Coherent Structures in the Flow Field around a Circular Cylinder with Scour Hole
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Abstract
Large-eddy simulation (LES) and laboratory-flume visualizations were used to investigate coherent structures present in the flow field around a circular cylinder located in a scour hole. The bathymetry corresponds to equilibrium scour conditions and is fixed in LES. The flow parameters in the simulation correspond to the experimental conditions in which the approach flow is fully turbulent. Detailed consideration is given to the interaction of the horseshoe vortex (HV) system within the scour hole with the detached shear layers formed from the cylinder, and the near bed turbulence. It is found that the overall structure of the HV system varies considerably in space and time, though a large, relatively stable, primary necklace vortex is present at practically all times inside the scour hole. The simulation captures the presence of bimodal chaotic oscillations inside the HV system, as well as the sharp increase in the resolved turbulent kinetic energy levels and pressure fluctuations reported in prior experimental investigations. High levels of the mean bed shear stress are observed beneath the primary necklace vortex, especially over the region where the bimodal oscillations are strong, as well as beneath the small junction vortex at the base of the cylinder. It is also found that the detachment and advection of patches of vorticity from the downstream part of the legs of the necklace vortices can induce large instantaneous bed shear stress values. When the critical bed shear stress value for sediment entrainment on a flat surface is adjusted for bed slope effects, the LES simulation correctly predicts that the distribution of the mean bed shear stress is consistent with equilibrium scour conditions.
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Acknowledgments
The writers thank Dr. Marian Muste from IIHR-Hydroscience and Engineering for his advice related to the experimental part of the study and help in using the LSPIV technique. The writers would also like to thank the National Center for High Performance Computing (NCHC) in Taiwan for providing the computational resources needed to perform some of the simulations as part of a collaboration program between the two institutions. The writers would also like to thank the anonymous reviewers for their comments, which helped to improve the paper.
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Published In
Journal of Hydraulic Engineering
Volume 134 • Issue 5 • May 2008
Pages: 572 - 587
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© 2008 ASCE.
History
Received: Jun 16, 2006
Accepted: Aug 15, 2007
Published online: May 1, 2008
Published in print: May 2008
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