Effects of Bed Roughness on Flow around Bed-Mounted Cylinders in Open Channels
Publication: Journal of Engineering Mechanics
Volume 135, Issue 2
Abstract
This paper presents the results of an experimental study of flow around cylindrical objects on a rough bed in an open channel. This is an extension of a previous study of flow around cylinders on a smooth bed. The purpose of this study is to explore the effects of bed roughness on the characteristics of the deflected flow around cylindrical objects and the resulting bed-shear stress distributions. Similar to the previous study cylindrical objects of equal diameter and four heights were tested under similar flow conditions producing four different levels of submergence. Bed shear stress and deflected flow velocities were measured by a thin yaw-type Preston probe after a set of flow visualization tests. Flow visualization tests showed that the horse-shoe vortex systems on the rough bed occupy a relatively greater width compared to the smooth bed. Unlike smooth bed observations, the flow separation point upstream of the cylinder was not dependent on the level of submergence as the separation points were found to appear within a short range of to . Bed shear stress has been found to increase significantly near the shoulder of the cylinders, and its ratio with respect to the approach bed-shear stress was twice as large compared to the smooth bed case. Mean velocity profiles were analyzed in terms of three-dimensional turbulent boundary layer theories. Bed roughness was found to oppose the effect of the lateral pressure gradient that causes skewing in the boundary layer. Perry and Joubert’s model has been found to be equally accurate on smooth and rough beds for predicting the deflected velocity magnitudes around cylinders. The present study will enhance the knowledge of hydraulics of flow around bed-mounted objects (e.g. fish-rocks) in natural streams.
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Acknowledgments
The experiments of this study were conducted at the Ellerslie River Engineering Laboratory of University of Alberta, Edmonton, Canada. The writers are grateful to Perry Fedun for building the experimental arrangement. The first writer is also grateful to the University of Alberta for a F. S. Chia Ph.D. Scholarship. Funds for this study were provided by the Natural Sciences and Engineering Research Council of Canada.
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Information & Authors
Information
Published In
Journal of Engineering Mechanics
Volume 135 • Issue 2 • February 2009
Pages: 100 - 110
Copyright
© 2009 ASCE.
History
Received: Apr 16, 2008
Accepted: Oct 14, 2008
Published online: Feb 1, 2009
Published in print: Feb 2009
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