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Coherent Structures in the Flow Around a Bridge Abutment and a Bridge Pier at Equilibirium Scour Conditions

Author(s): Mete Koken; Gokhan Kirkil; George Constantinescu

Linked Author(s): George Constantinescu

Keywords: No Keywords

Abstract: Large Eddy Simulation (LES) is used to investigate the role played by the coherent structures associated with the horseshoe vortex (HV) system and the detached shear layer (DSL) in the flow around a vertical bridge abutment and a circular bridge pier placed in a straight channel with equilibrium scour bathymetry. In both cases the equilibrium bathymetry is obtained from experiments conducted at the same channel Reynolds number. Large Scale Particle Image Velocimetry (LSPIV) together with dye visualizations are used to validate the numerical predictions. It is observed that random ejections of patches of vorticity from the legs of the necklace structures are present. These patches of vorticity travel past the abutment/pier while their axis remains approximately parallel to the bed and their coherence remains high. These vortical eddies move predominantly against the mean slope of the scour hole and can cause sediment entrainment as they are convected over the bed before they dissipate. This mechanism explains, at least partially, the lateral growth of the scour hole in the later stages of the scouring process for both piers and abutments. Additionally, interaction of the vortex tubes convected inside the DSL also helps sediment entrainment as the bed shear stress values are strongly amplified beneath those structures. Finally, it is observed that some of the DSL eddies are randomly entrained into the recirculation region behind the abutment/pier. This explains how the scour hole is growing in the wake region situated just behind the abutment/pier and how the entrained sediment is eventually transported in the deposition regions behind the structure


Year: 2007

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