity against its standard deviation and against the mean speed. The fraction of the samples that pass these tests increases with increasing spatial averaging and exceeds 95% for distances longer than 55 beam separations. The statistical uncertainty of the velocity and shear vector, averaged over 10 min and longer, stems from turbulent fluctuations rather than Doppler noise. Estimation of the vertical velocity requires a correction for the bias in the measured tilt.

The mean velocity and shear estimates from this natural tidal channel show more complex depth–time variations than found in idealized one-dimensional channel flow, which seldom occurs in nature. The ADCP measurements reveal the secondary circulation, bursts of up- and downwelling, shear reversals, and transverse velocity shear.

* Current affiliation: Department of Oceanography, Dalhousie University, Halifax, Nova Scotia, Canada.

Corresponding author address: Dr. Rolf G. Lueck, School of Earth and Ocean Sciences, University of Victoria, P.O. Box 1700, 3800 Finnerty Rd., Victoria, BC V8W 2Y2, Canada.

Email: rlueck@uvic.ca

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