Numerical Investigation of the Role of Turbulent Bursting in Sediment Entrainment
Publication: Journal of Hydraulic Engineering
Volume 133, Issue 3
Abstract
In this work we incorporate a Gram–Charlier-type joint probability distribution of near-bed two-dimensional instantaneous velocities into a simple mechanistic model to investigate the role of turbulent bursting in sediment entrainment. The results reveal that under typical values of bed-shear stress , the time fractions of Quadrants 1–4 (Q1–Q4) remain constantly as 16, 34, 19, and 31%, respectively. Entrainment of the fine sediment mixtures is dominated by the lifting mode, whereas entrainment of the coarse ones is dominated by rolling. Sweeps (Q4) are consistently the most significant contributor to entrainment under various types of sediment mixtures. As the standard deviation of grain-size distribution increases, the hiding effect exerted on the finer grains of the mixture is reduced, leading to the elevated correction factors for effective hydrodynamic forces, and thus the reduced threshold velocities for entrainment. The reduced thresholds would, in turn, enhance the fractional contributions of ejections and inward interactions (Q2 and Q3), which are associated with negative longitudinal velocity fluctuations, such that the fractional contribution of outward interactions (Q1) would become less significant.
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Acknowledgments
The writers acknowledge the research funding granted by the National Science Council, Taiwan, R.O.C. Comments from the ASCE reviewers helped improve the clarity of this work.
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© 2007 ASCE.
History
Received: May 3, 2005
Accepted: Aug 28, 2006
Published online: Mar 1, 2007
Published in print: Mar 2007
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