Turbulence Characteristics in Supercritical Open Channel Flows: Effects of Froude Number and Aspect Ratio
Publication: Journal of Hydraulic Engineering
Volume 140, Issue 4
Abstract
An experimental investigation of supercritical uniform and gradually varied open channel flows is presented for a wide range of Froude numbers and flume width-to-flow depth aspect ratios. The instantaneous streamwise and vertical flow velocities were measured in a laboratory flume over the entire width using a two dimensional–laser Doppler anemometry (2D-LDA) system to determine turbulence intensities, and bed and Reynolds shear stresses. The mean velocity patterns show undulation across the flume, indicating the presence of counterrotating secondary current cells. These currents redistribute turbulence intensities and bed and Reynolds shear stresses across the flume. For aspect ratios , i.e., narrow open channel flow, the velocity-dip phenomenon is identified both in the streamwise velocity and the Reynolds shear stress distributions. For high aspect ratios, i.e., wide open channel flow, the strength of secondary currents diminish toward the flume center, resulting in a 2D flow farther away from the walls and no velocity-dip phenomenon. Froude number effects on the flow characteristics are less pronounced compared to the aspect ratio effects. At high Froude numbers, the results for narrow and wide open channel flows agree well with literature data. The log-law holds in the inner region across the entire flume width for all investigated Froude numbers and aspect ratios. The Reynolds shear stress distribution agrees well with the computed spanwise bed shear stress distribution. At the flume side walls, the bed shear stresses are higher than the mean values. These results are verified with an engineering example in which high sediment transport and corresponding deep abrasion patterns at the side walls were observed.
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Acknowledgments
The authors like to address their sincere thanks to W. H. Hager for the internal review and acknowledge the financial support of both swisselectric research and the Swiss Federal Office of Energy.
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Published In
Journal of Hydraulic Engineering
Volume 140 • Issue 4 • April 2014
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© 2014 American Society of Civil Engineers.
History
Received: Apr 17, 2013
Accepted: Nov 6, 2013
Published online: Nov 8, 2013
Published in print: Apr 1, 2014
Discussion open until: Jul 12, 2014
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